util.log('Could not load task "' + taskName +'" from "' + taskFile + '", does not export factory function.');

Close

require spacing around infix operators (space-infix-ops)

While formatting preferences are very personal, a number of style guides require spaces around operators, such as:

var sum = 1 + 2;

The proponents of these extra spaces believe it make the code easier to read and can more easily highlight potential errors, such as:

var sum = i+++2;

While this is valid JavaScript syntax, it is hard to determine what the author intended.

Rule Details

This rule is aimed at ensuring there are spaces around infix operators.

Options

This rule accepts a single options argument with the following defaults:

"space-infix-ops": ["error", {"int32Hint": false}]

int32Hint

Set the int32Hint option to true (default is false) to allow write a|0 without space.

var foo = bar|0; // `foo` is forced to be signed 32 bit integer

Examples of incorrect code for this rule:

/*eslint space-infix-ops: "error"*/
/*eslint-env es6*/

a+b

a+ b

a +b

a?b:c

const a={b:1};

var {a=0}=bar;

function foo(a=0) { }

Examples of correct code for this rule:

/*eslint space-infix-ops: "error"*/
/*eslint-env es6*/

a + b

a       + b

a ? b : c

const a = {b:1};

var {a = 0} = bar;

function foo(a = 0) { }

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

    var clean = require('./clean').apply(null, args);

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/

        gulp: gulp,

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/

    var isAliased = Array.isArray(taskDefinition);

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 lint = require('./lint').apply(null, args);

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 copy() {

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 defaults = {

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 task = gulp.tasks[taskName];

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 gulp = require('gulp');

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/

        util.log('Could not initialize task function "' + taskName +'" from "' + taskFile + '":');

Close

require spacing around infix operators (space-infix-ops)

While formatting preferences are very personal, a number of style guides require spaces around operators, such as:

var sum = 1 + 2;

The proponents of these extra spaces believe it make the code easier to read and can more easily highlight potential errors, such as:

var sum = i+++2;

While this is valid JavaScript syntax, it is hard to determine what the author intended.

Rule Details

This rule is aimed at ensuring there are spaces around infix operators.

Options

This rule accepts a single options argument with the following defaults:

"space-infix-ops": ["error", {"int32Hint": false}]

int32Hint

Set the int32Hint option to true (default is false) to allow write a|0 without space.

var foo = bar|0; // `foo` is forced to be signed 32 bit integer

Examples of incorrect code for this rule:

/*eslint space-infix-ops: "error"*/
/*eslint-env es6*/

a+b

a+ b

a +b

a?b:c

const a={b:1};

var {a=0}=bar;

function foo(a=0) { }

Examples of correct code for this rule:

/*eslint space-infix-ops: "error"*/
/*eslint-env es6*/

a + b

a       + b

a ? b : c

const a = {b:1};

var {a = 0} = bar;

function foo(a = 0) { }

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

    var test = require('./test').apply(null, args);

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 path = require('path');

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 = item.name || item;

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 excludeGlobs = flatten(values(paths.exclude)).map(function (glob) {

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/

        )(function (err) {

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/

        // flatten cssRules

Close

enforce or disallow capitalization of the first letter of a comment (capitalized-comments)

Comments are useful for leaving information for future developers. In order for that information to be useful and not distracting, it is sometimes desirable for comments to follow a particular style. One element of comment formatting styles is whether the first word of a comment should be capitalized or lowercase.

In general, no comment style is any more or less valid than any others, but many developers would agree that a consistent style can improve a project's maintainability.

Rule Details

This rule aims to enforce a consistent style of comments across your codebase, specifically by either requiring or disallowing a capitalized letter as the first word character in a comment. This rule will not issue warnings when non-cased letters are used.

By default, this rule will require a non-lowercase letter at the beginning of comments.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error"] */

// lowercase comment

Examples of correct code for this rule:

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

Options

This rule has two options: a string value "always" or "never" which determines whether capitalization of the first word of a comment should be required or forbidden, and optionally an object containing more configuration parameters for the rule.

