var format = {

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    number_with_delimiter: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

    kbytes_to_human_size: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      return ordinalize(~~val);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    return sprintf('%.' + (~~precision) + 'f', val);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

  function number_with_delimiter(val, options) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      intpart = ~~( intpart / 1000 );

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    var numstr = val;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

        return _.capitalize( String(val) );

Close

Disallow or enforce spaces inside of parentheses (space-in-parens)

Some style guides require or disallow spaces inside of parentheses:

foo( 'bar' );
var x = ( 1 + 2 ) * 3;

foo('bar');
var x = (1 + 2) * 3;

Rule Details

This rule will enforce consistency of spacing directly inside of parentheses, by disallowing or requiring one or more spaces to the right of ( and to the left of ). In either case, () will still be allowed.

Options

There are two options for this rule:

• "never" (default) enforces zero spaces inside of parentheses
• "always" enforces a space inside of parentheses

Depending on your coding conventions, you can choose either option by specifying it in your configuration:

"space-in-parens": ["error", "always"]

"never"

Examples of incorrect code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo( 'bar');
foo('bar' );
foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Examples of correct code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo();

foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

"always"

Examples of incorrect code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo( 'bar');
foo('bar' );
foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

Examples of correct code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo();

foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Exceptions

An object literal may be used as a third array item to specify exceptions, with the key "exceptions" and an array as the value. These exceptions work in the context of the first option. That is, if "always" is set to enforce spacing, then any "exception" will disallow spacing. Conversely, if "never" is set to disallow spacing, then any "exception" will enforce spacing.

The following exceptions are available: ["{}", "[]", "()", "empty"].

Examples of incorrect code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo(1, {bar: 'baz'});

Examples of correct code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo(1, {bar: 'baz'} );

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo( 1, {bar: 'baz'} );

Examples of correct code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo( 1, {bar: 'baz'});

Examples of incorrect code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1);

Examples of correct code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1 );

Examples of incorrect code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo((1 + 2));
foo((1 + 2), 1);

Examples of correct code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo( (1 + 2) );
foo( (1 + 2), 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo( ( 1 + 2 ) );
foo( ( 1 + 2 ), 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo(( 1 + 2 ));
foo(( 1 + 2 ), 1 );

The "empty" exception concerns empty parentheses, and works the same way as the other exceptions, inverting the first option.

Example of incorrect code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo();

Example of correct code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo( );

Example of incorrect code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo( );

Example of correct code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo();

You can include multiple entries in the "exceptions" array.

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar( {bar:'baz'} );
baz( 1, [1,2] );
foo( {bar: 'baz'}, [1, 2] );

Examples of correct code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar({bar:'baz'});
baz( 1, [1,2]);
foo({bar: 'baz'}, [1, 2]);

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between parentheses.

Related Rules

• [space-in-brackets](space-in-brackets.md) (deprecated) Source: http://eslint.org/docs/rules/

      var stime;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var etime;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var days    = ~~(val / 86400);

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var days    = ~~(val / 86400);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

  function get_time_zone(default_tz) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      v[0] = v[0].replace(/0+$/, '').replace(/\.$/, '')

Close

require or disallow semicolons instead of ASI (semi)

JavaScript is unique amongst the C-like languages in that it doesn't require semicolons at the end of each statement. In many cases, the JavaScript engine can determine that a semicolon should be in a certain spot and will automatically add it. This feature is known as automatic semicolon insertion (ASI) and is considered one of the more controversial features of JavaScript. For example, the following lines are both valid:

var name = "ESLint"
var website = "eslint.org";

On the first line, the JavaScript engine will automatically insert a semicolon, so this is not considered a syntax error. The JavaScript engine still knows how to interpret the line and knows that the line end indicates the end of the statement.

In the debate over ASI, there are generally two schools of thought. The first is that we should treat ASI as if it didn't exist and always include semicolons manually. The rationale is that it's easier to always include semicolons than to try to remember when they are or are not required, and thus decreases the possibility of introducing an error.

However, the ASI mechanism can sometimes be tricky to people who are using semicolons. For example, consider this code:

return
{
    name: "ESLint"
};

This may look like a return statement that returns an object literal, however, the JavaScript engine will interpret this code as:

return;
{
    name: "ESLint";
}

Effectively, a semicolon is inserted after the return statement, causing the code below it (a labeled literal inside a block) to be unreachable. This rule and the [no-unreachable](no-unreachable.md) rule will protect your code from such cases.

On the other side of the argument are those who say that since semicolons are inserted automatically, they are optional and do not need to be inserted manually. However, the ASI mechanism can also be tricky to people who don't use semicolons. For example, consider this code:

var globalCounter = { }

(function () {
    var n = 0
    globalCounter.increment = function () {
        return ++n
    }
})()

In this example, a semicolon will not be inserted after the first line, causing a run-time error (because an empty object is called as if it's a function). The [no-unexpected-multiline](no-unexpected-multiline.md) rule can protect your code from such cases.

Although ASI allows for more freedom over your coding style, it can also make your code behave in an unexpected way, whether you use semicolons or not. Therefore, it is best to know when ASI takes place and when it does not, and have ESLint protect your code from these potentially unexpected cases. In short, as once described by Isaac Schlueter, a \n character always ends a statement (just like a semicolon) unless one of the following is true:

1. The statement has an unclosed paren, array literal, or object literal or ends in some other way that is not a valid way to end a statement. (For instance, ending with . or ,.)
2. The line is -- or ++ (in which case it will decrement/increment the next token.)
3. It is a for(), while(), do, if(), or else, and there is no {
4. The next line starts with [, (, +, *, /, -, ,, ., or some other binary operator that can only be found between two tokens in a single expression.

