Showing 270 of 270 total issues
Missing space before function parentheses. Open
tape('integration', async function(t) { // eslint-disable-line no-loop-func
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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/
Unexpected var, use let or const instead. Open
var b;
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Missing semicolon. Open
return `<li class="${className}"><a href="${item.href}">${item.name}</a></li>`
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require or disallow semicolons instead of ASI (semi)
JavaScript is unique amongst the C-like languages in that it doesn't require semicolons at the end of each statement. In many cases, the JavaScript engine can determine that a semicolon should be in a certain spot and will automatically add it. This feature is known as automatic semicolon insertion (ASI) and is considered one of the more controversial features of JavaScript. For example, the following lines are both valid:
var name = "ESLint"
var website = "eslint.org";
On the first line, the JavaScript engine will automatically insert a semicolon, so this is not considered a syntax error. The JavaScript engine still knows how to interpret the line and knows that the line end indicates the end of the statement.
In the debate over ASI, there are generally two schools of thought. The first is that we should treat ASI as if it didn't exist and always include semicolons manually. The rationale is that it's easier to always include semicolons than to try to remember when they are or are not required, and thus decreases the possibility of introducing an error.
However, the ASI mechanism can sometimes be tricky to people who are using semicolons. For example, consider this code:
return
{
name: "ESLint"
};
This may look like a return
statement that returns an object literal, however, the JavaScript engine will interpret this code as:
return;
{
name: "ESLint";
}
Effectively, a semicolon is inserted after the return
statement, causing the code below it (a labeled literal inside a block) to be unreachable. This rule and the [no-unreachable](no-unreachable.md) rule will protect your code from such cases.
On the other side of the argument are those who says that since semicolons are inserted automatically, they are optional and do not need to be inserted manually. However, the ASI mechanism can also be tricky to people who don't use semicolons. For example, consider this code:
var globalCounter = { }
(function () {
var n = 0
globalCounter.increment = function () {
return ++n
}
})()
In this example, a semicolon will not be inserted after the first line, causing a run-time error (because an empty object is called as if it's a function). The [no-unexpected-multiline](no-unexpected-multiline.md) rule can protect your code from such cases.
Although ASI allows for more freedom over your coding style, it can also make your code behave in an unexpected way, whether you use semicolons or not. Therefore, it is best to know when ASI takes place and when it does not, and have ESLint protect your code from these potentially unexpected cases. In short, as once described by Isaac Schlueter, a \n
character always ends a statement (just like a semicolon) unless one of the following is true:
- The statement has an unclosed paren, array literal, or object literal or ends in some other way that is not a valid way to end a statement. (For instance, ending with
.
or,
.) - The line is
--
or++
(in which case it will decrement/increment the next token.) - It is a
for()
,while()
,do
,if()
, orelse
, and there is no{
- The next line starts with
[
,(
,+
,*
,/
,-
,,
,.
, or some other binary operator that can only be found between two tokens in a single expression.
Rule Details
This rule enforces consistent use of semicolons.
Options
This rule has two options, a string option and an object option.
String option:
-
"always"
(default) requires semicolons at the end of statements -
"never"
disallows semicolons as the end of statements (except to disambiguate statements beginning with[
,(
,/
,+
, or-
)
Object option:
-
"omitLastInOneLineBlock": true
ignores the last semicolon in a block in which its braces (and therefore the content of the block) are in the same line
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint semi: ["error", "always"]*/
var name = "ESLint"
object.method = function() {
// ...
}
Examples of correct code for this rule with the default "always"
option:
/*eslint semi: "error"*/
var name = "ESLint";
object.method = function() {
// ...
};
never
Examples of incorrect code for this rule with the "never"
option:
/*eslint semi: ["error", "never"]*/
var name = "ESLint";
object.method = function() {
// ...
};
Examples of correct code for this rule with the "never"
option:
/*eslint semi: ["error", "never"]*/
var name = "ESLint"
object.method = function() {
// ...
}
var name = "ESLint"
;(function() {
// ...
})()
omitLastInOneLineBlock
Examples of additional correct code for this rule with the "always", { "omitLastInOneLineBlock": true }
options:
/*eslint semi: ["error", "always", { "omitLastInOneLineBlock": true}] */
if (foo) { bar() }
if (foo) { bar(); baz() }
When Not To Use It
If you do not want to enforce semicolon usage (or omission) in any particular way, then you can turn this rule off.
Further Reading
Related Rules
- [no-extra-semi](no-extra-semi.md)
- [no-unexpected-multiline](no-unexpected-multiline.md)
- [semi-spacing](semi-spacing.md) Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var pack = require('./pack')(gulp, paths, watchOptions, cli);
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Comments should not begin with a lowercase character Open
// unwrap cssRules
- Read upRead up
- Exclude checks
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"]
.
