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:

• comma-dangle: require or disallow trailing commas
• 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: disallow empty block statements
• no-empty-character-class: disallow empty character classes in regular expressions
• 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 function or var 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-negated-in-lhs: disallow negating the left operand in in expressions
• no-obj-calls: disallow calling global object properties as functions
• no-prototype-builtins: Disallow use of Object.prototypes builtins directly
• no-regex-spaces: disallow multiple spaces in regular expression literals
• no-sparse-arrays: disallow sparse arrays
• 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
• 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
• 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-implicit-coercion: disallow shorthand type conversions
• no-implicit-globals: disallow var and named 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-native-reassign: disallow reassigning native objects
• no-new: disallow new operators outside of assignments or comparisons
• no-new-func: disallow new operators with the Function object
• no-new-wrappers: disallow new operators with the String, Number, and Boolean objects
• no-octal: disallow octal literals
• no-octal-escape: disallow octal escape sequences in string literals
• no-param-reassign: disallow reassigning function parameters
• no-proto: disallow the use of the __proto__ property
• no-redeclare: disallow var redeclaration
• no-return-assign: disallow assignment operators in return statements
• 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-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()
• 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 var 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: disallow var declarations from shadowing variables in the outer scope
• no-shadow-restricted-names: disallow identifiers from shadowing restricted names
• no-undef: disallow the use of undeclared variables unless mentioned in /*global */ comments
• no-undef-init: disallow initializing variables to undefined
• 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 var 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
• 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: enforce at least one newline at the end of files
• 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
• linebreak-style: enforce consistent linebreak style
• lines-around-comment: require empty lines around comments
• max-depth: enforce a maximum depth that blocks can be nested
• max-len: enforce a maximum line length
• max-lines: enforce a maximum file length
• 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: enforce a maximum number of statements allowed in function blocks
• max-statements-per-line: enforce a maximum number of statements allowed per line
• new-cap: require constructor function 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 var 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 mixes of different 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-spaced-func: disallow spacing between function identifiers and their applications
• 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: enforce variables to be declared either together or separately in functions
• one-var-declaration-per-line: require or disallow newlines around var declarations
• 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: require or disallow semicolons instead of ASI
• semi-spacing: enforce consistent spacing before and after semicolons
• 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 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 the Unicode 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-reflect: require Reflect methods where applicable
• 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
• 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

Removed

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