Here are the supported object options:

• ignorePattern: A string representing a regular expression pattern of words that should be ignored by this rule. If the first word of a comment matches the pattern, this rule will not report that comment.
  • Note that the following words are always ignored by this rule: ["jscs", "jshint", "eslint", "istanbul", "global", "globals", "exported"].
• ignoreInlineComments: If this is true, the rule will not report on comments in the middle of code. By default, this is false.
• ignoreConsecutiveComments: If this is true, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this is false.

Here is an example configuration:

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "ignorePattern": "pragma|ignored",
            "ignoreInlineComments": true
        }
    ]
}

"always"

Using the "always" option means that this rule will report any comments which start with a lowercase letter. This is the default configuration for this rule.

Note that configuration comments and comments which start with URLs are never reported.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// lowercase comment

Examples of correct code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

"never"

Using the "never" option means that this rule will report any comments which start with an uppercase letter.

Examples of incorrect code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// Capitalized comment

Examples of correct code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// lowercase comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

ignorePattern

The ignorePattern object takes a string value, which is used as a regular expression applied to the first word of a comment.

Examples of correct code with the "ignorePattern" option set to "pragma":

/* eslint capitalized-comments: ["error", "always", { "ignorePattern": "pragma" }] */

function foo() {
    /* pragma wrap(true) */
}

ignoreInlineComments

Setting the ignoreInlineComments option to true means that comments in the middle of code (with a token on the same line as the beginning of the comment, and another token on the same line as the end of the comment) will not be reported by this rule.

Examples of correct code with the "ignoreInlineComments" option set to true:

/* eslint capitalized-comments: ["error", "always", { "ignoreInlineComments": true }] */

function foo(/* ignored */ a) {
}

ignoreConsecutiveComments

If the ignoreConsecutiveComments option is set to true, then comments which otherwise violate the rule will not be reported as long as they immediately follow another comment. This can be applied more than once.

Examples of correct code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// This comment is valid since it has the correct capitalization.
// this comment is ignored since it follows another comment,
// and this one as well because it follows yet another comment.

/* Here is a block comment which has the correct capitalization, */
/* but this one is ignored due to being consecutive; */
/*
 * in fact, even if any of these are multi-line, that is fine too.
 */

Examples of incorrect code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// this comment is invalid, but only on this line.
// this comment does NOT get reported, since it is a consecutive comment.

Using Different Options for Line and Block Comments

If you wish to have a different configuration for line comments and block comments, you can do so by using two different object configurations (note that the capitalization option will be enforced consistently for line and block comments):

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "line": {
                "ignorePattern": "pragma|ignored",
            },
            "block": {
                "ignoreInlineComments": true,
                "ignorePattern": "ignored"
            }
        }
    ]
}

Examples of incorrect code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// capitalized line comment, this is incorrect, blockignore does not help here
/* lowercased block comment, this is incorrect too */

Examples of correct code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// Uppercase line comment, this is correct
/* blockignore lowercase block comment, this is correct due to ignorePattern */

When Not To Use It

This rule can be disabled if you do not care about the grammatical style of comments in your codebase.

Compatibility

• JSCS: requireCapitalizedComments and disallowCapitalizedComments Source: http://eslint.org/docs/rules/

    tape('integration', async function(t) { // eslint-disable-line no-loop-func

Close

Require or disallow a space before function parenthesis (space-before-function-paren)

When formatting a function, whitespace is allowed between the function name or function keyword and the opening paren. Named functions also require a space between the function keyword and the function name, but anonymous functions require no whitespace. For example:

function withoutSpace(x) {
    // ...
}

function withSpace (x) {
    // ...
}

var anonymousWithoutSpace = function() {};

var anonymousWithSpace = function () {};

Style guides may require a space after the function keyword for anonymous functions, while others specify no whitespace. Similarly, the space after a function name may or may not be required.

Rule Details

This rule aims to enforce consistent spacing before function parentheses and as such, will warn whenever whitespace doesn't match the preferences specified.