- Note that the following words are always ignored by this rule:
-
ignoreInlineComments
: If this istrue
, the rule will not report on comments in the middle of code. By default, this isfalse
. -
ignoreConsecutiveComments
: If this istrue
, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this isfalse
.
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
Unexpected var, use let or const instead. Open
var flatten = require('lodash.flatten');
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Arrow function should not return assignment. Open
elements.map(element => element.style[prefix('animationName', window, document)] = '');
- Read upRead up
- Exclude checks
Disallow Assignment in return Statement (no-return-assign)
One of the interesting, and sometimes confusing, aspects of JavaScript is that assignment can happen at almost any point. Because of this, an errant equals sign can end up causing assignment when the true intent was to do a comparison. This is especially true when using a return
statement. For example:
function doSomething() {
return foo = bar + 2;
}
It is difficult to tell the intent of the return
statement here. It's possible that the function is meant to return the result of bar + 2
, but then why is it assigning to foo
? It's also possible that the intent was to use a comparison operator such as ==
and that this code is an error.
Because of this ambiguity, it's considered a best practice to not use assignment in return
statements.
Rule Details
This rule aims to eliminate assignments from return
statements. As such, it will warn whenever an assignment is found as part of return
.
Options
The rule takes one option, a string, which must contain one of the following values:
-
except-parens
(default): Disallow assignments unless they are enclosed in parentheses. -
always
: Disallow all assignments.
except-parens
This is the default option. It disallows assignments unless they are enclosed in parentheses.
Examples of incorrect code for the default "except-parens"
option:
/*eslint no-return-assign: "error"*/
function doSomething() {
return foo = bar + 2;
}
function doSomething() {
return foo += 2;
}
Examples of correct code for the default "except-parens"
option:
/*eslint no-return-assign: "error"*/
function doSomething() {
return foo == bar + 2;
}
function doSomething() {
return foo === bar + 2;
}
function doSomething() {
return (foo = bar + 2);
}
always
This option disallows all assignments in return
statements.
All assignments are treated as problems.
Examples of incorrect code for the "always"
option:
/*eslint no-return-assign: ["error", "always"]*/
function doSomething() {
return foo = bar + 2;
}
function doSomething() {
return foo += 2;
}
function doSomething() {
return (foo = bar + 2);
}
Examples of correct code for the "always"
option:
/*eslint no-return-assign: ["error", "always"]*/
function doSomething() {
return foo == bar + 2;
}
function doSomething() {
return foo === bar + 2;
}
When Not To Use It
If you want to allow the use of assignment operators in a return
statement, then you can safely disable this rule.
Source: http://eslint.org/docs/rules/
Unexpected function expression. Open
)(function (err) {
- Read upRead up
- Exclude checks
Suggest using arrow functions as callbacks. (prefer-arrow-callback)
Arrow functions are suited to callbacks, because:
-
this
keywords in arrow functions bind to the upper scope's. - The notation of the arrow function is shorter than function expression's.
Rule Details
This rule is aimed to flag usage of function expressions in an argument list.
The following patterns are considered problems:
/*eslint prefer-arrow-callback: "error"*/
foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));
The following patterns are not considered problems:
/*eslint prefer-arrow-callback: "error"*/
/*eslint-env es6*/
foo(a => a);
foo(function*() { yield; });
// this is not a callback.
var foo = function foo(a) { return a; };
// using `this` without `.bind(this)`.
foo(function() { return this.a; });
// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });
Options
This rule takes one optional argument, an object which is an options object.
allowNamedFunctions
This is a boolean
option and it is false
by default. When set to true
, the rule doesn't warn on named functions used as callbacks.
Examples of correct code for the { "allowNamedFunctions": true }
option:
/*eslint prefer-arrow-callback: ["error", { "allowNamedFunctions": true }]*/
foo(function bar() {});
allowUnboundThis
This is a boolean
option and it is true
by default. When set to false
, this option allows the use of this
without restriction and checks for dynamically assigned this
values such as when using Array.prototype.map
with a context
argument. Normally, the rule will flag the use of this
whenever a function does not use bind()
to specify the value of this
constantly.
Examples of incorrect code for the { "allowUnboundThis": false }
option:
/*eslint prefer-arrow-callback: ["error", { "allowUnboundThis": false }]*/
/*eslint-env es6*/
foo(function() { this.a; });
foo(function() { (() => this); });
someArray.map(function (itm) { return this.doSomething(itm); }, someObject);
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Missing space before function parentheses. Open
${props.navigation.map(function(item) {
- Read upRead up
- Exclude checks
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/
Unexpected function expression. Open
.map(function (key) {
- Read upRead up
- Exclude checks
Suggest using arrow functions as callbacks. (prefer-arrow-callback)
Arrow functions are suited to callbacks, because:
-
this
keywords in arrow functions bind to the upper scope's. - The notation of the arrow function is shorter than function expression's.