Rule Details

This rule enforces consistent use of semicolons.

Options

This rule has two options, a string option and an object option.

String option:

• "always" (default) requires semicolons at the end of statements
• "never" disallows semicolons as the end of statements (except to disambiguate statements beginning with [, (, /, +, or -)

Object option (when "always"):

• "omitLastInOneLineBlock": true ignores the last semicolon in a block in which its braces (and therefore the content of the block) are in the same line

Object option (when "never"):

• "beforeStatementContinuationChars": "any" (default) ignores semicolons (or lacking semicolon) at the end of statements if the next line starts with [, (, /, +, or -.
• "beforeStatementContinuationChars": "always" requires semicolons at the end of statements if the next line starts with [, (, /, +, or -.
• "beforeStatementContinuationChars": "never" disallows semicolons as the end of statements if it doesn't make ASI hazard even if the next line starts with [, (, /, +, or -.

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint semi: ["error", "always"]*/

var name = "ESLint"

object.method = function() {
    // ...
}

Examples of correct code for this rule with the default "always" option:

/*eslint semi: "error"*/

var name = "ESLint";

object.method = function() {
    // ...
};

never

Examples of incorrect code for this rule with the "never" option:

/*eslint semi: ["error", "never"]*/

var name = "ESLint";

object.method = function() {
    // ...
};

Examples of correct code for this rule with the "never" option:

/*eslint semi: ["error", "never"]*/

var name = "ESLint"

object.method = function() {
    // ...
}

var name = "ESLint"

;(function() {
    // ...
})()

import a from "a"
(function() {
    // ...
})()

import b from "b"
;(function() {
    // ...
})()

omitLastInOneLineBlock

Examples of additional correct code for this rule with the "always", { "omitLastInOneLineBlock": true } options:

/*eslint semi: ["error", "always", { "omitLastInOneLineBlock": true}] */

if (foo) { bar() }

if (foo) { bar(); baz() }

beforeStatementContinuationChars

Examples of additional incorrect code for this rule with the "never", { "beforeStatementContinuationChars": "always" } options:

/*eslint semi: ["error", "never", { "beforeStatementContinuationChars": "always"}] */
import a from "a"

(function() {
    // ...
})()

Examples of additional incorrect code for this rule with the "never", { "beforeStatementContinuationChars": "never" } options:

/*eslint semi: ["error", "never", { "beforeStatementContinuationChars": "never"}] */
import a from "a"

;(function() {
    // ...
})()

When Not To Use It

If you do not want to enforce semicolon usage (or omission) in any particular way, then you can turn this rule off.

Further Reading

• An Open Letter to JavaScript Leaders Regarding Semicolons
• JavaScript Semicolon Insertion

Related Rules

• [no-extra-semi](no-extra-semi.md)
• [no-unexpected-multiline](no-unexpected-multiline.md)
• [semi-spacing](semi-spacing.md) Source: http://eslint.org/docs/rules/

    bytes_to_human_size: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      var av_options = { precision: options.precision || 0 };  // Precision of 0 returns the significant digits

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      col = a[0];

Close

Prefer destructuring from arrays and objects (prefer-destructuring)

With JavaScript ES6, a new syntax was added for creating variables from an array index or object property, called destructuring. This rule enforces usage of destructuring instead of accessing a property through a member expression.

Rule Details

Options

This rule takes two sets of configuration objects. The first object parameter determines what types of destructuring the rule applies to.

The two properties, array and object, can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.

Alternatively, you can use separate configurations for different assignment types. It accepts 2 other keys instead of array and object.

One key is VariableDeclarator and the other is AssignmentExpression, which can be used to control the destructuring requirement for each of those types independently. Each property accepts an object that accepts two properties, array and object, which can be used to control the destructuring requirement for each of array and object independently for variable declarations and assignment expressions. By default, array and object are set to true for both VariableDeclarator and AssignmentExpression.

The rule has a second object with a single key, enforceForRenamedProperties, which determines whether the object destructuring applies to renamed variables.

Examples of incorrect code for this rule:

// With `array` enabled
var foo = array[0];

// With `object` enabled
var foo = object.foo;
var foo = object['foo'];

Examples of correct code for this rule:

// With `array` enabled
var [ foo ] = array;
var foo = array[someIndex];

// With `object` enabled
var { foo } = object;

var foo = object.bar;

let foo;
({ foo } = object);

Examples of incorrect code when enforceForRenamedProperties is enabled:

var foo = object.bar;

Examples of correct code when enforceForRenamedProperties is enabled:

var { bar: foo } = object;

An example configuration, with the defaults array and object filled in, looks like this:

{
  "rules": {
    "prefer-destructuring": ["error", {
      "array": true,
      "object": true
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

The two properties, array and object, which can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.

For example, the following configuration enforces only object destructuring, but not array destructuring:

{
  "rules": {
    "prefer-destructuring": ["error", {"object": true, "array": false}]
  }
}

An example configuration, with the defaults VariableDeclarator and AssignmentExpression filled in, looks like this:

{
  "rules": {
    "prefer-destructuring": ["error", {
      "VariableDeclarator": {
        "array": false,
        "object": true
      },
      "AssignmentExpression": {
        "array": true,
        "object": true
      }
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

The two properties, VariableDeclarator and AssignmentExpression, which can be used to turn on or off the destructuring requirement for array and object. By default, all values are true.

