'fs' is defined but never used. Open
import fs from "fs";
- Read upRead up
- Exclude checks
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
orcommonjs
- when
parserOptions.sourceType
ismodule
- when
ecmaFeatures.globalReturn
istrue
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/
Expected line before comment. Open
/**
- Read upRead up
- Exclude checks
require empty lines around comments (lines-around-comment)
Many style guides require empty lines before or after comments. The primary goal of these rules is to make the comments easier to read and improve readability of the code.
Rule Details
This rule requires empty lines before and/or after comments. It can be enabled separately for both block (/*
) and line (//
) comments. This rule does not apply to comments that appear on the same line as code and does not require empty lines at the beginning or end of a file.
Options
This rule has an object option:
-
"beforeBlockComment": true
(default) requires an empty line before block comments -
"afterBlockComment": true
requires an empty line after block comments -
"beforeLineComment": true
requires an empty line before line comments -
"afterLineComment": true
requires an empty line after line comments -
"allowBlockStart": true
allows comments to appear at the start of block statements -
"allowBlockEnd": true
allows comments to appear at the end of block statements -
"allowObjectStart": true
allows comments to appear at the start of object literals -
"allowObjectEnd": true
allows comments to appear at the end of object literals -
"allowArrayStart": true
allows comments to appear at the start of array literals -
"allowArrayEnd": true
allows comments to appear at the end of array literals -
"applyDefaultIgnorePatterns"
enables or disables the default comment patterns to be ignored by the rule -
"ignorePattern"
custom patterns to be ignored by the rule
beforeBlockComment
Examples of incorrect code for this rule with the default { "beforeBlockComment": true }
option:
/*eslint lines-around-comment: ["error", { "beforeBlockComment": true }]*/
var night = "long";
/* what a great and wonderful day */
var day = "great"
Examples of correct code for this rule with the default { "beforeBlockComment": true }
option:
/*eslint lines-around-comment: ["error", { "beforeBlockComment": true }]*/
var night = "long";
/* what a great and wonderful day */
var day = "great"
afterBlockComment
Examples of incorrect code for this rule with the { "afterBlockComment": true }
option:
/*eslint lines-around-comment: ["error", { "afterBlockComment": true }]*/
var night = "long";
/* what a great and wonderful day */
var day = "great"
Examples of correct code for this rule with the { "afterBlockComment": true }
option:
/*eslint lines-around-comment: ["error", { "afterBlockComment": true }]*/
var night = "long";
/* what a great and wonderful day */
var day = "great"
beforeLineComment
Examples of incorrect code for this rule with the { "beforeLineComment": true }
option:
/*eslint lines-around-comment: ["error", { "beforeLineComment": true }]*/
var night = "long";
// what a great and wonderful day
var day = "great"
Examples of correct code for this rule with the { "beforeLineComment": true }
option:
/*eslint lines-around-comment: ["error", { "beforeLineComment": true }]*/
var night = "long";
// what a great and wonderful day
var day = "great"
afterLineComment
Examples of incorrect code for this rule with the { "afterLineComment": true }
option:
/*eslint lines-around-comment: ["error", { "afterLineComment": true }]*/
var night = "long";
// what a great and wonderful day
var day = "great"
Examples of correct code for this rule with the { "afterLineComment": true }
option:
/*eslint lines-around-comment: ["error", { "afterLineComment": true }]*/
var night = "long";
// what a great and wonderful day
var day = "great"
allowBlockStart
Examples of correct code for this rule with the { "beforeLineComment": true, "allowBlockStart": true }
options:
/*eslint lines-around-comment: ["error", { "beforeLineComment": true, "allowBlockStart": true }]*/
function foo(){
// what a great and wonderful day
var day = "great"
return day;
}
Examples of correct code for this rule with the { "beforeBlockComment": true, "allowBlockStart": true }
options:
/*eslint lines-around-comment: ["error", { "beforeBlockComment": true, "allowBlockStart": true }]*/
function foo(){
/* what a great and wonderful day */
var day = "great"
return day;
}
allowBlockEnd
Examples of correct code for this rule with the { "afterLineComment": true, "allowBlockEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterLineComment": true, "allowBlockEnd": true }]*/
function foo(){
var day = "great"
return day;
// what a great and wonderful day
}
Examples of correct code for this rule with the { "afterBlockComment": true, "allowBlockEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterBlockComment": true, "allowBlockEnd": true }]*/
