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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
};

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 longform will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all longform 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 longform 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/

        names.forEach(function (name) {

Close

Suggest using arrow functions as callbacks. (prefer-arrow-callback)

Arrow functions are suited to callbacks, because:

• this keywords in arrow functions bind to the upper scope's.
• The notation of the arrow function is shorter than function expression's.

Rule Details

This rule is aimed to flag usage of function expressions in an argument list.

The following patterns are considered problems:

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

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

The following patterns are not considered problems:

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

foo(a => a);
foo(function*() { yield; });

// this is not a callback.
var foo = function foo(a) { return a; };

// using `this` without `.bind(this)`.
foo(function() { return this.a; });

// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });

Options

This rule takes one optional argument, an object which is an options object.

allowNamedFunctions

This is a boolean option and it is false by default. When set to true, the rule doesn't warn on named functions used as callbacks.

Examples of correct code for the { "allowNamedFunctions": true } option:

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

foo(function bar() {});

allowUnboundThis

This is a boolean option and it is true by default. When set to false, this option allows the use of this without restriction and checks for dynamically assigned this values such as when using Array.prototype.map with a context argument. Normally, the rule will flag the use of this whenever a function does not use bind() to specify the value of this constantly.

Examples of incorrect code for the { "allowUnboundThis": false } option:

/*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

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

In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/

    return function lint() {

Close

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

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

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

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

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

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

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

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

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

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

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

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

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

        tasks.forEach(function (task) {

Close

Suggest using arrow functions as callbacks. (prefer-arrow-callback)

Arrow functions are suited to callbacks, because:

• this keywords in arrow functions bind to the upper scope's.
• The notation of the arrow function is shorter than function expression's.

Rule Details

This rule is aimed to flag usage of function expressions in an argument list.

The following patterns are considered problems:

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

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

The following patterns are not considered problems:

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

foo(a => a);
foo(function*() { yield; });

// this is not a callback.
var foo = function foo(a) { return a; };

// using `this` without `.bind(this)`.
foo(function() { return this.a; });

// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });

Options

This rule takes one optional argument, an object which is an options object.

allowNamedFunctions

This is a boolean option and it is false by default. When set to true, the rule doesn't warn on named functions used as callbacks.

Examples of correct code for the { "allowNamedFunctions": true } option:

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

foo(function bar() {});

allowUnboundThis

This is a boolean option and it is true by default. When set to false, this option allows the use of this without restriction and checks for dynamically assigned this values such as when using Array.prototype.map with a context argument. Normally, the rule will flag the use of this whenever a function does not use bind() to specify the value of this constantly.

Examples of incorrect code for the { "allowUnboundThis": false } option:

/*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

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

In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/

            var aliases = tasks

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 resolve = require('path').resolve;

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 taskFactory, taskFunction;

Close

enforce variables to be declared either together or separately in functions (one-var)

Variables can be declared at any point in JavaScript code using var, let, or const. There are many styles and preferences related to the declaration of variables, and one of those is deciding on how many variable declarations should be allowed in a single function.

There are two schools of thought in this regard:

1. There should be just one variable declaration for all variables in the function. That declaration typically appears at the top of the function.
2. You should use one variable declaration for each variable you want to define.

For instance:

// one variable declaration per function
function foo() {
    var bar, baz;
}

// multiple variable declarations per function
function foo() {
    var bar;
    var baz;
}

The single-declaration school of thought is based in pre-ECMAScript 6 behaviors, where there was no such thing as block scope, only function scope. Since all var statements are hoisted to the top of the function anyway, some believe that declaring all variables in a single declaration at the top of the function removes confusion around scoping rules.

Rule Details

This rule enforces variables to be declared either together or separately per function ( for var) or block (for let and const) scope.

Options

This rule has one option, which can be a string option or an object option.

