port = window.location.port;

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/

window.$ = window.jQuery = require('jquery');

Close

Disallow Use of Chained Assignment Expressions (no-multi-assign)

Chaining the assignment of variables can lead to unexpected results and be difficult to read.

a = b = c = d;

Rule Details

This rule disallows using multiple assignments within a single statement.

Examples of incorrect code for this rule:

/*eslint no-multi-assign: "error"*/

var a = b = c = 5;

var foo = bar = "baz";

var a =
    b =
    c;

Examples of correct code for this rule:

/*eslint no-multi-assign: "error"*/
var a = 5;
var b = 5;
var c = 5;

var foo = "baz";
var bar = "baz";

var a = c;
var b = c;

Related Rules

• [max-statements-per-line](max-statements-per-line.md) Source: http://eslint.org/docs/rules/

  var port = encrypt ? 443 : 80;

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 spice = new SpiceMainConn({

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/

    onsuccess: function() {

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/

    uri: (encrypt ? 'wss://' : 'ws://') + host + ':' + port + '/' + $('#remote-console').attr('data-url'),

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/

    onerror: function(e) {

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/

$(function() {

Close

Require using arrow functions for callbacks (prefer-arrow-callback)

Arrow functions can be an attractive alternative to function expressions for callbacks or function arguments.

For example, arrow functions are automatically bound to their surrounding scope/context. This provides an alternative to the pre-ES6 standard of explicitly binding function expressions to achieve similar behavior.

Additionally, arrow functions are:

• less verbose, and easier to reason about.

• bound lexically regardless of where or when they are invoked.

Rule Details

This rule locates function expressions used as callbacks or function arguments. An error will be produced for any that could be replaced by an arrow function without changing the result.

The following examples will be flagged:

/* eslint prefer-arrow-callback: "error" */

foo(function(a) { return a; }); // ERROR
// prefer: foo(a => a)

foo(function() { return this.a; }.bind(this)); // ERROR
// prefer: foo(() => this.a)

Instances where an arrow function would not produce identical results will be ignored.

The following examples will not be flagged:

/* eslint prefer-arrow-callback: "error" */
/* eslint-env es6 */

// arrow function callback
foo(a => a); // OK

// generator as callback
foo(function*() { yield; }); // OK

// function expression not used as callback or function argument
var foo = function foo(a) { return a; }; // OK

// unbound function expression callback
foo(function() { return this.a; }); // OK

// recursive named function callback
foo(function bar(n) { return n && n + bar(n - 1); }); // OK

Options

Access further control over this rule's behavior via an options object.

Default: { allowNamedFunctions: false, allowUnboundThis: true }

allowNamedFunctions

By default { "allowNamedFunctions": false }, this boolean option prohibits using named functions as callbacks or function arguments.

Changing this value to true will reverse this option's behavior by allowing use of named functions without restriction.

{ "allowNamedFunctions": true } will not flag the following example:

/* eslint prefer-arrow-callback: [ "error", { "allowNamedFunctions": true } ] */

foo(function bar() {});

allowUnboundThis

By default { "allowUnboundThis": true }, this boolean option allows function expressions containing this to be used as callbacks, as long as the function in question has not been explicitly bound.

When set to false this option prohibits the use of function expressions as callbacks or function arguments entirely, without exception.

{ "allowUnboundThis": false } will flag the following examples:

/* eslint prefer-arrow-callback: [ "error", { "allowUnboundThis": false } ] */
/* eslint-env es6 */

foo(function() { this.a; });

foo(function() { (() => this); });

someArray.map(function(itm) { return this.doSomething(itm); }, someObject);

When Not To Use It

• In environments that have not yet adopted ES6 language features (ES3/5).

• In ES6+ environments that allow the use of function expressions when describing callbacks or function arguments.

Further Reading

• More on ES6 arrow functions Source: http://eslint.org/docs/rules/

  var spice = new SpiceMainConn({

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 host = window.location.hostname;

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 encrypt = window.location.protocol === 'https:';

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/

  $('#ctrlaltdel').click(function() {

Close

Require using arrow functions for callbacks (prefer-arrow-callback)

Arrow functions can be an attractive alternative to function expressions for callbacks or function arguments.

For example, arrow functions are automatically bound to their surrounding scope/context. This provides an alternative to the pre-ES6 standard of explicitly binding function expressions to achieve similar behavior.

Additionally, arrow functions are:

• less verbose, and easier to reason about.

• bound lexically regardless of where or when they are invoked.

Rule Details

This rule locates function expressions used as callbacks or function arguments. An error will be produced for any that could be replaced by an arrow function without changing the result.

The following examples will be flagged:

/* eslint prefer-arrow-callback: "error" */

foo(function(a) { return a; }); // ERROR
// prefer: foo(a => a)

foo(function() { return this.a; }.bind(this)); // ERROR
// prefer: foo(() => this.a)

Instances where an arrow function would not produce identical results will be ignored.