For example, the following configuration enforces object destructuring in variable declarations and enforces array destructuring in assignment expressions.

{
  "rules": {
    "prefer-destructuring": ["error", {
      "VariableDeclarator": {
        "array": false,
        "object": true
      },
      "AssignmentExpression": {
        "array": true,
        "object": false
      }
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

Examples of correct code when object destructuring in VariableDeclarator is enforced:

/* eslint prefer-destructuring: ["error", {VariableDeclarator: {object: true}}] */
var {bar: foo} = object;

Examples of correct code when array destructuring in AssignmentExpression is enforced:

/* eslint prefer-destructuring: ["error", {AssignmentExpression: {array: true}}] */
[bar] = array;

When Not To Use It

If you want to be able to access array indices or object properties directly, you can either configure the rule to your tastes or disable the rule entirely.

Additionally, if you intend to access large array indices directly, like:

var foo = array[100];

Then the array part of this rule is not recommended, as destructuring does not match this use case very well.

Or for non-iterable 'array-like' objects:

var $ = require('jquery');
var foo = $('body')[0];
var [bar] = $('body'); // fails with a TypeError

Further Reading

If you want to learn more about destructuring, check out the links below:

• Destructuring Assignment (MDN)
• Destructuring and parameter handling in ECMAScript 6 (2ality blog) Source: http://eslint.org/docs/rules/

      var hours   = ~~((val / 3600) - (days * 24));

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    var fmt = '0.0';

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    mhz_to_human_size: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      var col = options.column;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var col = options.column;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      return '(' + stime.strftime(options.format) + ' - ' + etime.strftime(options.format) + ')';

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

    set: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      var days    = ~~(val / 86400);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

  function get_time_zone(default_tz) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      intpart = ~~val;

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    var numstr = val;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var p = options.precision - 1;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    var v = str_val.split(separator);

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    datetime: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

    datetime_range: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

    number_ordinal: function(val, _options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      return ordinalize(~~val);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    return sprintf('%.' + (~~precision) + 'f', val);

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

      intpart = Math.abs(~~val);

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

      intpart = ~~( intpart / 1000 );

Close

Disallow or enforce spaces inside of parentheses (space-in-parens)

Some style guides require or disallow spaces inside of parentheses:

foo( 'bar' );
var x = ( 1 + 2 ) * 3;

foo('bar');
var x = (1 + 2) * 3;

Rule Details

This rule will enforce consistency of spacing directly inside of parentheses, by disallowing or requiring one or more spaces to the right of ( and to the left of ). In either case, () will still be allowed.

Options

There are two options for this rule:

• "never" (default) enforces zero spaces inside of parentheses
• "always" enforces a space inside of parentheses

Depending on your coding conventions, you can choose either option by specifying it in your configuration:

"space-in-parens": ["error", "always"]

"never"

Examples of incorrect code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo( 'bar');
foo('bar' );
foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Examples of correct code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo();

foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

"always"

Examples of incorrect code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo( 'bar');
foo('bar' );
foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

Examples of correct code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo();

foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Exceptions

An object literal may be used as a third array item to specify exceptions, with the key "exceptions" and an array as the value. These exceptions work in the context of the first option. That is, if "always" is set to enforce spacing, then any "exception" will disallow spacing. Conversely, if "never" is set to disallow spacing, then any "exception" will enforce spacing.

The following exceptions are available: ["{}", "[]", "()", "empty"].

Examples of incorrect code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo(1, {bar: 'baz'});

Examples of correct code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo(1, {bar: 'baz'} );

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo( 1, {bar: 'baz'} );

Examples of correct code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo( 1, {bar: 'baz'});

Examples of incorrect code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1);

Examples of correct code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1 );

Examples of incorrect code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo((1 + 2));
foo((1 + 2), 1);

Examples of correct code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo( (1 + 2) );
foo( (1 + 2), 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo( ( 1 + 2 ) );
foo( ( 1 + 2 ), 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo(( 1 + 2 ));
foo(( 1 + 2 ), 1 );

The "empty" exception concerns empty parentheses, and works the same way as the other exceptions, inverting the first option.

Example of incorrect code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo();

Example of correct code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo( );

Example of incorrect code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo( );

Example of correct code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo();

You can include multiple entries in the "exceptions" array.

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar( {bar:'baz'} );
baz( 1, [1,2] );
foo( {bar: 'baz'}, [1, 2] );

Examples of correct code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar({bar:'baz'});
baz( 1, [1,2]);
foo({bar: 'baz'}, [1, 2]);

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between parentheses.

Related Rules

• [space-in-brackets](space-in-brackets.md) (deprecated) Source: http://eslint.org/docs/rules/

      options.negative_format = '-' + options.format;

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Disallow or enforce spaces inside of brackets (array-bracket-spacing)

A number of style guides require or disallow spaces between array brackets and other tokens. This rule applies to both array literals and destructuring assignments (ECMAScript 6).

/*eslint-env es6*/

var arr = [ 'foo', 'bar' ];
var [ x, y ] = z;

var arr = ['foo', 'bar'];
var [x,y] = z;

Rule Details

This rule enforces consistent spacing inside array brackets.