Rule Details
This rule is aimed to flag usage of function expressions in an argument list.
The following patterns are considered problems:
/*eslint prefer-arrow-callback: "error"*/
foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));
The following patterns are not considered problems:
/*eslint prefer-arrow-callback: "error"*/
/*eslint-env es6*/
foo(a => a);
foo(function*() { yield; });
// this is not a callback.
var foo = function foo(a) { return a; };
// using `this` without `.bind(this)`.
foo(function() { return this.a; });
// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });
Options
This rule takes one optional argument, an object which is an options object.
allowNamedFunctions
This is a boolean
option and it is false
by default. When set to true
, the rule doesn't warn on named functions used as callbacks.
Examples of correct code for the { "allowNamedFunctions": true }
option:
/*eslint prefer-arrow-callback: ["error", { "allowNamedFunctions": true }]*/
foo(function bar() {});
allowUnboundThis
This is a boolean
option and it is true
by default. When set to false
, this option allows the use of this
without restriction and checks for dynamically assigned this
values such as when using Array.prototype.map
with a context
argument. Normally, the rule will flag the use of this
whenever a function does not use bind()
to specify the value of this
constantly.
Examples of incorrect code for the { "allowUnboundThis": false }
option:
/*eslint prefer-arrow-callback: ["error", { "allowUnboundThis": false }]*/
/*eslint-env es6*/
foo(function() { this.a; });
foo(function() { (() => this); });
someArray.map(function (itm) { return this.doSomething(itm); }, someObject);
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var copyStatic = require('./static')(gulp, paths, watchOptions, cli);
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Comments should not begin with a lowercase character Open
// jogwheel does not support asynchronously running animations
- Read upRead up
- Exclude checks
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"]
.
- Note that the following words are always ignored by this rule:
-
ignoreInlineComments
: If this istrue
, the rule will not report on comments in the middle of code. By default, this isfalse
. -
ignoreConsecutiveComments
: If this istrue
, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this isfalse
.
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
Comments should not begin with a lowercase character Open
/**
- Read upRead up
- Exclude checks
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"]
.
- Note that the following words are always ignored by this rule:
-
ignoreInlineComments
: If this istrue
, the rule will not report on comments in the middle of code. By default, this isfalse
. -
ignoreConsecutiveComments
: If this istrue
, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this isfalse
.
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
Empty block statement. Open
if (process.env.NODE_ENV !== 'production') {
- Read upRead up
- Exclude checks
disallow empty block statements (no-empty)
Empty block statements, while not technically errors, usually occur due to refactoring that wasn't completed. They can cause confusion when reading code.
Rule Details
This rule disallows empty block statements. This rule ignores block statements which contain a comment (for example, in an empty catch
or finally
block of a try
statement to indicate that execution should continue regardless of errors).
Examples of incorrect code for this rule:
/*eslint no-empty: "error"*/
if (foo) {
}
while (foo) {
}
switch(foo) {
}
try {
doSomething();
} catch(ex) {
} finally {
}
Examples of correct code for this rule:
/*eslint no-empty: "error"*/
if (foo) {
// empty
}
while (foo) {
/* empty */
}
try {
doSomething();
} catch (ex) {
// continue regardless of error
}
try {
doSomething();
} finally {
/* continue regardless of error */
}
Options
This rule has an object option for exceptions:
-
"allowEmptyCatch": true
allows emptycatch
clauses (that is, which do not contain a comment)
allowEmptyCatch
Examples of additional correct code for this rule with the { "allowEmptyCatch": true }
option:
/* eslint no-empty: ["error", { "allowEmptyCatch": true }] */
try {
doSomething();
} catch (ex) {}
try {
doSomething();
}
catch (ex) {}
finally {
/* continue regardless of error */
}
When Not To Use It
If you intentionally use empty block statements then you can disable this rule.
Related Rules
- [no-empty-function](./no-empty-function.md) Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var util = require('gulp-util');
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var test = require('./test')(gulp, paths, watchOptions, cli);
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Comments should not begin with a lowercase character Open
// filter with user-provided predecate
- Read upRead up
- Exclude checks
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"]
.
- Note that the following words are always ignored by this rule:
-
ignoreInlineComments
: If this istrue
, the rule will not report on comments in the middle of code. By default, this isfalse
. -
ignoreConsecutiveComments
: If this istrue
, the rule will not report on a comment which violates the rule, as long as the comment immediately follows another comment. By default, this isfalse
.
