Rules

Rules in ESLint are grouped by category to help you understand their purpose.

No rules are enabled by default. The "extends": "eslint:recommended" property in a configuration file enables rules that report common problems, which have a check mark below.

The --fix option on the command line automatically fixes problems (currently mostly whitespace) reported by rules which have a wrench below.

Possible Errors

These rules relate to possible syntax or logic errors in JavaScript code:

no-await-in-loop

disallow await inside of loops

no-cond-assign

disallow assignment operators in conditional expressions

no-console

disallow the use of console

no-constant-condition

disallow constant expressions in conditions

no-control-regex

disallow control characters in regular expressions

no-debugger

disallow the use of debugger

no-dupe-args

disallow duplicate arguments in function definitions

no-dupe-keys

disallow duplicate keys in object literals

no-duplicate-case

disallow duplicate case labels

no-empty-character-class

disallow empty character classes in regular expressions

no-empty

disallow empty block statements

no-ex-assign

disallow reassigning exceptions in catch clauses

no-extra-boolean-cast

disallow unnecessary boolean casts

no-extra-parens

disallow unnecessary parentheses

no-extra-semi

disallow unnecessary semicolons

no-func-assign

disallow reassigning function declarations

no-inner-declarations

disallow variable or function declarations in nested blocks

no-invalid-regexp

disallow invalid regular expression strings in RegExp constructors

no-irregular-whitespace

disallow irregular whitespace outside of strings and comments

no-obj-calls

disallow calling global object properties as functions

no-prototype-builtins

disallow calling some Object.prototype methods directly on objects

no-regex-spaces

disallow multiple spaces in regular expressions

no-sparse-arrays

disallow sparse arrays

no-template-curly-in-string

disallow template literal placeholder syntax in regular strings

no-unexpected-multiline

disallow confusing multiline expressions

no-unreachable

disallow unreachable code after return, throw, continue, and break statements

no-unsafe-finally

disallow control flow statements in finally blocks

no-unsafe-negation

disallow negating the left operand of relational operators

use-isnan

require calls to isNaN() when checking for NaN

valid-jsdoc

enforce valid JSDoc comments

valid-typeof

enforce comparing typeof expressions against valid strings

Best Practices

These rules relate to better ways of doing things to help you avoid problems:

accessor-pairs

enforce getter and setter pairs in objects

array-callback-return

enforce return statements in callbacks of array methods

block-scoped-var

enforce the use of variables within the scope they are defined

class-methods-use-this

enforce that class methods utilize this

complexity

enforce a maximum cyclomatic complexity allowed in a program

consistent-return

require return statements to either always or never specify values

curly

enforce consistent brace style for all control statements

default-case

require default cases in switch statements

dot-location

enforce consistent newlines before and after dots

dot-notation

enforce dot notation whenever possible

eqeqeq

require the use of === and !==

guard-for-in

require for-in loops to include an if statement

no-alert

disallow the use of alert, confirm, and prompt

no-caller

disallow the use of arguments.caller or arguments.callee

no-case-declarations

disallow lexical declarations in case clauses

no-div-regex

disallow division operators explicitly at the beginning of regular expressions

no-else-return

disallow else blocks after return statements in if statements

no-empty-function

disallow empty functions

no-empty-pattern

disallow empty destructuring patterns

no-eq-null

disallow null comparisons without type-checking operators

no-eval

disallow the use of eval()

no-extend-native

disallow extending native types

no-extra-bind

disallow unnecessary calls to .bind()