Options

This rule has a string option:

• "never" (default) disallows spaces inside array brackets
• "always" requires one or more spaces or newlines inside array brackets

This rule has an object option for exceptions to the "never" option:

• "singleValue": true requires one or more spaces or newlines inside brackets of array literals that contain a single element
• "objectsInArrays": true requires one or more spaces or newlines between brackets of array literals and braces of their object literal elements [ { or } ]
• "arraysInArrays": true requires one or more spaces or newlines between brackets of array literals and brackets of their array literal elements [ [ or ] ]

This rule has an object option for exceptions to the "always" option:

• "singleValue": false disallows spaces inside brackets of array literals that contain a single element
• "objectsInArrays": false disallows spaces between brackets of array literals and braces of their object literal elements [{ or }]
• "arraysInArrays": false disallows spaces between brackets of array literals and brackets of their array literal elements [[ or ]]

This rule has built-in exceptions:

• "never" (and also the exceptions to the "always" option) allows newlines inside array brackets, because this is a common pattern
• "always" does not require spaces or newlines in empty array literals []

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint array-bracket-spacing: ["error", "never"]*/
/*eslint-env es6*/

var arr = [ 'foo', 'bar' ];
var arr = ['foo', 'bar' ];
var arr = [ ['foo'], 'bar'];
var arr = [[ 'foo' ], 'bar'];
var arr = [ 'foo',
  'bar'
];
var [ x, y ] = z;
var [ x,y ] = z;
var [ x, ...y ] = z;
var [ ,,x, ] = z;

Examples of correct code for this rule with the default "never" option:

/*eslint array-bracket-spacing: ["error", "never"]*/
/*eslint-env es6*/

var arr = [];
var arr = ['foo', 'bar', 'baz'];
var arr = [['foo'], 'bar', 'baz'];
var arr = [
  'foo',
  'bar',
  'baz'
];
var arr = ['foo',
  'bar'
];
var arr = [
  'foo',
  'bar'];

var [x, y] = z;
var [x,y] = z;
var [x, ...y] = z;
var [,,x,] = z;

always

Examples of incorrect code for this rule with the "always" option:

/*eslint array-bracket-spacing: ["error", "always"]*/
/*eslint-env es6*/

var arr = ['foo', 'bar'];
var arr = ['foo', 'bar' ];
var arr = [ ['foo'], 'bar' ];
var arr = ['foo',
  'bar'
];
var arr = [
  'foo',
  'bar'];

var [x, y] = z;
var [x,y] = z;
var [x, ...y] = z;
var [,,x,] = z;

Examples of correct code for this rule with the "always" option:

/*eslint array-bracket-spacing: ["error", "always"]*/
/*eslint-env es6*/

var arr = [];
var arr = [ 'foo', 'bar', 'baz' ];
var arr = [ [ 'foo' ], 'bar', 'baz' ];
var arr = [ 'foo',
  'bar'
];
var arr = [
  'foo',
  'bar' ];
var arr = [
  'foo',
  'bar',
  'baz'
];

var [ x, y ] = z;
var [ x,y ] = z;
var [ x, ...y ] = z;
var [ ,,x, ] = z;

singleValue

Examples of incorrect code for this rule with the "always", { "singleValue": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "singleValue": false }]*/

var foo = [ 'foo' ];
var foo = [ 'foo'];
var foo = ['foo' ];
var foo = [ 1 ];
var foo = [ 1];
var foo = [1 ];
var foo = [ [ 1, 2 ] ];
var foo = [ { 'foo': 'bar' } ];

Examples of correct code for this rule with the "always", { "singleValue": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "singleValue": false }]*/

var foo = ['foo'];
var foo = [1];
var foo = [[ 1, 1 ]];
var foo = [{ 'foo': 'bar' }];

objectsInArrays

Examples of incorrect code for this rule with the "always", { "objectsInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "objectsInArrays": false }]*/

var arr = [ { 'foo': 'bar' } ];
var arr = [ {
  'foo': 'bar'
} ]

Examples of correct code for this rule with the "always", { "objectsInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "objectsInArrays": false }]*/

var arr = [{ 'foo': 'bar' }];
var arr = [{
  'foo': 'bar'
}];

arraysInArrays

Examples of incorrect code for this rule with the "always", { "arraysInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "arraysInArrays": false }]*/

var arr = [ [ 1, 2 ], 2, 3, 4 ];
var arr = [ [ 1, 2 ], 2, [ 3, 4 ] ];

Examples of correct code for this rule with the "always", { "arraysInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "arraysInArrays": false }]*/

var arr = [[ 1, 2 ], 2, 3, 4 ];
var arr = [[ 1, 2 ], 2, [ 3, 4 ]];

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between array brackets.

Related Rules

• [space-in-parens](space-in-parens.md)
• [object-curly-spacing](object-curly-spacing.md)
• [computed-property-spacing](computed-property-spacing.md) Source: http://eslint.org/docs/rules/

    var intpart;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      var a = String(col).split('__');

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      intpart = ~~val;

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    var fmt = (numstr[0] === '-') ? options.negative_format : options.format;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Disallow or enforce spaces inside of brackets (array-bracket-spacing)

A number of style guides require or disallow spaces between array brackets and other tokens. This rule applies to both array literals and destructuring assignments (ECMAScript 6).

