Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

enforce a maximum line length (max-len)

Very long lines of code in any language can be difficult to read. In order to aid in readability and maintainability many coders have developed a convention to limit lines of code to X number of characters (traditionally 80 characters).

var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" }, "difficult": "to read" }; // very long

Rule Details

This rule enforces a maximum line length to increase code readability and maintainability. The length of a line is defined as the number of Unicode characters in the line.

Options

This rule has a number or object option:

• "code" (default 80) enforces a maximum line length
• "tabWidth" (default 4) specifies the character width for tab characters
• "comments" enforces a maximum line length for comments; defaults to value of code
• "ignorePattern" ignores lines matching a regular expression; can only match a single line and need to be double escaped when written in YAML or JSON
• "ignoreComments": true ignores all trailing comments and comments on their own line
• "ignoreTrailingComments": true ignores only trailing comments
• "ignoreUrls": true ignores lines that contain a URL
• "ignoreStrings": true ignores lines that contain a double-quoted or single-quoted string
• "ignoreTemplateLiterals": true ignores lines that contain a template literal
• "ignoreRegExpLiterals": true ignores lines that contain a RegExp literal

code

Examples of incorrect code for this rule with the default { "code": 80 } option:

/*eslint max-len: ["error", 80]*/

var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" }, "difficult": "to read" };

Examples of correct code for this rule with the default { "code": 80 } option:

/*eslint max-len: ["error", 80]*/

var foo = {
  "bar": "This is a bar.",
  "baz": { "qux": "This is a qux" },
  "easier": "to read"
};

tabWidth

Examples of incorrect code for this rule with the default { "tabWidth": 4 } option:

/*eslint max-len: ["error", 80, 4]*/

\t  \t  var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" } };

Examples of correct code for this rule with the default { "tabWidth": 4 } option:

/*eslint max-len: ["error", 80, 4]*/

\t  \t  var foo = {
\t  \t  \t  \t  "bar": "This is a bar.",
\t  \t  \t  \t  "baz": { "qux": "This is a qux" }
\t  \t  };

comments

Examples of incorrect code for this rule with the { "comments": 65 } option:

/*eslint max-len: ["error", { "comments": 65 }]*/

/**
 * This is a comment that violates the maximum line length we have specified
**/

ignoreComments

Examples of correct code for this rule with the { "ignoreComments": true } option:

/*eslint max-len: ["error", { "ignoreComments": true }]*/

/**
 * This is a really really really really really really really really really long comment
**/

ignoreTrailingComments

Examples of correct code for this rule with the { "ignoreTrailingComments": true } option:

/*eslint max-len: ["error", { "ignoreTrailingComments": true }]*/

var foo = 'bar'; // This is a really really really really really really really long comment

ignoreUrls

Examples of correct code for this rule with the { "ignoreUrls": true } option:

/*eslint max-len: ["error", { "ignoreUrls": true }]*/

var url = 'https://www.example.com/really/really/really/really/really/really/really/long';

ignoreStrings

Examples of correct code for this rule with the { "ignoreStrings": true } option:

/*eslint max-len: ["error", { "ignoreStrings": true }]*/

var longString = 'this is a really really really really really long string!';

ignoreTemplateLiterals

Examples of correct code for this rule with the { "ignoreTemplateLiterals": true } option:

/*eslint max-len: ["error", { "ignoreTemplateLiterals": true }]*/

var longTemplateLiteral = `this is a really really really really really long template literal!`;

ignoreRegExpLiterals

Examples of correct code for this rule with the { "ignoreRegExpLiterals": true } option:

/*eslint max-len: ["error", { "ignoreRegExpLiterals": true }]*/

var longRegExpLiteral = /this is a really really really really really long regular expression!/;

ignorePattern

Examples of correct code for this rule with the { "ignorePattern": true } option:

/*eslint max-len: ["error", { "ignorePattern": "^\\s*var\\s.+=\\s*require\\s*\\(/" }]*/

var dep = require('really/really/really/really/really/really/really/really/long/module');