Options

This rule has a string option or an object option:

{
    "space-before-function-paren": ["error", "always"],
    // or
    "space-before-function-paren": ["error", {
        "anonymous": "always",
        "named": "always",
        "asyncArrow": "ignore"
    }],
}

• always (default) requires a space followed by the ( of arguments.
• never disallows any space followed by the ( of arguments.

The string option does not check async arrow function expressions for backward compatibility.

You can also use a separate option for each type of function. Each of the following options can be set to "always", "never", or "ignore". Default is "always" basically.

• anonymous is for anonymous function expressions (e.g. function () {}).
• named is for named function expressions (e.g. function foo () {}).
• asyncArrow is for async arrow function expressions (e.g. async () => {}). asyncArrow is set to "ignore" by default for backwards compatibility.

"always"

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

/*eslint space-before-function-paren: "error"*/
/*eslint-env es6*/

function foo() {
    // ...
}

var bar = function() {
    // ...
};

var bar = function foo() {
    // ...
};

class Foo {
    constructor() {
        // ...
    }
}

var foo = {
    bar() {
        // ...
    }
};

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

/*eslint space-before-function-paren: "error"*/
/*eslint-env es6*/

function foo () {
    // ...
}

var bar = function () {
    // ...
};

var bar = function foo () {
    // ...
};

class Foo {
    constructor () {
        // ...
    }
}

var foo = {
    bar () {
        // ...
    }
};

// async arrow function expressions are ignored by default.
var foo = async () => 1
var foo = async() => 1

"never"

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

/*eslint space-before-function-paren: ["error", "never"]*/
/*eslint-env es6*/

function foo () {
    // ...
}

var bar = function () {
    // ...
};

var bar = function foo () {
    // ...
};

class Foo {
    constructor () {
        // ...
    }
}

var foo = {
    bar () {
        // ...
    }
};

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

/*eslint space-before-function-paren: ["error", "never"]*/
/*eslint-env es6*/

function foo() {
    // ...
}

var bar = function() {
    // ...
};

var bar = function foo() {
    // ...
};

class Foo {
    constructor() {
        // ...
    }
}

var foo = {
    bar() {
        // ...
    }
};

// async arrow function expressions are ignored by default.
var foo = async () => 1
var foo = async() => 1

{"anonymous": "always", "named": "never", "asyncArrow": "always"}

Examples of incorrect code for this rule with the {"anonymous": "always", "named": "never", "asyncArrow": "always"} option:

/*eslint space-before-function-paren: ["error", {"anonymous": "always", "named": "never", "asyncArrow": "always"}]*/
/*eslint-env es6*/

function foo () {
    // ...
}

var bar = function() {
    // ...
};

class Foo {
    constructor () {
        // ...
    }
}

var foo = {
    bar () {
        // ...
    }
};

var foo = async(a) => await a

Examples of correct code for this rule with the {"anonymous": "always", "named": "never", "asyncArrow": "always"} option:

/*eslint space-before-function-paren: ["error", {"anonymous": "always", "named": "never", "asyncArrow": "always"}]*/
/*eslint-env es6*/

function foo() {
    // ...
}

var bar = function () {
    // ...
};

class Foo {
    constructor() {
        // ...
    }
}

var foo = {
    bar() {
        // ...
    }
};

var foo = async (a) => await a

{"anonymous": "never", "named": "always"}

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

/*eslint space-before-function-paren: ["error", { "anonymous": "never", "named": "always" }]*/
/*eslint-env es6*/

function foo() {
    // ...
}

var bar = function () {
    // ...
};

class Foo {
    constructor() {
        // ...
    }
}

var foo = {
    bar() {
        // ...
    }
};

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

/*eslint space-before-function-paren: ["error", { "anonymous": "never", "named": "always" }]*/
/*eslint-env es6*/

function foo () {
    // ...
}

var bar = function() {
    // ...
};

class Foo {
    constructor () {
        // ...
    }
}

var foo = {
    bar () {
        // ...
    }
};

{"anonymous": "ignore", "named": "always"}

Examples of incorrect code for this rule with the {"anonymous": "ignore", "named": "always"} option:

/*eslint space-before-function-paren: ["error", { "anonymous": "ignore", "named": "always" }]*/
/*eslint-env es6*/

function foo() {
    // ...
}

class Foo {
    constructor() {
        // ...
    }
}

var foo = {
    bar() {
        // ...
    }
};

Examples of correct code for this rule with the {"anonymous": "ignore", "named": "always"} option:

/*eslint space-before-function-paren: ["error", { "anonymous": "ignore", "named": "always" }]*/
/*eslint-env es6*/

var bar = function() {
    // ...
};

var bar = function () {
    // ...
};

function foo () {
    // ...
}

class Foo {
    constructor () {
        // ...
    }
}

var foo = {
    bar () {
        // ...
    }
};

When Not To Use It

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

Related Rules

• [space-after-keywords](space-after-keywords.md)
• [space-return-throw-case](space-return-throw-case.md) Source: http://eslint.org/docs/rules/

                    // fetch test markup

Close

enforce or disallow capitalization of the first letter of a comment (capitalized-comments)

Comments are useful for leaving information for future developers. In order for that information to be useful and not distracting, it is sometimes desirable for comments to follow a particular style. One element of comment formatting styles is whether the first word of a comment should be capitalized or lowercase.

In general, no comment style is any more or less valid than any others, but many developers would agree that a consistent style can improve a project's maintainability.

Rule Details

This rule aims to enforce a consistent style of comments across your codebase, specifically by either requiring or disallowing a capitalized letter as the first word character in a comment. This rule will not issue warnings when non-cased letters are used.

By default, this rule will require a non-lowercase letter at the beginning of comments.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error"] */

// lowercase comment

Examples of correct code for this rule:

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

Options

This rule has two options: a string value "always" or "never" which determines whether capitalization of the first word of a comment should be required or forbidden, and optionally an object containing more configuration parameters for the rule.

Here are the supported object options:

• ignorePattern: A string representing a regular expression pattern of words that should be ignored by this rule. If the first word of a comment matches the pattern, this rule will not report that comment.
  • Note that the following words are always ignored by this rule: ["jscs", "jshint", "eslint", "istanbul", "global", "globals", "exported"].
• ignoreInlineComments: If this is true, the rule will not report on comments in the middle of code. By default, this is false.
• ignoreConsecutiveComments: If this is true, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this is false.

Here is an example configuration:

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "ignorePattern": "pragma|ignored",
            "ignoreInlineComments": true
        }
    ]
}

"always"

Using the "always" option means that this rule will report any comments which start with a lowercase letter. This is the default configuration for this rule.

Note that configuration comments and comments which start with URLs are never reported.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// lowercase comment

Examples of correct code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

"never"

Using the "never" option means that this rule will report any comments which start with an uppercase letter.

Examples of incorrect code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// Capitalized comment

Examples of correct code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// lowercase comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

ignorePattern

The ignorePattern object takes a string value, which is used as a regular expression applied to the first word of a comment.

Examples of correct code with the "ignorePattern" option set to "pragma":

/* eslint capitalized-comments: ["error", "always", { "ignorePattern": "pragma" }] */

function foo() {
    /* pragma wrap(true) */
}

ignoreInlineComments

Setting the ignoreInlineComments option to true means that comments in the middle of code (with a token on the same line as the beginning of the comment, and another token on the same line as the end of the comment) will not be reported by this rule.

Examples of correct code with the "ignoreInlineComments" option set to true:

/* eslint capitalized-comments: ["error", "always", { "ignoreInlineComments": true }] */

function foo(/* ignored */ a) {
}

ignoreConsecutiveComments

If the ignoreConsecutiveComments option is set to true, then comments which otherwise violate the rule will not be reported as long as they immediately follow another comment. This can be applied more than once.

Examples of correct code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// This comment is valid since it has the correct capitalization.
// this comment is ignored since it follows another comment,
// and this one as well because it follows yet another comment.

/* Here is a block comment which has the correct capitalization, */
/* but this one is ignored due to being consecutive; */
/*
 * in fact, even if any of these are multi-line, that is fine too.
 */

Examples of incorrect code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// this comment is invalid, but only on this line.
// this comment does NOT get reported, since it is a consecutive comment.