/*eslint-env es6*/

var arr = [ 'foo', 'bar' ];
var [ x, y ] = z;

var arr = ['foo', 'bar'];
var [x,y] = z;

Rule Details

This rule enforces consistent spacing inside array brackets.

Options

This rule has a string option:

• "never" (default) disallows spaces inside array brackets
• "always" requires one or more spaces or newlines inside array brackets

This rule has an object option for exceptions to the "never" option:

• "singleValue": true requires one or more spaces or newlines inside brackets of array literals that contain a single element
• "objectsInArrays": true requires one or more spaces or newlines between brackets of array literals and braces of their object literal elements [ { or } ]
• "arraysInArrays": true requires one or more spaces or newlines between brackets of array literals and brackets of their array literal elements [ [ or ] ]

This rule has an object option for exceptions to the "always" option:

• "singleValue": false disallows spaces inside brackets of array literals that contain a single element
• "objectsInArrays": false disallows spaces between brackets of array literals and braces of their object literal elements [{ or }]
• "arraysInArrays": false disallows spaces between brackets of array literals and brackets of their array literal elements [[ or ]]

This rule has built-in exceptions:

• "never" (and also the exceptions to the "always" option) allows newlines inside array brackets, because this is a common pattern
• "always" does not require spaces or newlines in empty array literals []

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint array-bracket-spacing: ["error", "never"]*/
/*eslint-env es6*/

var arr = [ 'foo', 'bar' ];
var arr = ['foo', 'bar' ];
var arr = [ ['foo'], 'bar'];
var arr = [[ 'foo' ], 'bar'];
var arr = [ 'foo',
  'bar'
];
var [ x, y ] = z;
var [ x,y ] = z;
var [ x, ...y ] = z;
var [ ,,x, ] = z;

Examples of correct code for this rule with the default "never" option:

/*eslint array-bracket-spacing: ["error", "never"]*/
/*eslint-env es6*/

var arr = [];
var arr = ['foo', 'bar', 'baz'];
var arr = [['foo'], 'bar', 'baz'];
var arr = [
  'foo',
  'bar',
  'baz'
];
var arr = ['foo',
  'bar'
];
var arr = [
  'foo',
  'bar'];

var [x, y] = z;
var [x,y] = z;
var [x, ...y] = z;
var [,,x,] = z;

always

Examples of incorrect code for this rule with the "always" option:

/*eslint array-bracket-spacing: ["error", "always"]*/
/*eslint-env es6*/

var arr = ['foo', 'bar'];
var arr = ['foo', 'bar' ];
var arr = [ ['foo'], 'bar' ];
var arr = ['foo',
  'bar'
];
var arr = [
  'foo',
  'bar'];

var [x, y] = z;
var [x,y] = z;
var [x, ...y] = z;
var [,,x,] = z;

Examples of correct code for this rule with the "always" option:

/*eslint array-bracket-spacing: ["error", "always"]*/
/*eslint-env es6*/

var arr = [];
var arr = [ 'foo', 'bar', 'baz' ];
var arr = [ [ 'foo' ], 'bar', 'baz' ];
var arr = [ 'foo',
  'bar'
];
var arr = [
  'foo',
  'bar' ];
var arr = [
  'foo',
  'bar',
  'baz'
];

var [ x, y ] = z;
var [ x,y ] = z;
var [ x, ...y ] = z;
var [ ,,x, ] = z;

singleValue

Examples of incorrect code for this rule with the "always", { "singleValue": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "singleValue": false }]*/

var foo = [ 'foo' ];
var foo = [ 'foo'];
var foo = ['foo' ];
var foo = [ 1 ];
var foo = [ 1];
var foo = [1 ];
var foo = [ [ 1, 2 ] ];
var foo = [ { 'foo': 'bar' } ];

Examples of correct code for this rule with the "always", { "singleValue": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "singleValue": false }]*/

var foo = ['foo'];
var foo = [1];
var foo = [[ 1, 1 ]];
var foo = [{ 'foo': 'bar' }];

objectsInArrays

Examples of incorrect code for this rule with the "always", { "objectsInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "objectsInArrays": false }]*/

var arr = [ { 'foo': 'bar' } ];
var arr = [ {
  'foo': 'bar'
} ]

Examples of correct code for this rule with the "always", { "objectsInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "objectsInArrays": false }]*/

var arr = [{ 'foo': 'bar' }];
var arr = [{
  'foo': 'bar'
}];

arraysInArrays

Examples of incorrect code for this rule with the "always", { "arraysInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "arraysInArrays": false }]*/

var arr = [ [ 1, 2 ], 2, 3, 4 ];
var arr = [ [ 1, 2 ], 2, [ 3, 4 ] ];

Examples of correct code for this rule with the "always", { "arraysInArrays": false } options:

/*eslint array-bracket-spacing: ["error", "always", { "arraysInArrays": false }]*/

var arr = [[ 1, 2 ], 2, 3, 4 ];
var arr = [[ 1, 2 ], 2, [ 3, 4 ]];

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between array brackets.