function foo(){
var day = "great"
return day;
/* what a great and wonderful day */
}
allowObjectStart
Examples of correct code for this rule with the { "beforeLineComment": true, "allowObjectStart": true }
option:
/*eslint lines-around-comment: ["error", { "beforeLineComment": true, "allowObjectStart": true }]*/
var foo = {
// what a great and wonderful day
day: "great"
};
const {
// what a great and wonderful day
foo: someDay
} = {foo: "great"};
const {
// what a great and wonderful day
day
} = {day: "great"};
Examples of correct code for this rule with the { "beforeBlockComment": true, "allowObjectStart": true }
option:
/*eslint lines-around-comment: ["error", { "beforeBlockComment": true, "allowObjectStart": true }]*/
var foo = {
/* what a great and wonderful day */
day: "great"
};
const {
/* what a great and wonderful day */
foo: someDay
} = {foo: "great"};
const {
/* what a great and wonderful day */
day
} = {day: "great"};
allowObjectEnd
Examples of correct code for this rule with the { "afterLineComment": true, "allowObjectEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterLineComment": true, "allowObjectEnd": true }]*/
var foo = {
day: "great"
// what a great and wonderful day
};
const {
foo: someDay
// what a great and wonderful day
} = {foo: "great"};
const {
day
// what a great and wonderful day
} = {day: "great"};
Examples of correct code for this rule with the { "afterBlockComment": true, "allowObjectEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterBlockComment": true, "allowObjectEnd": true }]*/
var foo = {
day: "great"
/* what a great and wonderful day */
};
const {
foo: someDay
/* what a great and wonderful day */
} = {foo: "great"};
const {
day
/* what a great and wonderful day */
} = {day: "great"};
allowArrayStart
Examples of correct code for this rule with the { "beforeLineComment": true, "allowArrayStart": true }
option:
/*eslint lines-around-comment: ["error", { "beforeLineComment": true, "allowArrayStart": true }]*/
var day = [
// what a great and wonderful day
"great",
"wonderful"
];
const [
// what a great and wonderful day
someDay
] = ["great", "not great"];
Examples of correct code for this rule with the { "beforeBlockComment": true, "allowArrayStart": true }
option:
/*eslint lines-around-comment: ["error", { "beforeBlockComment": true, "allowArrayStart": true }]*/
var day = [
/* what a great and wonderful day */
"great",
"wonderful"
];
const [
/* what a great and wonderful day */
someDay
] = ["great", "not great"];
allowArrayEnd
Examples of correct code for this rule with the { "afterLineComment": true, "allowArrayEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterLineComment": true, "allowArrayEnd": true }]*/
var day = [
"great",
"wonderful"
// what a great and wonderful day
];
const [
someDay
// what a great and wonderful day
] = ["great", "not great"];
Examples of correct code for this rule with the { "afterBlockComment": true, "allowArrayEnd": true }
option:
/*eslint lines-around-comment: ["error", { "afterBlockComment": true, "allowArrayEnd": true }]*/
var day = [
"great",
"wonderful"
/* what a great and wonderful day */
];
const [
someDay
/* what a great and wonderful day */
] = ["great", "not great"];
ignorePattern
By default this rule ignores comments starting with the following words: eslint
, jshint
, jslint
, istanbul
, global
, exported
, jscs
. An alternative regular expression can be provided.
Examples of correct code for the ignorePattern
option:
/*eslint lines-around-comment: ["error"]*/
foo();
/* eslint mentioned in this comment */,
bar();
/*eslint lines-around-comment: ["error", { "ignorePattern": "pragma" }] */
foo();
/* a valid comment using pragma in it */
Examples of incorrect code for the ignorePattern
option:
/*eslint lines-around-comment: ["error", { "ignorePattern": "pragma" }] */
1 + 1;
/* something else */
applyDefaultIgnorePatterns
Default ignore patterns are applied even when ignorePattern
is provided. If you want to omit default patterns, set this option to false
.
Examples of correct code for the { "applyDefaultIgnorePatterns": false }
option:
/*eslint lines-around-comment: ["error", { "ignorePattern": "pragma", applyDefaultIgnorePatterns: false }] */
foo();
/* a valid comment using pragma in it */
Examples of incorrect code for the { "applyDefaultIgnorePatterns": false }
option:
/*eslint lines-around-comment: ["error", { "applyDefaultIgnorePatterns": false }] */
foo();
/* eslint mentioned in comment */
When Not To Use It
Many people enjoy a terser code style and don't mind comments bumping up against code. If you fall into that category this rule is not for you.
Related Rules
- [space-before-blocks](space-before-blocks.md)
- [spaced-comment](spaced-comment.md) Source: http://eslint.org/docs/rules/