The following examples will not be flagged:

/* eslint prefer-arrow-callback: "error" */
/* eslint-env es6 */

// arrow function callback
foo(a => a); // OK

// generator as callback
foo(function*() { yield; }); // OK

// function expression not used as callback or function argument
var foo = function foo(a) { return a; }; // OK

// unbound function expression callback
foo(function() { return this.a; }); // OK

// recursive named function callback
foo(function bar(n) { return n && n + bar(n - 1); }); // OK

Options

Access further control over this rule's behavior via an options object.

Default: { allowNamedFunctions: false, allowUnboundThis: true }

allowNamedFunctions

By default { "allowNamedFunctions": false }, this boolean option prohibits using named functions as callbacks or function arguments.

Changing this value to true will reverse this option's behavior by allowing use of named functions without restriction.

{ "allowNamedFunctions": true } will not flag the following example:

/* eslint prefer-arrow-callback: [ "error", { "allowNamedFunctions": true } ] */

foo(function bar() {});

allowUnboundThis

By default { "allowUnboundThis": true }, this boolean option allows function expressions containing this to be used as callbacks, as long as the function in question has not been explicitly bound.

When set to false this option prohibits the use of function expressions as callbacks or function arguments entirely, without exception.

{ "allowUnboundThis": false } will flag the following examples:

/* eslint prefer-arrow-callback: [ "error", { "allowUnboundThis": false } ] */
/* eslint-env es6 */

foo(function() { this.a; });

foo(function() { (() => this); });

someArray.map(function(itm) { return this.doSomething(itm); }, someObject);

When Not To Use It

• In environments that have not yet adopted ES6 language features (ES3/5).

• In ES6+ environments that allow the use of function expressions when describing callbacks or function arguments.

Further Reading

• More on ES6 arrow functions Source: http://eslint.org/docs/rules/

  var spice = new SpiceMainConn({

Close

Disallow Undeclared Variables (no-undef)

This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).

Rule Details

Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment, or specified in the globals key in the configuration file. A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).

Examples of incorrect code for this rule:

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

var a = someFunction();
b = 10;

Examples of correct code for this rule with global declaration:

/*global someFunction b:true*/
/*eslint no-undef: "error"*/

var a = someFunction();
b = 10;

The b:true syntax in /*global */ indicates that assignment to b is correct.

Examples of incorrect code for this rule with global declaration:

/*global b*/
/*eslint no-undef: "error"*/

b = 10;

By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.

Options

• typeof set to true will warn for variables used inside typeof check (Default false).

typeof

Examples of correct code for the default { "typeof": false } option:

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

if (typeof UndefinedIdentifier === "undefined") {
    // do something ...
}

You can use this option if you want to prevent typeof check on a variable which has not been declared.

Examples of incorrect code for the { "typeof": true } option:

/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Examples of correct code for the { "typeof": true } option with global declaration:

/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Environments

For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in [Specifying Environments](../user-guide/configuring.md#specifying-environments). A few examples are given below.

browser

Examples of correct code for this rule with browser environment:

/*eslint no-undef: "error"*/
/*eslint-env browser*/

setTimeout(function() {
    alert("Hello");
});

Node.js

Examples of correct code for this rule with node environment:

/*eslint no-undef: "error"*/
/*eslint-env node*/

var fs = require("fs");
module.exports = function() {
    console.log(fs);
};

When Not To Use It

If explicit declaration of global variables is not to your taste.

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint. Source: http://eslint.org/docs/rules/

    sendCtrlAltDel(spice);

Close

Disallow Undeclared Variables (no-undef)

This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).

Rule Details

Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment, or specified in the globals key in the configuration file. A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).

Examples of incorrect code for this rule:

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

var a = someFunction();
b = 10;

Examples of correct code for this rule with global declaration:

/*global someFunction b:true*/
/*eslint no-undef: "error"*/

var a = someFunction();
b = 10;

The b:true syntax in /*global */ indicates that assignment to b is correct.

Examples of incorrect code for this rule with global declaration:

/*global b*/
/*eslint no-undef: "error"*/

b = 10;

By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.

Options

• typeof set to true will warn for variables used inside typeof check (Default false).

typeof

Examples of correct code for the default { "typeof": false } option:

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

if (typeof UndefinedIdentifier === "undefined") {
    // do something ...
}

You can use this option if you want to prevent typeof check on a variable which has not been declared.

Examples of incorrect code for the { "typeof": true } option:

/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Examples of correct code for the { "typeof": true } option with global declaration:

/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Environments

For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in [Specifying Environments](../user-guide/configuring.md#specifying-environments). A few examples are given below.

browser

Examples of correct code for this rule with browser environment:

/*eslint no-undef: "error"*/
/*eslint-env browser*/

setTimeout(function() {
    alert("Hello");
});

Node.js

Examples of correct code for this rule with node environment:

/*eslint no-undef: "error"*/
/*eslint-env node*/

var fs = require("fs");
module.exports = function() {
    console.log(fs);
};

When Not To Use It

If explicit declaration of global variables is not to your taste.

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint. Source: http://eslint.org/docs/rules/

    onsuccess: function() {
      $('#connection-status').removeClass('label-danger label-warning').addClass('label-success');
      $('#connection-status').text(__('Connected'));
    },

Close

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 45.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76