Related Rules

• [complexity](complexity.md)
• [max-depth](max-depth.md)
• [max-nested-callbacks](max-nested-callbacks.md)
• [max-params](max-params.md)
• [max-statements](max-statements.md) Source: http://eslint.org/docs/rules/

enforce a maximum line length (max-len)

Very long lines of code in any language can be difficult to read. In order to aid in readability and maintainability many coders have developed a convention to limit lines of code to X number of characters (traditionally 80 characters).

var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" }, "difficult": "to read" }; // very long

Rule Details

This rule enforces a maximum line length to increase code readability and maintainability. The length of a line is defined as the number of Unicode characters in the line.

Options

This rule has a number or object option:

• "code" (default 80) enforces a maximum line length
• "tabWidth" (default 4) specifies the character width for tab characters
• "comments" enforces a maximum line length for comments; defaults to value of code
• "ignorePattern" ignores lines matching a regular expression; can only match a single line and need to be double escaped when written in YAML or JSON
• "ignoreComments": true ignores all trailing comments and comments on their own line
• "ignoreTrailingComments": true ignores only trailing comments
• "ignoreUrls": true ignores lines that contain a URL
• "ignoreStrings": true ignores lines that contain a double-quoted or single-quoted string
• "ignoreTemplateLiterals": true ignores lines that contain a template literal
• "ignoreRegExpLiterals": true ignores lines that contain a RegExp literal

code

Examples of incorrect code for this rule with the default { "code": 80 } option:

/*eslint max-len: ["error", 80]*/

var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" }, "difficult": "to read" };

Examples of correct code for this rule with the default { "code": 80 } option:

/*eslint max-len: ["error", 80]*/

var foo = {
  "bar": "This is a bar.",
  "baz": { "qux": "This is a qux" },
  "easier": "to read"
};

tabWidth

Examples of incorrect code for this rule with the default { "tabWidth": 4 } option:

/*eslint max-len: ["error", 80, 4]*/

\t  \t  var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" } };

Examples of correct code for this rule with the default { "tabWidth": 4 } option:

/*eslint max-len: ["error", 80, 4]*/

\t  \t  var foo = {
\t  \t  \t  \t  "bar": "This is a bar.",
\t  \t  \t  \t  "baz": { "qux": "This is a qux" }
\t  \t  };

comments

Examples of incorrect code for this rule with the { "comments": 65 } option:

/*eslint max-len: ["error", { "comments": 65 }]*/

/**
 * This is a comment that violates the maximum line length we have specified
**/

ignoreComments

Examples of correct code for this rule with the { "ignoreComments": true } option:

/*eslint max-len: ["error", { "ignoreComments": true }]*/

/**
 * This is a really really really really really really really really really long comment
**/

ignoreTrailingComments

Examples of correct code for this rule with the { "ignoreTrailingComments": true } option:

/*eslint max-len: ["error", { "ignoreTrailingComments": true }]*/

var foo = 'bar'; // This is a really really really really really really really long comment

ignoreUrls

Examples of correct code for this rule with the { "ignoreUrls": true } option:

/*eslint max-len: ["error", { "ignoreUrls": true }]*/

var url = 'https://www.example.com/really/really/really/really/really/really/really/long';

ignoreStrings

Examples of correct code for this rule with the { "ignoreStrings": true } option:

/*eslint max-len: ["error", { "ignoreStrings": true }]*/

var longString = 'this is a really really really really really long string!';

ignoreTemplateLiterals

Examples of correct code for this rule with the { "ignoreTemplateLiterals": true } option:

/*eslint max-len: ["error", { "ignoreTemplateLiterals": true }]*/

var longTemplateLiteral = `this is a really really really really really long template literal!`;

ignoreRegExpLiterals

Examples of correct code for this rule with the { "ignoreRegExpLiterals": true } option:

/*eslint max-len: ["error", { "ignoreRegExpLiterals": true }]*/

var longRegExpLiteral = /this is a really really really really really long regular expression!/;

ignorePattern

Examples of correct code for this rule with the { "ignorePattern": true } option:

/*eslint max-len: ["error", { "ignorePattern": "^\\s*var\\s.+=\\s*require\\s*\\(/" }]*/

var dep = require('really/really/really/really/really/really/really/really/long/module');

Related Rules

• [complexity](complexity.md)
• [max-depth](max-depth.md)
• [max-nested-callbacks](max-nested-callbacks.md)
• [max-params](max-params.md)
• [max-statements](max-statements.md) Source: http://eslint.org/docs/rules/

Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

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

JavaScript allows you to define strings in one of three ways: double quotes, single quotes, and backticks (as of ECMAScript 6). For example:

/*eslint-env es6*/

var double = "double";
var single = 'single';
var backtick = `backtick`;    // ES6 only

Each of these lines creates a string and, in some cases, can be used interchangeably. The choice of how to define strings in a codebase is a stylistic one outside of template literals (which allow embedded of expressions to be interpreted).

Many codebases require strings to be defined in a consistent manner.

Rule Details

This rule enforces the consistent use of either backticks, double, or single quotes.

Options

This rule has two options, a string option and an object option.

String option:

• "double" (default) requires the use of double quotes wherever possible
• "single" requires the use of single quotes wherever possible
• "backtick" requires the use of backticks wherever possible

Object option:

• "avoidEscape": true allows strings to use single-quotes or double-quotes so long as the string contains a quote that would have to be escaped otherwise
• "allowTemplateLiterals": true allows strings to use backticks

Deprecated: The object property avoid-escape is deprecated; please use the object property avoidEscape instead.

double

Examples of incorrect code for this rule with the default "double" option:

/*eslint quotes: ["error", "double"]*/

var single = 'single';
var unescaped = 'a string containing "double" quotes';

Examples of correct code for this rule with the default "double" option:

/*eslint quotes: ["error", "double"]*/
/*eslint-env es6*/

var double = "double";
var backtick = `back\ntick`;  // backticks are allowed due to newline
var backtick = tag`backtick`; // backticks are allowed due to tag

single

Examples of incorrect code for this rule with the "single" option:

/*eslint quotes: ["error", "single"]*/

var double = "double";
var unescaped = "a string containing 'single' quotes";

Examples of correct code for this rule with the "single" option:

/*eslint quotes: ["error", "single"]*/
/*eslint-env es6*/

var single = 'single';
var backtick = `back${x}tick`; // backticks are allowed due to substitution

backticks

Examples of incorrect code for this rule with the "backtick" option:

/*eslint quotes: ["error", "backtick"]*/

var single = 'single';
var double = "double";
var unescaped = 'a string containing `backticks`';

Examples of correct code for this rule with the "backtick" option:

/*eslint quotes: ["error", "backtick"]*/
/*eslint-env es6*/

var backtick = `backtick`;

avoidEscape

Examples of additional correct code for this rule with the "double", { "avoidEscape": true } options:

/*eslint quotes: ["error", "double", { "avoidEscape": true }]*/

var single = 'a string containing "double" quotes';

Examples of additional correct code for this rule with the "single", { "avoidEscape": true } options:

/*eslint quotes: ["error", "single", { "avoidEscape": true }]*/

var double = "a string containing 'single' quotes";

Examples of additional correct code for this rule with the "backtick", { "avoidEscape": true } options:

/*eslint quotes: ["error", "backtick", { "avoidEscape": true }]*/

var double = "a string containing `backtick` quotes"

allowTemplateLiterals

Examples of additional correct code for this rule with the "double", { "allowTemplateLiterals": true } options:

/*eslint quotes: ["error", "double", { "allowTemplateLiterals": true }]*/

var double = "double";
var double = `double`;

Examples of additional correct code for this rule with the "single", { "allowTemplateLiterals": true } options:

/*eslint quotes: ["error", "single", { "allowTemplateLiterals": true }]*/

var single = 'single';
var single = `single`;

When Not To Use It

If you do not need consistency in your string styles, you can safely disable this rule. Source: http://eslint.org/docs/rules/

enforce consistent spacing inside braces (object-curly-spacing)

While formatting preferences are very personal, a number of style guides require or disallow spaces between curly braces in the following situations:

// simple object literals
var obj = { foo: "bar" };

// nested object literals
var obj = { foo: { zoo: "bar" } };

// destructuring assignment (EcmaScript 6)
var { x, y } = y;

// import/export declarations (EcmaScript 6)
import { foo } from "bar";
export { foo };

Rule Details

This rule enforce consistent spacing inside braces of object literals, destructuring assignments, and import/export specifiers.