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
Multiple spaces found before '('. Open
if (!isLeader(job)) {
- Read upRead up
- Exclude checks
Disallow multiple spaces (no-multi-spaces)
Multiple spaces in a row that are not used for indentation are typically mistakes. For example:
if(foo === "bar") {}
It's hard to tell, but there are two spaces between foo
and ===
. Multiple spaces such as this are generally frowned upon in favor of single spaces:
if(foo === "bar") {}
Rule Details
This rule aims to disallow multiple whitespace around logical expressions, conditional expressions, declarations, array elements, object properties, sequences and function parameters.
Examples of incorrect code for this rule:
/*eslint no-multi-spaces: "error"*/
var a = 1;
if(foo === "bar") {}
a << b
var arr = [1, 2];
a ? b: c
Examples of correct code for this rule:
/*eslint no-multi-spaces: "error"*/
var a = 1;
if(foo === "bar") {}
a << b
var arr = [1, 2];
a ? b: c
Options
To avoid contradictions if some other rules require multiple spaces, this rule has an option to ignore certain node types in the abstract syntax tree (AST) of JavaScript code.
exceptions
The exceptions
object expects property names to be AST node types as defined by ESTree. The easiest way to determine the node types for exceptions
is to use the online demo.
Only the Property
node type is ignored by default, because for the [key-spacing](key-spacing.md) rule some alignment options require multiple spaces in properties of object literals.
Examples of correct code for the default "exceptions": { "Property": true }
option:
/*eslint no-multi-spaces: "error"*/
/*eslint key-spacing: ["error", { align: "value" }]*/
var obj = {
first: "first",
second: "second"
};
Examples of incorrect code for the "exceptions": { "Property": false }
option:
/*eslint no-multi-spaces: ["error", { exceptions: { "Property": false } }]*/
/*eslint key-spacing: ["error", { align: "value" }]*/
var obj = {
first: "first",
second: "second"
};
Examples of correct code for the "exceptions": { "BinaryExpression": true }
option:
/*eslint no-multi-spaces: ["error", { exceptions: { "BinaryExpression": true } }]*/
var a = 1 * 2;
Examples of correct code for the "exceptions": { "VariableDeclarator": true }
option:
/*eslint no-multi-spaces: ["error", { exceptions: { "VariableDeclarator": true } }]*/
var someVar = 'foo';
var someOtherVar = 'barBaz';
Examples of correct code for the "exceptions": { "ImportDeclaration": true }
option:
/*eslint no-multi-spaces: ["error", { exceptions: { "ImportDeclaration": true } }]*/
import mod from 'mod';
import someOtherMod from 'some-other-mod';
When Not To Use It
If you don't want to check and disallow multiple spaces, then you should turn this rule off.
Related Rules
- [key-spacing](key-spacing.md)
- [space-infix-ops](space-infix-ops.md)
- [space-in-brackets](space-in-brackets.md) (deprecated)
- [space-in-parens](space-in-parens.md)
- [space-after-keywords](space-after-keywords)
- [space-unary-ops](space-unary-ops)
- [space-return-throw-case](space-return-throw-case) Source: http://eslint.org/docs/rules/
Unexpected named function 'postcss'. Open
return function postcss() {
- Read upRead up
- Exclude checks
Require or disallow named function
expressions (func-names)
A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:
Foo.prototype.bar = function bar() {};
Adding the second bar
in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function
in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.
Rule Details
This rule can enforce or disallow the use of named function expressions.
Options
This rule has a string option:
-
"always"
(default) requires function expressions to have a name -
"as-needed"
requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment -
"never"
disallows named function expressions, except in recursive functions, where a name is needed
always
Examples of incorrect code for this rule with the default "always"
option:
/*eslint func-names: ["error", "always"]*/
Foo.prototype.bar = function() {};
(function() {
// ...
}())
Examples of correct code for this rule with the default "always"
option:
/*eslint func-names: ["error", "always"]*/
Foo.prototype.bar = function bar() {};
(function bar() {
// ...
}())
as-needed
ECMAScript 6 introduced a name
property on all functions. The value of name
is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name
property equal to the name of the variable. The value of name
is then used in stack traces for easier debugging.
Examples of incorrect code for this rule with the default "as-needed"
option:
/*eslint func-names: ["error", "as-needed"]*/
Foo.prototype.bar = function() {};
(function() {
// ...
}())
Examples of correct code for this rule with the default "as-needed"
option:
/*eslint func-names: ["error", "as-needed"]*/
var bar = function() {};
(function bar() {
// ...
}())
never
Examples of incorrect code for this rule with the "never"
option:
/*eslint func-names: ["error", "never"]*/
Foo.prototype.bar = function bar() {};
(function bar() {
// ...
}())
Examples of correct code for this rule with the "never"
option:
/*eslint func-names: ["error", "never"]*/
Foo.prototype.bar = function() {};
(function() {
// ...
}())
Further Reading
Compatibility
- JSCS: requireAnonymousFunctions
- JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var remember = require('gulp-remember');
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/