Using Different Options for Line and Block Comments

If you wish to have a different configuration for line comments and block comments, you can do so by using two different object configurations (note that the capitalization option will be enforced consistently for line and block comments):

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "line": {
                "ignorePattern": "pragma|ignored",
            },
            "block": {
                "ignoreInlineComments": true,
                "ignorePattern": "ignored"
            }
        }
    ]
}

Examples of incorrect code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// capitalized line comment, this is incorrect, blockignore does not help here
/* lowercased block comment, this is incorrect too */

Examples of correct code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// Uppercase line comment, this is correct
/* blockignore lowercase block comment, this is correct due to ignorePattern */

When Not To Use It

This rule can be disabled if you do not care about the grammatical style of comments in your codebase.

Compatibility

• JSCS: requireCapitalizedComments and disallowCapitalizedComments Source: http://eslint.org/docs/rules/

                    // inject js when css and html is injected

Close

enforce or disallow capitalization of the first letter of a comment (capitalized-comments)

Comments are useful for leaving information for future developers. In order for that information to be useful and not distracting, it is sometimes desirable for comments to follow a particular style. One element of comment formatting styles is whether the first word of a comment should be capitalized or lowercase.

In general, no comment style is any more or less valid than any others, but many developers would agree that a consistent style can improve a project's maintainability.

Rule Details

This rule aims to enforce a consistent style of comments across your codebase, specifically by either requiring or disallowing a capitalized letter as the first word character in a comment. This rule will not issue warnings when non-cased letters are used.

By default, this rule will require a non-lowercase letter at the beginning of comments.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error"] */

// lowercase comment

Examples of correct code for this rule:

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

Options

This rule has two options: a string value "always" or "never" which determines whether capitalization of the first word of a comment should be required or forbidden, and optionally an object containing more configuration parameters for the rule.

Here are the supported object options:

• ignorePattern: A string representing a regular expression pattern of words that should be ignored by this rule. If the first word of a comment matches the pattern, this rule will not report that comment.
  • Note that the following words are always ignored by this rule: ["jscs", "jshint", "eslint", "istanbul", "global", "globals", "exported"].
• ignoreInlineComments: If this is true, the rule will not report on comments in the middle of code. By default, this is false.
• ignoreConsecutiveComments: If this is true, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this is false.

Here is an example configuration:

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "ignorePattern": "pragma|ignored",
            "ignoreInlineComments": true
        }
    ]
}

"always"

Using the "always" option means that this rule will report any comments which start with a lowercase letter. This is the default configuration for this rule.

Note that configuration comments and comments which start with URLs are never reported.

Examples of incorrect code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// lowercase comment

Examples of correct code for this rule:

/* eslint capitalized-comments: ["error", "always"] */

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

/* eslint semi:off */
/* eslint-env node */
/* eslint-disable */
/* eslint-enable */
/* istanbul ignore next */
/* jscs:enable */
/* jshint asi:true */
/* global foo */
/* globals foo */
/* exported myVar */
// eslint-disable-line
// eslint-disable-next-line
// https://github.com

"never"

Using the "never" option means that this rule will report any comments which start with an uppercase letter.

Examples of incorrect code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// Capitalized comment

Examples of correct code with the "never" option:

/* eslint capitalized-comments: ["error", "never"] */

// lowercase comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

ignorePattern

The ignorePattern object takes a string value, which is used as a regular expression applied to the first word of a comment.

Examples of correct code with the "ignorePattern" option set to "pragma":

/* eslint capitalized-comments: ["error", "always", { "ignorePattern": "pragma" }] */

function foo() {
    /* pragma wrap(true) */
}

ignoreInlineComments

Setting the ignoreInlineComments option to true means that comments in the middle of code (with a token on the same line as the beginning of the comment, and another token on the same line as the end of the comment) will not be reported by this rule.