Options

This rule has two options, a string option and an object option.

String option:

• "never" (default) disallows spacing inside of braces
• "always" requires spacing inside of braces (except {})

Object option:

• "arraysInObjects": true requires spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to never)
• "arraysInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to always)
• "objectsInObjects": true requires spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to never)
• "objectsInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to always)

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = { 'foo': 'bar' };
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux'}, bar};
var {x } = y;
import { foo } from 'bar';

Examples of correct code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': {'bar': 'baz'}, 'qux': 'quxx'};
var obj = {
  'foo': 'bar'
};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var obj = {};
var {x} = y;
import {foo} from 'bar';

always

Examples of incorrect code for this rule with the "always" option:

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux' }, bar};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var {x} = y;
import {foo } from 'bar';

Examples of correct code for this rule with the "always" option:

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {};
var obj = { 'foo': 'bar' };
var obj = { 'foo': { 'bar': 'baz' }, 'qux': 'quxx' };
var obj = {
  'foo': 'bar'
};
var { x } = y;
import { foo } from 'bar';

arraysInObjects

Examples of additional correct code for this rule with the "never", { "arraysInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "arraysInObjects": true }]*/

var obj = {"foo": [ 1, 2 ] };
var obj = {"foo": [ "baz", "bar" ] };

Examples of additional correct code for this rule with the "always", { "arraysInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "arraysInObjects": false }]*/

var obj = { "foo": [ 1, 2 ]};
var obj = { "foo": [ "baz", "bar" ]};

objectsInObjects

Examples of additional correct code for this rule with the "never", { "objectsInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "objectsInObjects": true }]*/

var obj = {"foo": {"baz": 1, "bar": 2} };

Examples of additional correct code for this rule with the "always", { "objectsInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "objectsInObjects": false }]*/

var obj = { "foo": { "baz": 1, "bar": 2 }};

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between curly braces.

Related Rules

• [comma-spacing](comma-spacing.md)
• [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/

Enforce require() on the top-level module scope (global-require)

In Node.js, module dependencies are included using the require() function, such as:

var fs = require("fs");

While require() may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:

function foo() {

    if (condition) {
        var fs = require("fs");
    }
}

Since require() does a synchronous load, it can cause performance problems when used in other locations.

Further, ES6 modules mandate that import and export statements can only occur in the top level of the module's body.

Rule Details

This rule requires all calls to require() to be at the top level of the module, similar to ES6 import and export statements, which also can occur only at the top level.

Examples of incorrect code for this rule:

/*eslint global-require: "error"*/
/*eslint-env es6*/

// calling require() inside of a function is not allowed
function readFile(filename, callback) {
    var fs = require('fs');
    fs.readFile(filename, callback)
}

// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }

// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }

// you may not require() inside an arrow function body
var getModule = (name) => require(name);

// you may not require() inside of a function body as well
function getModule(name) { return require(name); }

// you may not require() inside of a try/catch block
try {
    require(unsafeModule);
} catch(e) {
    console.log(e);
}

Examples of correct code for this rule:

/*eslint global-require: "error"*/

// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();

// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
    fs.readFile(filename, callback)
}

// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');

// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
    z = require("z");

When Not To Use It

If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require() an optional dependency inside of a try/catch, you can disable this rule for just that dependency using the // eslint-disable-line global-require comment. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

Examples of correct code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

Examples of incorrect code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

Examples of correct code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

Enforce require() on the top-level module scope (global-require)

In Node.js, module dependencies are included using the require() function, such as:

var fs = require("fs");

While require() may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:

function foo() {

    if (condition) {
        var fs = require("fs");
    }
}

Since require() does a synchronous load, it can cause performance problems when used in other locations.