no-extra-label

disallow unnecessary labels

no-fallthrough

disallow fallthrough of case statements

no-floating-decimal

disallow leading or trailing decimal points in numeric literals

no-global-assign

disallow assignments to native objects or read-only global variables

no-implicit-coercion

disallow shorthand type conversions

no-implicit-globals

disallow variable and function declarations in the global scope

no-implied-eval

disallow the use of eval()-like methods

no-invalid-this

disallow this keywords outside of classes or class-like objects

no-iterator

disallow the use of the __iterator__ property

no-labels

disallow labeled statements

no-lone-blocks

disallow unnecessary nested blocks

no-loop-func

disallow function declarations and expressions inside loop statements

no-magic-numbers

disallow magic numbers

no-multi-spaces

disallow multiple spaces

no-multi-str

disallow multiline strings

no-new-func

disallow new operators with the Function object

no-new-wrappers

disallow new operators with the String, Number, and Boolean objects

no-new

disallow new operators outside of assignments or comparisons

no-octal-escape

disallow octal escape sequences in string literals

no-octal

disallow octal literals

no-param-reassign

disallow reassigning function parameters

no-proto

disallow the use of the __proto__ property

no-redeclare

disallow variable redeclaration

no-restricted-properties

disallow certain properties on certain objects

no-return-assign

disallow assignment operators in return statements

no-return-await

disallow unnecessary return await

no-script-url

disallow javascript: urls

no-self-assign

disallow assignments where both sides are exactly the same

no-self-compare

disallow comparisons where both sides are exactly the same

no-sequences

disallow comma operators

no-throw-literal

disallow throwing literals as exceptions

no-unmodified-loop-condition

disallow unmodified loop conditions

no-unused-expressions

disallow unused expressions

no-unused-labels

disallow unused labels

no-useless-call

disallow unnecessary calls to .call() and .apply()

no-useless-concat

disallow unnecessary concatenation of literals or template literals

no-useless-escape

disallow unnecessary escape characters

no-useless-return

disallow redundant return statements

no-void

disallow void operators

no-warning-comments

disallow specified warning terms in comments

no-with

disallow with statements

radix

enforce the consistent use of the radix argument when using parseInt()

require-await

disallow async functions which have no await expression

vars-on-top

require var declarations be placed at the top of their containing scope

wrap-iife

require parentheses around immediate function invocations

yoda

require or disallow “Yoda” conditions

Strict Mode

These rules relate to strict mode directives:

strict

require or disallow strict mode directives

Variables

These rules relate to variable declarations:

init-declarations

require or disallow initialization in variable declarations

no-catch-shadow

disallow catch clause parameters from shadowing variables in the outer scope

no-delete-var

disallow deleting variables

no-label-var

disallow labels that share a name with a variable

no-restricted-globals

disallow specified global variables

no-shadow-restricted-names

disallow identifiers from shadowing restricted names

no-shadow

disallow variable declarations from shadowing variables declared in the outer scope

no-undef-init

disallow initializing variables to undefined

no-undef

disallow the use of undeclared variables unless mentioned in /*global */ comments

no-undefined

disallow the use of undefined as an identifier

no-unused-vars

disallow unused variables

no-use-before-define

disallow the use of variables before they are defined

Node.js and CommonJS

These rules relate to code running in Node.js, or in browsers with CommonJS:

callback-return

require return statements after callbacks

global-require

require require() calls to be placed at top-level module scope

handle-callback-err

require error handling in callbacks

no-mixed-requires

disallow require calls to be mixed with regular variable declarations

no-new-require

disallow new operators with calls to require

no-path-concat

disallow string concatenation with __dirname and __filename

no-process-env

disallow the use of process.env

no-process-exit

disallow the use of process.exit()

no-restricted-modules

disallow specified modules when loaded by require

no-sync

disallow synchronous methods

Stylistic Issues

These rules relate to style guidelines, and are therefore quite subjective:

array-bracket-spacing

enforce consistent spacing inside array brackets

block-spacing

enforce consistent spacing inside single-line blocks

brace-style

enforce consistent brace style for blocks

camelcase

enforce camelcase naming convention

capitalized-comments

enforce or disallow capitalization of the first letter of a comment

comma-dangle

require or disallow trailing commas

comma-spacing

enforce consistent spacing before and after commas

comma-style

enforce consistent comma style

computed-property-spacing

enforce consistent spacing inside computed property brackets

consistent-this

enforce consistent naming when capturing the current execution context

eol-last

require or disallow newline at the end of files

func-call-spacing

require or disallow spacing between function identifiers and their invocations

func-name-matching

require function names to match the name of the variable or property to which they are assigned