Related Rules

• [space-in-parens](space-in-parens.md)
• [object-curly-spacing](object-curly-spacing.md)
• [computed-property-spacing](computed-property-spacing.md) Source: http://eslint.org/docs/rules/

      var stime;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    elapsed_time_human: function(val, _options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

    var floatpart;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

  function number_to_currency(val, options) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

    var s = val < 0 ? '-' : '';

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      var hours   = ~~((val / 3600) - (days * 24));

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

  function apply_prefix_and_suffix(val, options) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

    return default_tz;

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

    var floatpart;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    var s = '';

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      intpart = ~~( intpart / 1000 );

Close

Disallow or enforce spaces inside of parentheses (space-in-parens)

Some style guides require or disallow spaces inside of parentheses:

foo( 'bar' );
var x = ( 1 + 2 ) * 3;

foo('bar');
var x = (1 + 2) * 3;

Rule Details

This rule will enforce consistency of spacing directly inside of parentheses, by disallowing or requiring one or more spaces to the right of ( and to the left of ). In either case, () will still be allowed.

Options

There are two options for this rule:

• "never" (default) enforces zero spaces inside of parentheses
• "always" enforces a space inside of parentheses

Depending on your coding conventions, you can choose either option by specifying it in your configuration:

"space-in-parens": ["error", "always"]

"never"

Examples of incorrect code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo( 'bar');
foo('bar' );
foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Examples of correct code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo();

foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

"always"

Examples of incorrect code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo( 'bar');
foo('bar' );
foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

Examples of correct code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo();

foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Exceptions

An object literal may be used as a third array item to specify exceptions, with the key "exceptions" and an array as the value. These exceptions work in the context of the first option. That is, if "always" is set to enforce spacing, then any "exception" will disallow spacing. Conversely, if "never" is set to disallow spacing, then any "exception" will enforce spacing.

The following exceptions are available: ["{}", "[]", "()", "empty"].

Examples of incorrect code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo(1, {bar: 'baz'});

Examples of correct code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo(1, {bar: 'baz'} );

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo( 1, {bar: 'baz'} );

Examples of correct code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo( 1, {bar: 'baz'});

Examples of incorrect code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1);

Examples of correct code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1 );

Examples of incorrect code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo((1 + 2));
foo((1 + 2), 1);

Examples of correct code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo( (1 + 2) );
foo( (1 + 2), 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo( ( 1 + 2 ) );
foo( ( 1 + 2 ), 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo(( 1 + 2 ));
foo(( 1 + 2 ), 1 );

The "empty" exception concerns empty parentheses, and works the same way as the other exceptions, inverting the first option.

Example of incorrect code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo();

Example of correct code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo( );

Example of incorrect code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo( );

Example of correct code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo();

You can include multiple entries in the "exceptions" array.

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar( {bar:'baz'} );
baz( 1, [1,2] );
foo( {bar: 'baz'}, [1, 2] );

Examples of correct code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar({bar:'baz'});
baz( 1, [1,2]);
foo({bar: 'baz'}, [1, 2]);

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between parentheses.

Related Rules

• [space-in-brackets](space-in-brackets.md) (deprecated) Source: http://eslint.org/docs/rules/

  function number_to_human_size(val, options) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      var av_options = { precision: options.precision || 0 };  // Precision of 0 returns the significant digits

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      s += sprintf(precision + ' GHz', val / 1000000000);

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

        return _.capitalize( String(val) );

Close

Disallow or enforce spaces inside of parentheses (space-in-parens)

Some style guides require or disallow spaces inside of parentheses:

foo( 'bar' );
var x = ( 1 + 2 ) * 3;

foo('bar');
var x = (1 + 2) * 3;

Rule Details

This rule will enforce consistency of spacing directly inside of parentheses, by disallowing or requiring one or more spaces to the right of ( and to the left of ). In either case, () will still be allowed.

Options

There are two options for this rule:

• "never" (default) enforces zero spaces inside of parentheses
• "always" enforces a space inside of parentheses

Depending on your coding conventions, you can choose either option by specifying it in your configuration:

"space-in-parens": ["error", "always"]

"never"

Examples of incorrect code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo( 'bar');
foo('bar' );
foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Examples of correct code for this rule with the default "never" option:

/*eslint space-in-parens: ["error", "never"]*/

foo();

foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

"always"

Examples of incorrect code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo( 'bar');
foo('bar' );
foo('bar');

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

Examples of correct code for this rule with the "always" option:

/*eslint space-in-parens: ["error", "always"]*/

foo();

foo( 'bar' );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

Exceptions

An object literal may be used as a third array item to specify exceptions, with the key "exceptions" and an array as the value. These exceptions work in the context of the first option. That is, if "always" is set to enforce spacing, then any "exception" will disallow spacing. Conversely, if "never" is set to disallow spacing, then any "exception" will enforce spacing.

The following exceptions are available: ["{}", "[]", "()", "empty"].

Examples of incorrect code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo(1, {bar: 'baz'});

Examples of correct code for this rule with the "never", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo(1, {bar: 'baz'} );

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo( {bar: 'baz'} );
foo( 1, {bar: 'baz'} );

Examples of correct code for this rule with the "always", { "exceptions": ["{}"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}"] }]*/

foo({bar: 'baz'});
foo( 1, {bar: 'baz'});

Examples of incorrect code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1);

Examples of correct code for this rule with the "never", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo( [bar, baz] );
foo( [bar, baz], 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["[]"] } option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["[]"] }]*/

foo([bar, baz]);
foo([bar, baz], 1 );

Examples of incorrect code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo((1 + 2));
foo((1 + 2), 1);

Examples of correct code for this rule with the "never", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["()"] }]*/

foo( (1 + 2) );
foo( (1 + 2), 1);

Examples of incorrect code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo( ( 1 + 2 ) );
foo( ( 1 + 2 ), 1 );

Examples of correct code for this rule with the "always", { "exceptions": ["()"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["()"] }]*/

foo(( 1 + 2 ));
foo(( 1 + 2 ), 1 );

The "empty" exception concerns empty parentheses, and works the same way as the other exceptions, inverting the first option.

Example of incorrect code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo();

Example of correct code for this rule with the "never", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "never", { "exceptions": ["empty"] }]*/

foo( );

Example of incorrect code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo( );

Example of correct code for this rule with the "always", { "exceptions": ["empty"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["empty"] }]*/

foo();

You can include multiple entries in the "exceptions" array.

Examples of incorrect code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar( {bar:'baz'} );
baz( 1, [1,2] );
foo( {bar: 'baz'}, [1, 2] );

Examples of correct code for this rule with the "always", { "exceptions": ["{}", "[]"] }] option:

/*eslint space-in-parens: ["error", "always", { "exceptions": ["{}", "[]"] }]*/

bar({bar:'baz'});
baz( 1, [1,2]);
foo({bar: 'baz'}, [1, 2]);

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between parentheses.

Related Rules

• [space-in-brackets](space-in-brackets.md) (deprecated) Source: http://eslint.org/docs/rules/

      var hours   = ~~((val / 3600) - (days * 24));

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

      var minutes = ~~((val / 60) - (hours * 60) - (days * 1440));

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    var minus;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      intpart = ~~( intpart / 1000 );

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    precision = '%.' + precision + 'f';

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    gbytes_to_human_size: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      val = ~~val;

Close

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

    var intpart;

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    currency_with_delimiter: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      var av_options = { precision: options.precision || 0 };  // Precision of 0 returns the significant digits

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      var a = String(col).split('__');

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    var minus;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

  function mhz_to_human_size(val, precision) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

      var av_options = _.pick(options, [ 'delimiter', 'separator' ]);

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      var sfx = a[1]; // The suffix (month, quarter, year) defines the range

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      var hours   = ~~((val / 3600) - (days * 24));

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    var s = '';

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    var fmt = (numstr[0] === '-') ? options.negative_format : options.format;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      var p = options.precision - 1;

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

    var s = val < 0 ? '-' : '';

Close

require let or const instead of var (no-var)

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let and const keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes such as:

var count = people.length;
var enoughFood = count > sandwiches.length;

if (enoughFood) {
    var count = sandwiches.length; // accidentally overriding the count variable
    console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}

// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

/*eslint no-var: "error"*/

var x = "y";
var CONFIG = {};

Examples of correct code for this rule:

/*eslint no-var: "error"*/
/*eslint-env es6*/

let x = "y";
const CONFIG = {};

When Not To Use It

In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their codebase may not want to apply this rule if the cost of migrating from var to let is too costly. Source: http://eslint.org/docs/rules/

      s += sprintf(precision + ' THz', val / 1000000000000);

Close

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

  function remove_right_side_zeros(str_val, separator) {

Close

Require CamelCase (camelcase)

When it comes to naming variables, style guides generally fall into one of two camps: camelcase (variableName) and underscores (variable_name). This rule focuses on using the camelcase approach. If your style guide calls for camelCasing your variable names, then this rule is for you!

Rule Details

This rule looks for any underscores (_) located within the source code. It ignores leading and trailing underscores and only checks those in the middle of a variable name. If ESLint decides that the variable is a constant (all uppercase), then no warning will be thrown. Otherwise, a warning will be thrown. This rule only flags definitions and assignments but not function calls. In case of ES6 import statements, this rule only targets the name of the variable that will be imported into the local module scope.

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names
• "ignoreDestructuring": false (default) enforces camelcase style for destructured identifiers
• "ignoreDestructuring": true does not check destructured identifiers
• allow (string[]) list of properties to accept. Accept regex.

properties: "always"

Examples of incorrect code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

obj.do_something = function() {
    // ...
};

function foo({ no_camelcased }) {
    // ...
};

function foo({ isCamelcased: no_camelcased }) {
    // ...
}

function foo({ no_camelcased = 'default value' }) {
    // ...
};

var obj = {
    my_pref: 1
};

var { category_id = 1 } = query;

var { foo: no_camelcased } = bar;

var { foo: bar_baz = 1 } = quz;

Examples of correct code for this rule with the default { "properties": "always" } option:

/*eslint camelcase: "error"*/

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";
var foo = bar.baz_boom;
var foo = { qux: bar.baz_boom };

obj.do_something();
do_something();
new do_something();

var { category_id: category } = query;

function foo({ isCamelCased }) {
    // ...
};

function foo({ isCamelCased: isAlsoCamelCased }) {
    // ...
}

function foo({ isCamelCased = 'default value' }) {
    // ...
};

var { categoryId = 1 } = query;

var { foo: isCamelCased } = bar;

var { foo: isCamelCased = 1 } = quz;

properties: "never"

Examples of correct code for this rule with the { "properties": "never" } option:

/*eslint camelcase: ["error", {properties: "never"}]*/

var obj = {
    my_pref: 1
};

ignoreDestructuring: false

Examples of incorrect code for this rule with the default { "ignoreDestructuring": false } option:

/*eslint camelcase: "error"*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

var { category_id: category_alias } = query;

var { category_id: categoryId, ...other_props } = query;

ignoreDestructuring: true

Examples of incorrect code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id: category_alias } = query;

var { category_id, ...other_props } = query;

