Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

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/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

disallow multiple empty lines (no-multiple-empty-lines)

Some developers prefer to have multiple blank lines removed, while others feel that it helps improve readability. Whitespace is useful for separating logical sections of code, but excess whitespace takes up more of the screen.

Rule Details

This rule aims to reduce the scrolling required when reading through your code. It will warn when the maximum amount of empty lines has been exceeded.

Options

This rule has an object option:

• "max" (default: 2) enforces a maximum number of consecutive empty lines.
• "maxEOF" enforces a maximum number of consecutive empty lines at the end of files.
• "maxBOF" enforces a maximum number of consecutive empty lines at the beginning of files.

max

Examples of incorrect code for this rule with the default { "max": 2 } option:

/*eslint no-multiple-empty-lines: "error"*/

var foo = 5;



var bar = 3;

Examples of correct code for this rule with the default { "max": 2 } option:

/*eslint no-multiple-empty-lines: "error"*/

var foo = 5;


var bar = 3;

maxEOF

Examples of incorrect code for this rule with the { max: 2, maxEOF: 1 } options:

/*eslint no-multiple-empty-lines: ["error", { "max": 2, "maxEOF": 1 }]*/

var foo = 5;


var bar = 3;

Examples of correct code for this rule with the { max: 2, maxEOF: 1 } options:

/*eslint no-multiple-empty-lines: ["error", { "max": 2, "maxEOF": 1 }]*/

var foo = 5;


var bar = 3;

maxBOF

Examples of incorrect code for this rule with the { max: 2, maxBOF: 1 } options:

/*eslint no-multiple-empty-lines: ["error", { "max": 2, "maxBOF": 1 }]*/


var foo = 5;


var bar = 3;

Examples of correct code for this rule with the { max: 2, maxBOF: 1 } options:

/*eslint no-multiple-empty-lines: ["error", { "max": 2, "maxBOF": 1}]*/

var foo = 5;


var bar = 3;

When Not To Use It

If you do not care about extra blank lines, turn this off. Source: http://eslint.org/docs/rules/

require or disallow padding within blocks (padded-blocks)

Some style guides require block statements to start and end with blank lines. The goal is to improve readability by visually separating the block content and the surrounding code.

if (a) {

    b();

}

Since it's good to have a consistent code style, you should either always write padded blocks or never do it.

Rule Details

This rule enforces consistent empty line padding within blocks.

Options

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

String option:

• "always" (default) requires empty lines at the beginning and ending of block statements (except switch statements and classes)
• "never" disallows empty lines at the beginning and ending of block statements (except switch statements and classes)

Object option:

• "blocks" require or disallow padding within block statements
• "classes" require or disallow padding within classes
• "switches" require or disallow padding within switch statements

always

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

/*eslint padded-blocks: ["error", "always"]*/

if (a) {
    b();
}

if (a) { b(); }

if (a)
{
    b();
}

if (a) {

    b();
}

if (a) {
    b();

}

if (a) {
    // comment
    b();

}

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

/*eslint padded-blocks: ["error", "always"]*/

if (a) {

    b();

}

if (a)
{

    b();

}

if (a) {

    // comment
    b();

}

never

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

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

if (a) {

    b();

}

if (a)
{

    b();

}

if (a) {

    b();
}

if (a) {
    b();

}

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

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

if (a) {
    b();
}

if (a)
{
    b();
}

blocks

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

/*eslint padded-blocks: ["error", { "blocks": "always" }]*/

if (a) {
    b();
}

if (a) { b(); }

if (a)
{
    b();
}

if (a) {

    b();
}

if (a) {
    b();

}

if (a) {
    // comment
    b();

}

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

/*eslint padded-blocks: ["error", { "blocks": "always" }]*/

if (a) {

    b();

}

if (a)
{

    b();

}

if (a) {

    // comment
    b();

}

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

/*eslint padded-blocks: ["error", { "blocks": "never" }]*/

if (a) {

    b();

}

if (a)
{

    b();

}

if (a) {

    b();
}

if (a) {
    b();