String option:

• "always" (default) requires one variable declaration per scope
• "never" requires multiple variable declarations per scope

Object option:

• "var": "always" requires one var declaration per function
• "var": "never" requires multiple var declarations per function
• "let": "always" requires one let declaration per block
• "let": "never" requires multiple let declarations per block
• "const": "always" requires one const declaration per block
• "const": "never" requires multiple const declarations per block

Alternate object option:

• "initialized": "always" requires one variable declaration for initialized variables per scope
• "initialized": "never" requires multiple variable declarations for initialized variables per scope
• "uninitialized": "always" requires one variable declaration for uninitialized variables per scope
• "uninitialized": "never" requires multiple variable declarations for uninitialized variables per scope

always

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

/*eslint one-var: ["error", "always"]*/
/*eslint-env es6*/

function foo() {
    var bar;
    var baz;
    let qux;
    let norf;
}

function foo(){
    const bar = false;
    const baz = true;
    let qux;
    let norf;
}

function foo() {
    var bar;

    if (baz) {
        var qux = true;
    }
}

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

/*eslint one-var: ["error", "always"]*/
/*eslint-env es6*/

function foo() {
    var bar,
        baz;
    let qux,
        norf;
}

function foo(){
    const bar = true,
        baz = false;
    let qux,
        norf;
}

function foo() {
    var bar,
        qux;

    if (baz) {
        qux = true;
    }
}

function foo(){
    let bar;

    if (baz) {
        let qux;
    }
}

never

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

/*eslint one-var: ["error", "never"]*/
/*eslint-env es6*/

function foo() {
    var bar,
        baz;
    const bar = true,
        baz = false;
}

function foo() {
    var bar,
        qux;

    if (baz) {
        qux = true;
    }
}

function foo(){
    let bar = true,
        baz = false;
}

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

/*eslint one-var: ["error", "never"]*/
/*eslint-env es6*/

function foo() {
    var bar;
    var baz;
}

function foo() {
    var bar;

    if (baz) {
        var qux = true;
    }
}

function foo() {
    let bar;

    if (baz) {
        let qux = true;
    }
}

var, let, and const

Examples of incorrect code for this rule with the { var: "always", let: "never", const: "never" } option:

/*eslint one-var: ["error", { var: "always", let: "never", const: "never" }]*/
/*eslint-env es6*/

function foo() {
    var bar;
    var baz;
    let qux,
        norf;
}

function foo() {
    const bar = 1,
          baz = 2;
    let qux,
        norf;
}

Examples of correct code for this rule with the { var: "always", let: "never", const: "never" } option:

/*eslint one-var: ["error", { var: "always", let: "never", const: "never" }]*/
/*eslint-env es6*/

function foo() {
    var bar,
        baz;
    let qux;
    let norf;
}

function foo() {
    const bar = 1;
    const baz = 2;
    let qux;
    let norf;
}

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

/*eslint one-var: ["error", { var: "never" }]*/
/*eslint-env es6*/

function foo() {
    var bar,
        baz;
}

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

/*eslint one-var: ["error", { var: "never" }]*/
/*eslint-env es6*/

function foo() {
    var bar,
        baz;
    const bar = 1; // `const` and `let` declarations are ignored if they are not specified
    const baz = 2;
    let qux;
    let norf;
}

initialized and uninitialized

Examples of incorrect code for this rule with the { "initialized": "always", "uninitialized": "never" } option:

/*eslint one-var: ["error", { "initialized": "always", "uninitialized": "never" }]*/
/*eslint-env es6*/

function foo() {
    var a, b, c;
    var foo = true;
    var bar = false;
}

Examples of correct code for this rule with the { "initialized": "always", "uninitialized": "never" } option:

/*eslint one-var: ["error", { "initialized": "always", "uninitialized": "never" }]*/

function foo() {
    var a;
    var b;
    var c;
    var foo = true,
        bar = false;
}

for (let z of foo) {
    doSomething(z);
}

let z;
for (z of foo) {
    doSomething(z);
}

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

/*eslint one-var: ["error", { "initialized": "never" }]*/
/*eslint-env es6*/

function foo() {
    var foo = true,
        bar = false;
}

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

/*eslint one-var: ["error", { initialized: "never" }]*/

function foo() {
    var foo = true;
    var bar = false;
    var a, b, c; // Uninitialized variables are ignored
}