Further, ES6 modules mandate that import and export statements can only occur in the top level of the module's body.

Rule Details

This rule requires all calls to require() to be at the top level of the module, similar to ES6 import and export statements, which also can occur only at the top level.

Examples of incorrect code for this rule:

/*eslint global-require: "error"*/
/*eslint-env es6*/

// calling require() inside of a function is not allowed
function readFile(filename, callback) {
    var fs = require('fs');
    fs.readFile(filename, callback)
}

// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }

// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }

// you may not require() inside an arrow function body
var getModule = (name) => require(name);

// you may not require() inside of a function body as well
function getModule(name) { return require(name); }

// you may not require() inside of a try/catch block
try {
    require(unsafeModule);
} catch(e) {
    console.log(e);
}

Examples of correct code for this rule:

/*eslint global-require: "error"*/

// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();

// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
    fs.readFile(filename, callback)
}

// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');

// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
    z = require("z");

When Not To Use It

If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require() an optional dependency inside of a try/catch, you can disable this rule for just that dependency using the // eslint-disable-line global-require comment. Source: http://eslint.org/docs/rules/

require or disallow trailing commas (comma-dangle)

Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec. However, IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.

var foo = {
    bar: "baz",
    qux: "quux",
};

Trailing commas simplify adding and removing items to objects and arrays, since only the lines you are modifying must be touched. Another argument in favor of trailing commas is that it improves the clarity of diffs when an item is added or removed from an object or array:

Less clear:

var foo = {
-    bar: "baz",
-    qux: "quux"
+    bar: "baz"
 };

More clear:

var foo = {
     bar: "baz",
-    qux: "quux",
 };

Rule Details

This rule enforces consistent use of trailing commas in object and array literals.

Options

This rule has a string option or an object option:

{
    "comma-dangle": ["error", "never"],
    // or
    "comma-dangle": ["error", {
        "arrays": "never",
        "objects": "never",
        "imports": "never",
        "exports": "never",
        "functions": "ignore",
    }]
}

• "never" (default) disallows trailing commas
• "always" requires trailing commas
• "always-multiline" requires trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }
• "only-multiline" allows (but does not require) trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }

Trailing commas in function declarations and function calls are valid syntax since ECMAScript 2017; however, the string option does not check these situations for backwards compatibility.

You can also use an object option to configure this rule for each type of syntax. Each of the following options can be set to "never", "always", "always-multiline", "only-multiline", or "ignore". The default for each option is "never" unless otherwise specified.

• arrays is for array literals and array patterns of destructuring. (e.g. let [a,] = [1,];)
• objects is for object literals and object patterns of destructuring. (e.g. let {a,} = {a: 1};)
• imports is for import declarations of ES Modules. (e.g. import {a,} from "foo";)
• exports is for export declarations of ES Modules. (e.g. export {a,};)
• functions is for function declarations and function calls. (e.g. (function(a,){ })(b,);)
  functions is set to "ignore" by default for consistency with the string option.

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

Examples of correct code for this rule with the default "never" option:

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

always

Examples of incorrect code for this rule with the "always" option:

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

Examples of correct code for this rule with the "always" option:

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

always-multiline

Examples of incorrect code for this rule with the "always-multiline" option:

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux"
});

Examples of correct code for this rule with the "always-multiline" option:

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

foo({
  bar: "baz",
  qux: "quux",
});

only-multiline

Examples of incorrect code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

Examples of correct code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux",
});

foo({
  bar: "baz",
  qux: "quux"
});

functions

Examples of incorrect code for this rule with the {"functions": "never"} option:

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

Examples of correct code for this rule with the {"functions": "never"} option:

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

Examples of incorrect code for this rule with the {"functions": "always"} option:

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

Examples of correct code for this rule with the {"functions": "always"} option:

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

When Not To Use It

You can turn this rule off if you are not concerned with dangling commas. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

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/