}

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

/*eslint padded-blocks: ["error", { "blocks": "never" }]*/

if (a) {
    b();
}

if (a)
{
    b();
}

classes

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

/*eslint padded-blocks: ["error", { "classes": "always" }]*/

class  A {
    constructor(){
    }
}

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

/*eslint padded-blocks: ["error", { "classes": "always" }]*/

class  A {

    constructor(){
    }

}

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

/*eslint padded-blocks: ["error", { "classes": "never" }]*/

class  A {

    constructor(){
    }

}

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

/*eslint padded-blocks: ["error", { "classes": "never" }]*/

class  A {
    constructor(){
    }
}

switches

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

/*eslint padded-blocks: ["error", { "switches": "always" }]*/

switch (a) {
    case 0: foo();
}

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

/*eslint padded-blocks: ["error", { "switches": "always" }]*/

switch (a) {

    case 0: foo();

}

if (a) {
    b();
}

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

/*eslint padded-blocks: ["error", { "switches": "never" }]*/

switch (a) {

    case 0: foo();

}

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

/*eslint padded-blocks: ["error", { "switches": "never" }]*/

switch (a) {
    case 0: foo();
}

if (a) {

    b();

}

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of padding within blocks. Source: http://eslint.org/docs/rules/

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

For more information visit Source: http://eslint.org/docs/rules/

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

For more information visit Source: http://eslint.org/docs/rules/

Disallow Unused Variables (no-unused-vars)

Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

Rule Details

This rule is aimed at eliminating unused variables, functions, and parameters of functions.

A variable is considered to be used if any of the following are true:

• It represents a function that is called (doSomething())
• It is read (var y = x)
• It is passed into a function as an argument (doSomething(x))
• It is read inside of a function that is passed to another function (doSomething(function() { foo(); }))

A variable is not considered to be used if it is only ever assigned to (var x = 5) or declared.

Examples of incorrect code for this rule:

/*eslint no-unused-vars: "error"*/
/*global some_unused_var*/

// It checks variables you have defined as global
some_unused_var = 42;

var x;

// Write-only variables are not considered as used.
var y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;

// By default, unused arguments cause warnings.
(function(foo) {
    return 5;
})();

// Unused recursive functions also cause warnings.
function fact(n) {
    if (n < 2) return 1;
    return n * fact(n - 1);
}

// When a function definition destructures an array, unused entries from the array also cause warnings.
function getY([x, y]) {
    return y;
}

Examples of correct code for this rule:

/*eslint no-unused-vars: "error"*/

var x = 10;
alert(x);

// foo is considered used here
myFunc(function foo() {
    // ...
}.bind(this));

(function(foo) {
    return foo;
})();

var myFunc;
myFunc = setTimeout(function() {
    // myFunc is considered used
    myFunc();
}, 50);

// Only the second argument from the descructured array is used.
function getY([, y]) {
    return y;
}

exported

In environments outside of CommonJS or ECMAScript modules, you may use var to create a global variable that may be used by other scripts. You can use the /* exported variableName */ comment block to indicate that this variable is being exported and therefore should not be considered unused.

Note that /* exported */ has no effect for any of the following:

• when the environment is node or commonjs
• when parserOptions.sourceType is module
• when ecmaFeatures.globalReturn is true

The line comment // exported variableName will not work as exported is not line-specific.

Examples of correct code for /* exported variableName */ operation:

/* exported global_var */

var global_var = 42;

Options

This rule takes one argument which can be a string or an object. The string settings are the same as those of the vars property (explained below).

By default this rule is enabled with all option for variables and after-used for arguments.

{
    "rules": {
        "no-unused-vars": ["error", { "vars": "all", "args": "after-used", "ignoreRestSiblings": false }]
    }
}

vars

The vars option has two settings:

• all checks all variables for usage, including those in the global scope. This is the default setting.
• local checks only that locally-declared variables are used but will allow global variables to be unused.

vars: local

Examples of correct code for the { "vars": "local" } option:

/*eslint no-unused-vars: ["error", { "vars": "local" }]*/
/*global some_unused_var */

some_unused_var = 42;

varsIgnorePattern

The varsIgnorePattern option specifies exceptions not to check for usage: variables whose names match a regexp pattern. For example, variables whose names contain ignored or Ignored.