Compatibility

• JSHint: This rule maps to the onevar JSHint rule, but allows let and const to be configured separately.
• JSCS: This rule roughly maps to disallowMultipleVarDecl Source: http://eslint.org/docs/rules/

    return function postcss() {

Close

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

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

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

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

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

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

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

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

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

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

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

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

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

            var meta = util.colors.grey(' [' + err.fileName + ':' + err.lineNumber + ']');

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 name = forced || functionName(fn);

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 tasks = Object.keys(gulp.tasks || {})

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/

                .filter(function (alias) {

Close

Suggest using arrow functions as callbacks. (prefer-arrow-callback)

Arrow functions are suited to callbacks, because:

• this keywords in arrow functions bind to the upper scope's.
• The notation of the arrow function is shorter than function expression's.

Rule Details

This rule is aimed to flag usage of function expressions in an argument list.

The following patterns are considered problems:

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

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

The following patterns are not considered problems:

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

foo(a => a);
foo(function*() { yield; });

// this is not a callback.
var foo = function foo(a) { return a; };

// using `this` without `.bind(this)`.
foo(function() { return this.a; });

// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });

Options

This rule takes one optional argument, an object which is an options object.

allowNamedFunctions

This is a boolean option and it is false by default. When set to true, the rule doesn't warn on named functions used as callbacks.

Examples of correct code for the { "allowNamedFunctions": true } option:

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

foo(function bar() {});

allowUnboundThis

This is a boolean option and it is true by default. When set to false, this option allows the use of this without restriction and checks for dynamically assigned this values such as when using Array.prototype.map with a context argument. Normally, the rule will flag the use of this whenever a function does not use bind() to specify the value of this constantly.

Examples of incorrect code for the { "allowUnboundThis": false } option:

/*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

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

In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/

    return function copyExamples() {

Close

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

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

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

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

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

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

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

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

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

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

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

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

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

    return function list() {

Close

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

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

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

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

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

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

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

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

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

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

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

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

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

                var match = String(task.fn).match(/\/\*\s@desc\s(.*)?\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/

config.tasks.public.forEach(function(taskDefinition){

Close

Require Or Disallow Space Before Blocks (space-before-blocks)

Consistency is an important part of any style guide. While it is a personal preference where to put the opening brace of blocks, it should be consistent across a whole project. Having an inconsistent style distracts the reader from seeing the important parts of the code.

Rule Details

This rule will enforce consistency of spacing before blocks. It is only applied on blocks that don’t begin on a new line.

• This rule ignores spacing which is between => and a block. The spacing is handled by the arrow-spacing rule.
• This rule ignores spacing which is between a keyword and a block. The spacing is handled by the keyword-spacing rule.

Options

This rule takes one argument. If it is "always" then blocks must always have at least one preceding space. If "never" then all blocks should never have any preceding space. If different spacing is desired for function blocks, keyword blocks and classes, an optional configuration object can be passed as the rule argument to configure the cases separately.

( e.g. { "functions": "never", "keywords": "always", "classes": "always" } )

The default is "always".

"always"

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

/*eslint space-before-blocks: "error"*/

if (a){
    b();
}

function a(){}

for (;;){
    b();
}

try {} catch(a){}

class Foo{
  constructor(){}
}

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

/*eslint space-before-blocks: "error"*/

if (a) {
    b();
}

if (a) {
    b();
} else{ /*no error. this is checked by `keyword-spacing` rule.*/
    c();
}


function a() {}

for (;;) {
    b();
}

try {} catch(a) {}

"never"