Examples of correct code for this rule with the { "ignoreDestructuring": true } option:

/*eslint camelcase: ["error", {ignoreDestructuring: true}]*/

var { category_id } = query;

var { category_id = 1 } = query;

var { category_id: category_id } = query;

allow

Examples of correct code for this rule with the allow option:

/*eslint camelcase: ["error", {allow: ["UNSAFE_componentWillMount"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

/*eslint camelcase: ["error", {allow: ["^UNSAFE_"]}]*/

function UNSAFE_componentWillMount() {
    // ...
}

function UNSAFE_componentWillMount() {
    // ...
}

When Not To Use It

If you have established coding standards using a different naming convention (separating words with underscores), turn this rule off. Source: http://eslint.org/docs/rules/

    boolean: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/

      var sfx = a[1]; // The suffix (month, quarter, year) defines the range

Close

Require Variable Declarations to be at the top of their scope (vars-on-top)

The vars-on-top rule generates warnings when variable declarations are not used serially at the top of a function scope or the top of a program. By default variable declarations are always moved (“hoisted”) invisibly to the top of their containing scope by the JavaScript interpreter. This rule forces the programmer to represent that behavior by manually moving the variable declaration to the top of its containing scope.

Rule Details

This rule aims to keep all variable declarations in the leading series of statements. Allowing multiple declarations helps promote maintainability and is thus allowed.

Examples of incorrect code for this rule:

/*eslint vars-on-top: "error"*/

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;
}

// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

// Variables after other statements:
f();
var a;

Examples of correct code for this rule:

/*eslint vars-on-top: "error"*/

function doSomething() {
    var first;
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}

function doSomething() {
    var i;
    for (i=0; i<10; i++) {}
}

/*eslint vars-on-top: "error"*/

var a;
f();

/*eslint vars-on-top: "error"*/

// Directives may precede variable declarations.
"use strict";
var a;
f();

// Comments can describe variables.
function doSomething() {
    // this is the first var.
    var first;
    // this is the second var.
    var second
}

Further Reading

• JavaScript Scoping and Hoisting
• var Hoisting
• A criticism of the Single Var Pattern in JavaScript, and a simple alternative
• Multiple var statements in JavaScript, not superfluous Source: http://eslint.org/docs/rules/

    datetime_ordinal: function(val, options) {

Close

Require Object Literal Shorthand Syntax (object-shorthand)

ECMAScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.

Here are a few common examples using the ES5 syntax:

// properties
var foo = {
    x: x,
    y: y,
    z: z,
};

// methods
var foo = {
    a: function() {},
    b: function() {}
};

Now here are ES6 equivalents:

/*eslint-env es6*/

// properties
var foo = {x, y, z};

// methods
var foo = {
    a() {},
    b() {}
};

Rule Details

This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.

Each of the following properties would warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w: function() {},
    x: function *() {},
    [y]: function() {},
    z: z
};

In that case the expected syntax would have been:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    w() {},
    *x() {},
    [y]() {},
    z
};

This rule does not flag arrow functions inside of object literals. The following will not warn:

/*eslint object-shorthand: "error"*/
/*eslint-env es6*/

var foo = {
    x: (y) => y
};

See Also:

• no-useless-rename which disallows renaming import, export, and destructured assignments to the same name.

Options

The rule takes an option which specifies when it should be applied. It can be set to one of the following values:

• "always" (default) expects that the shorthand will be used whenever possible.
• "methods" ensures the method shorthand is used (also applies to generators).
• "properties" ensures the property shorthand is used (where the key and variable name match).
• "never" ensures that no property or method shorthand is used in any object literal.
• "consistent" ensures that either all shorthand or all long-form will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all long-form will be used in an object literal, but ensures all shorthand whenever possible.

You can set the option in configuration like this:

{
    "object-shorthand": ["error", "always"]
}

Additionally, the rule takes an optional object configuration:

• "avoidQuotes": true indicates that long-form syntax is preferred whenever the object key is a string literal (default: false). Note that this option can only be enabled when the string option is set to "always", "methods", or "properties".
• "ignoreConstructors": true can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to "always" or "methods".
• "avoidExplicitReturnArrows": true indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to "always" or "methods".

avoidQuotes

{
    "object-shorthand": ["error", "always", { "avoidQuotes": true }]
}

Example of incorrect code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz"() {}
};

Example of correct code for this rule with the "always", { "avoidQuotes": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/

var foo = {
    "bar-baz": function() {},
    "qux": qux
};

ignoreConstructors

{
    "object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}

Example of correct code for this rule with the "always", { "ignoreConstructors": true } option:

/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/

var foo = {
    ConstructorFunction: function() {}
};

avoidExplicitReturnArrows

{
    "object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}

Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo: (bar, baz) => {
    return bar + baz;
  },

  qux: (foobar) => {
    return foobar * 2;
  }
};

Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true } option:

/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/

var foo = {
  foo(bar, baz) {
    return bar + baz;
  },

  qux: foobar => foobar * 2
};

Example of incorrect code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a,
    b: "foo",
};

Examples of correct code for this rule with the "consistent" option:

/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: "foo"
};

var bar = {
    a,
    b,
};

Example of incorrect code with the "consistent-as-needed" option, which is very similar to "consistent":

/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/

var foo = {
    a: a,
    b: b,
};

When Not To Use It

Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.

Further Reading

Object initializer - MDN Source: http://eslint.org/docs/rules/