Examples of correct code for the { "varsIgnorePattern": "[iI]gnored" } option:

/*eslint no-unused-vars: ["error", { "varsIgnorePattern": "[iI]gnored" }]*/

var firstVarIgnored = 1;
var secondVar = 2;
console.log(secondVar);

args

The args option has three settings:

• after-used - only the last argument must be used. This allows you, for instance, to have two named parameters to a function and as long as you use the second argument, ESLint will not warn you about the first. This is the default setting.
• all - all named arguments must be used.
• none - do not check arguments.

args: after-used

Examples of incorrect code for the default { "args": "after-used" } option:

/*eslint no-unused-vars: ["error", { "args": "after-used" }]*/

// 1 error
// "baz" is defined but never used
(function(foo, bar, baz) {
    return bar;
})();

Examples of correct code for the default { "args": "after-used" } option:

/*eslint no-unused-vars: ["error", {"args": "after-used"}]*/

(function(foo, bar, baz) {
    return baz;
})();

args: all

Examples of incorrect code for the { "args": "all" } option:

/*eslint no-unused-vars: ["error", { "args": "all" }]*/

// 2 errors
// "foo" is defined but never used
// "baz" is defined but never used
(function(foo, bar, baz) {
    return bar;
})();

args: none

Examples of correct code for the { "args": "none" } option:

/*eslint no-unused-vars: ["error", { "args": "none" }]*/

(function(foo, bar, baz) {
    return bar;
})();

ignoreRestSiblings

The ignoreRestSiblings option is a boolean (default: false). Using a Rest Property it is possible to "omit" properties from an object, but by default the sibling properties are marked as "unused". With this option enabled the rest property's siblings are ignored.

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

/*eslint no-unused-vars: ["error", { "ignoreRestSiblings": true }]*/
// 'type' is ignored because it has a rest property sibling.
var { type, ...coords } = data;

argsIgnorePattern

The argsIgnorePattern option specifies exceptions not to check for usage: arguments whose names match a regexp pattern. For example, variables whose names begin with an underscore.

Examples of correct code for the { "argsIgnorePattern": "^_" } option:

/*eslint no-unused-vars: ["error", { "argsIgnorePattern": "^_" }]*/

function foo(x, _y) {
    return x + 1;
}
foo();

caughtErrors

The caughtErrors option is used for catch block arguments validation.

It has two settings:

• none - do not check error objects. This is the default setting.
• all - all named arguments must be used.

caughtErrors: none

Not specifying this rule is equivalent of assigning it to none.

Examples of correct code for the { "caughtErrors": "none" } option:

/*eslint no-unused-vars: ["error", { "caughtErrors": "none" }]*/

try {
    //...
} catch (err) {
    console.error("errors");
}

caughtErrors: all

Examples of incorrect code for the { "caughtErrors": "all" } option:

/*eslint no-unused-vars: ["error", { "caughtErrors": "all" }]*/

// 1 error
// "err" is defined but never used
try {
    //...
} catch (err) {
    console.error("errors");
}

caughtErrorsIgnorePattern

The caughtErrorsIgnorePattern option specifies exceptions not to check for usage: catch arguments whose names match a regexp pattern. For example, variables whose names begin with a string 'ignore'.

Examples of correct code for the { "caughtErrorsIgnorePattern": "^ignore" } option:

/*eslint no-unused-vars: ["error", { "caughtErrorsIgnorePattern": "^ignore" }]*/

try {
    //...
} catch (ignoreErr) {
    console.error("errors");
}

When Not To Use It

If you don't want to be notified about unused variables or function arguments, you can safely turn this rule off. Source: http://eslint.org/docs/rules/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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

Require Camelcase (camelcase)

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

Rule Details

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

Options

This rule has an object option:

• "properties": "always" (default) enforces camelcase style for property names
• "properties": "never" does not check property names

always

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

/*eslint camelcase: "error"*/

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

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

var obj = {
    my_pref: 1
};

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

/*eslint camelcase: "error"*/

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

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

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

var { category_id: category } = query;

never

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

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

var obj = {
    my_pref: 1
};

When Not To Use It

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