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

/*eslint space-before-blocks: ["error", "never"]*/

if (a) {
    b();
}

function a() {}

for (;;) {
    b();
}

try {} catch(a) {}

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

/*eslint space-before-blocks: ["error", "never"]*/

if (a){
    b();
}

function a(){}

for (;;){
    b();
}

try{} catch(a){}

class Foo{
  constructor(){}
}

Examples of incorrect code for this rule when configured { "functions": "never", "keywords": "always", "classes": "never" }:

/*eslint space-before-blocks: ["error", { "functions": "never", "keywords": "always", "classes": "never" }]*/
/*eslint-env es6*/

function a() {}

try {} catch(a){}

class Foo{
  constructor() {}
}

Examples of correct code for this rule when configured { "functions": "never", "keywords": "always", "classes": "never" }:

/*eslint space-before-blocks: ["error", { "functions": "never", "keywords": "always", "classes": "never" }]*/
/*eslint-env es6*/

for (;;) {
  // ...
}

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

class Foo {
  constructor(){}
}

Examples of incorrect code for this rule when configured { "functions": "always", "keywords": "never", "classes": "never" }:

/*eslint space-before-blocks: ["error", { "functions": "always", "keywords": "never", "classes": "never" }]*/
/*eslint-env es6*/

function a(){}

try {} catch(a) {}

class Foo {
  constructor(){}
}

Examples of correct code for this rule when configured { "functions": "always", "keywords": "never", "classes": "never" }:

/*eslint space-before-blocks: ["error", { "functions": "always", "keywords": "never", "classes": "never" }]*/
/*eslint-env es6*/

if (a){
  b();
}

var a = function() {}

class Foo{
  constructor() {}
}

Examples of incorrect code for this rule when configured { "functions": "never", "keywords": "never", "classes": "always" }:

/*eslint space-before-blocks: ["error", { "functions": "never", "keywords": "never", "classes": "always" }]*/
/*eslint-env es6*/

class Foo{
  constructor(){}
}

Examples of correct code for this rule when configured { "functions": "never", "keywords": "never", "classes": "always" }:

/*eslint space-before-blocks: ["error", { "functions": "never", "keywords": "never", "classes": "always" }]*/
/*eslint-env es6*/

class Foo {
  constructor(){}
}

When Not To Use It

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

Related Rules

• [keyword-spacing](keyword-spacing.md)
• [arrow-spacing](arrow-spacing.md)
• [brace-style](brace-style.md) Source: http://eslint.org/docs/rules/

        var cleanPaths = [paths.clean.documentation, paths.clean.distribution];

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 function documentation(done) {

Close

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

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

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

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

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

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

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

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

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

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

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

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

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

                    examples: examples

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
};

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 longform will be used in an object literal.
• "consistent-as-needed" ensures that either all shorthand or all longform 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 longform 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/

            .reduce(function (results, taskName) {

Close

Suggest using arrow functions as callbacks. (prefer-arrow-callback)

Arrow functions are suited to callbacks, because:

• this keywords in arrow functions bind to the upper scope's.
• The notation of the arrow function is shorter than function expression's.

Rule Details

This rule is aimed to flag usage of function expressions in an argument list.

The following patterns are considered problems:

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

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

The following patterns are not considered problems:

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

foo(a => a);
foo(function*() { yield; });

// this is not a callback.
var foo = function foo(a) { return a; };

// using `this` without `.bind(this)`.
foo(function() { return this.a; });

// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });

Options

This rule takes one optional argument, an object which is an options object.

allowNamedFunctions

This is a boolean option and it is false by default. When set to true, the rule doesn't warn on named functions used as callbacks.

Examples of correct code for the { "allowNamedFunctions": true } option:

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

foo(function bar() {});

allowUnboundThis

This is a boolean option and it is true by default. When set to false, this option allows the use of this without restriction and checks for dynamically assigned this values such as when using Array.prototype.map with a context argument. Normally, the rule will flag the use of this whenever a function does not use bind() to specify the value of this constantly.

Examples of incorrect code for the { "allowUnboundThis": false } option:

/*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

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

In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/