Examples of correct code with the "ignoreInlineComments" option set to true:

/* eslint capitalized-comments: ["error", "always", { "ignoreInlineComments": true }] */

function foo(/* ignored */ a) {
}

ignoreConsecutiveComments

If the ignoreConsecutiveComments option is set to true, then comments which otherwise violate the rule will not be reported as long as they immediately follow another comment. This can be applied more than once.

Examples of correct code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// This comment is valid since it has the correct capitalization.
// this comment is ignored since it follows another comment,
// and this one as well because it follows yet another comment.

/* Here is a block comment which has the correct capitalization, */
/* but this one is ignored due to being consecutive; */
/*
 * in fact, even if any of these are multi-line, that is fine too.
 */

Examples of incorrect code with ignoreConsecutiveComments set to true:

/* eslint capitalize-comments: ["error", "always", { "ignoreConsecutiveComments": true }] */

// this comment is invalid, but only on this line.
// this comment does NOT get reported, since it is a consecutive comment.

Using Different Options for Line and Block Comments

If you wish to have a different configuration for line comments and block comments, you can do so by using two different object configurations (note that the capitalization option will be enforced consistently for line and block comments):

{
    "capitalized-comments": [
        "error",
        "always",
        {
            "line": {
                "ignorePattern": "pragma|ignored",
            },
            "block": {
                "ignoreInlineComments": true,
                "ignorePattern": "ignored"
            }
        }
    ]
}

Examples of incorrect code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// capitalized line comment, this is incorrect, blockignore does not help here
/* lowercased block comment, this is incorrect too */

Examples of correct code with different line and block comment configuration:

/* eslint capitalized-comments: ["error", "always", { "block": { "ignorePattern": "blockignore" } }] */

// Uppercase line comment, this is correct
/* blockignore lowercase block comment, this is correct due to ignorePattern */

When Not To Use It

This rule can be disabled if you do not care about the grammatical style of comments in your codebase.

Compatibility

• JSCS: requireCapitalizedComments and disallowCapitalizedComments Source: http://eslint.org/docs/rules/

                .forEach(propertyName => element.style[propertyName] = styles[propertyName]);

Close

Disallow Assignment in return Statement (no-return-assign)

One of the interesting, and sometimes confusing, aspects of JavaScript is that assignment can happen at almost any point. Because of this, an errant equals sign can end up causing assignment when the true intent was to do a comparison. This is especially true when using a return statement. For example:

function doSomething() {
    return foo = bar + 2;
}

It is difficult to tell the intent of the return statement here. It's possible that the function is meant to return the result of bar + 2, but then why is it assigning to foo? It's also possible that the intent was to use a comparison operator such as == and that this code is an error.

Because of this ambiguity, it's considered a best practice to not use assignment in return statements.

Rule Details

This rule aims to eliminate assignments from return statements. As such, it will warn whenever an assignment is found as part of return.

Options

The rule takes one option, a string, which must contain one of the following values:

• except-parens (default): Disallow assignments unless they are enclosed in parentheses.
• always: Disallow all assignments.

except-parens

This is the default option. It disallows assignments unless they are enclosed in parentheses.

Examples of incorrect code for the default "except-parens" option:

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

function doSomething() {
    return foo = bar + 2;
}

function doSomething() {
    return foo += 2;
}

Examples of correct code for the default "except-parens" option:

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

function doSomething() {
    return foo == bar + 2;
}

function doSomething() {
    return foo === bar + 2;
}

function doSomething() {
    return (foo = bar + 2);
}

always

This option disallows all assignments in return statements. All assignments are treated as problems.

Examples of incorrect code for the "always" option:

/*eslint no-return-assign: ["error", "always"]*/

function doSomething() {
    return foo = bar + 2;
}

function doSomething() {
    return foo += 2;
}

function doSomething() {
    return (foo = bar + 2);
}

Examples of correct code for the "always" option:

/*eslint no-return-assign: ["error", "always"]*/

function doSomething() {
    return foo == bar + 2;
}

function doSomething() {
    return foo === bar + 2;
}

When Not To Use It

If you want to allow the use of assignment operators in a return statement, then you can safely disable this rule. Source: http://eslint.org/docs/rules/