func-names

require or disallow named function expressions

func-style

enforce the consistent use of either function declarations or expressions

id-blacklist

disallow specified identifiers

id-length

enforce minimum and maximum identifier lengths

id-match

require identifiers to match a specified regular expression

indent

enforce consistent indentation

jsx-quotes

enforce the consistent use of either double or single quotes in JSX attributes

key-spacing

enforce consistent spacing between keys and values in object literal properties

keyword-spacing

enforce consistent spacing before and after keywords

line-comment-position

enforce position of line comments

linebreak-style

enforce consistent linebreak style

lines-around-comment

require empty lines around comments

lines-around-directive

require or disallow newlines around directives

max-depth

enforce a maximum depth that blocks can be nested

max-len

enforce a maximum line length

max-lines

enforce a maximum number of lines per file

max-nested-callbacks

enforce a maximum depth that callbacks can be nested

max-params

enforce a maximum number of parameters in function definitions

max-statements-per-line

enforce a maximum number of statements allowed per line

max-statements

enforce a maximum number of statements allowed in function blocks

multiline-ternary

enforce newlines between operands of ternary expressions

new-cap

require constructor names to begin with a capital letter

new-parens

require parentheses when invoking a constructor with no arguments

newline-after-var

require or disallow an empty line after variable declarations

newline-before-return

require an empty line before return statements

newline-per-chained-call

require a newline after each call in a method chain

no-array-constructor

disallow Array constructors

no-bitwise

disallow bitwise operators

no-continue

disallow continue statements

no-inline-comments

disallow inline comments after code

no-lonely-if

disallow if statements as the only statement in else blocks

no-mixed-operators

disallow mixed binary operators

no-mixed-spaces-and-tabs

disallow mixed spaces and tabs for indentation

no-multiple-empty-lines

disallow multiple empty lines

no-negated-condition

disallow negated conditions

no-nested-ternary

disallow nested ternary expressions

no-new-object

disallow Object constructors

no-plusplus

disallow the unary operators ++ and --

no-restricted-syntax

disallow specified syntax

no-tabs

disallow all tabs

no-ternary

disallow ternary operators

no-trailing-spaces

disallow trailing whitespace at the end of lines

no-underscore-dangle

disallow dangling underscores in identifiers

no-unneeded-ternary

disallow ternary operators when simpler alternatives exist

no-whitespace-before-property

disallow whitespace before properties

object-curly-newline

enforce consistent line breaks inside braces

object-curly-spacing

enforce consistent spacing inside braces

object-property-newline

enforce placing object properties on separate lines

one-var-declaration-per-line

require or disallow newlines around variable declarations

one-var

enforce variables to be declared either together or separately in functions

operator-assignment

require or disallow assignment operator shorthand where possible

operator-linebreak

enforce consistent linebreak style for operators

padded-blocks

require or disallow padding within blocks

quote-props

require quotes around object literal property names

quotes

enforce the consistent use of either backticks, double, or single quotes

require-jsdoc

require JSDoc comments

semi-spacing

enforce consistent spacing before and after semicolons

semi

require or disallow semicolons instead of ASI

sort-keys

require object keys to be sorted

sort-vars

require variables within the same declaration block to be sorted

space-before-blocks

enforce consistent spacing before blocks

space-before-function-paren

enforce consistent spacing before function definition opening parenthesis

space-in-parens

enforce consistent spacing inside parentheses

space-infix-ops

require spacing around infix operators

space-unary-ops

enforce consistent spacing before or after unary operators

spaced-comment

enforce consistent spacing after the // or /* in a comment

unicode-bom

require or disallow Unicode byte order mark (BOM)

wrap-regex

require parenthesis around regex literals

ECMAScript 6

These rules relate to ES6, also known as ES2015:

arrow-body-style

require braces around arrow function bodies

arrow-parens

require parentheses around arrow function arguments

arrow-spacing

enforce consistent spacing before and after the arrow in arrow functions

constructor-super

require super() calls in constructors

generator-star-spacing

enforce consistent spacing around * operators in generator functions

no-class-assign

disallow reassigning class members

no-confusing-arrow

disallow arrow functions where they could be confused with comparisons

no-const-assign

disallow reassigning const variables

no-dupe-class-members

disallow duplicate class members

no-duplicate-imports

disallow duplicate module imports

no-new-symbol

disallow new operators with the Symbol object

no-restricted-imports

disallow specified modules when loaded by import

no-this-before-super

disallow this/super before calling super() in constructors

no-useless-computed-key

disallow unnecessary computed property keys in object literals

no-useless-constructor

disallow unnecessary constructors

no-useless-rename

disallow renaming import, export, and destructured assignments to the same name

no-var

require let or const instead of var

object-shorthand

require or disallow method and property shorthand syntax for object literals

prefer-arrow-callback

require arrow functions as callbacks

prefer-const

require const declarations for variables that are never reassigned after declared

prefer-numeric-literals

disallow parseInt() in favor of binary, octal, and hexadecimal literals

prefer-rest-params

require rest parameters instead of arguments

prefer-spread

require spread operators instead of .apply()

prefer-template

require template literals instead of string concatenation

require-yield

require generator functions to contain yield

rest-spread-spacing

enforce spacing between rest and spread operators and their expressions

sort-imports

enforce sorted import declarations within modules

symbol-description

require symbol descriptions

template-curly-spacing

require or disallow spacing around embedded expressions of template strings

yield-star-spacing

require or disallow spacing around the * in yield* expressions

Deprecated

These rules have been deprecated and replaced by newer rules:

Removed

These rules from older versions of ESLint have been replaced by newer rules: