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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/

/* let state = {

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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/