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

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

require or disallow newlines around directives (lines-around-directive)

Directives are used in JavaScript to indicate to the execution environment that a script would like to opt into a feature such as "strict mode". Directives are grouped together in a directive prologue at the top of either a file or function block and are applied to the scope in which they occur.

// Strict mode is invoked for the entire script
"use strict";

var foo;

function bar() {
  var baz;
}

var foo;

function bar() {
  // Strict mode is only invoked within this function
  "use strict";

  var baz;
}

Rule Details

This rule requires or disallows blank newlines around directive prologues. This rule does not enforce any conventions about blank newlines between the individual directives. In addition, it does not require blank newlines before directive prologues unless they are preceded by a comment. Please use the [padded-blocks](padded-blocks.md) rule if this is a style you would like to enforce.

Options

This rule has one option. It can either be a string or an object:

• "always" (default) enforces blank newlines around directives.
• "never" disallows blank newlines around directives.

or

{
  "before": "always" or "never"
  "after": "always" or "never",
}

always

This is the default option.

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment

"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment

  "use strict";

  var bar;
}

never

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */

"use strict";

var foo;


/* Top of file */
// comment

"use strict";
"use asm";

var foo;


function foo() {
  "use strict";
  "use asm";

  var bar;
}


function foo() {
  // comment

  "use strict";

  var bar;
}

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

before & after

Examples of incorrect code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */

"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

Examples of correct code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of incorrect code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of correct code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

When Not To Use It

You can safely disable this rule if you do not have any strict conventions about whether or not directive prologues should have blank newlines before or after them.

Related Rules

• [lines-around-comment](lines-around-comment.md)
• [padded-blocks](padded-blocks.md)

Compatibility

• JSCS: requirePaddingNewLinesAfterUseStrict
• JSCS: disallowPaddingNewLinesAfterUseStrict Source: http://eslint.org/docs/rules/

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

require or disallow newlines around directives (lines-around-directive)

Directives are used in JavaScript to indicate to the execution environment that a script would like to opt into a feature such as "strict mode". Directives are grouped together in a directive prologue at the top of either a file or function block and are applied to the scope in which they occur.

// Strict mode is invoked for the entire script
"use strict";

var foo;

function bar() {
  var baz;
}

var foo;

function bar() {
  // Strict mode is only invoked within this function
  "use strict";

  var baz;
}

Rule Details

This rule requires or disallows blank newlines around directive prologues. This rule does not enforce any conventions about blank newlines between the individual directives. In addition, it does not require blank newlines before directive prologues unless they are preceded by a comment. Please use the [padded-blocks](padded-blocks.md) rule if this is a style you would like to enforce.

Options

This rule has one option. It can either be a string or an object:

• "always" (default) enforces blank newlines around directives.
• "never" disallows blank newlines around directives.

or

{
  "before": "always" or "never"
  "after": "always" or "never",
}

always

This is the default option.

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment

"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment

  "use strict";

  var bar;
}

never

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */

"use strict";

var foo;


/* Top of file */
// comment

"use strict";
"use asm";

var foo;


function foo() {
  "use strict";
  "use asm";

  var bar;
}


function foo() {
  // comment

  "use strict";

  var bar;
}

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

before & after

Examples of incorrect code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */

"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

Examples of correct code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of incorrect code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of correct code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

When Not To Use It

You can safely disable this rule if you do not have any strict conventions about whether or not directive prologues should have blank newlines before or after them.

Related Rules

• [lines-around-comment](lines-around-comment.md)
• [padded-blocks](padded-blocks.md)

Compatibility

• JSCS: requirePaddingNewLinesAfterUseStrict
• JSCS: disallowPaddingNewLinesAfterUseStrict 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/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

require or disallow newlines around directives (lines-around-directive)

Directives are used in JavaScript to indicate to the execution environment that a script would like to opt into a feature such as "strict mode". Directives are grouped together in a directive prologue at the top of either a file or function block and are applied to the scope in which they occur.

// Strict mode is invoked for the entire script
"use strict";

var foo;

function bar() {
  var baz;
}

var foo;

function bar() {
  // Strict mode is only invoked within this function
  "use strict";

  var baz;
}

Rule Details

This rule requires or disallows blank newlines around directive prologues. This rule does not enforce any conventions about blank newlines between the individual directives. In addition, it does not require blank newlines before directive prologues unless they are preceded by a comment. Please use the [padded-blocks](padded-blocks.md) rule if this is a style you would like to enforce.

Options

This rule has one option. It can either be a string or an object:

• "always" (default) enforces blank newlines around directives.
• "never" disallows blank newlines around directives.

or

{
  "before": "always" or "never"
  "after": "always" or "never",
}

always

This is the default option.

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

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

/* eslint lines-around-directive: ["error", "always"] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment

"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment

  "use strict";

  var bar;
}

never

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */

"use strict";

var foo;


/* Top of file */
// comment

"use strict";
"use asm";

var foo;


function foo() {
  "use strict";
  "use asm";

  var bar;
}


function foo() {
  // comment

  "use strict";

  var bar;
}

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

/* eslint lines-around-directive: ["error", "never"] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment
"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment
  "use strict";
  var bar;
}

before & after

Examples of incorrect code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */

"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

Examples of correct code for this rule with the { "before": "never", "after": "always" } option:

/* eslint lines-around-directive: ["error", { "before": "never", "after": "always" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of incorrect code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";

var foo;

/* Top of file */
// comment
"use strict";
"use asm";

var foo;

function foo() {
  "use strict";
  "use asm";

  var bar;
}

function foo() {
  // comment
  "use strict";

  var bar;
}

Examples of correct code for this rule with the { "before": "always", "after": "never" } option:

/* eslint lines-around-directive: ["error", { "before": "always", "after": "never" }] */

/* Top of file */
"use strict";
var foo;

/* Top of file */
// comment

"use strict";
"use asm";
var foo;

function foo() {
  "use strict";
  "use asm";
  var bar;
}

function foo() {
  // comment

  "use strict";
  var bar;
}

When Not To Use It

You can safely disable this rule if you do not have any strict conventions about whether or not directive prologues should have blank newlines before or after them.

Related Rules

• [lines-around-comment](lines-around-comment.md)
• [padded-blocks](padded-blocks.md)

Compatibility

• JSCS: requirePaddingNewLinesAfterUseStrict
• JSCS: disallowPaddingNewLinesAfterUseStrict Source: http://eslint.org/docs/rules/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. 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/

disallow if statements as the only statement in else blocks (no-lonely-if)

If an if statement is the only statement in the else block, it is often clearer to use an else if form.

if (foo) {
    // ...
} else {
    if (bar) {
        // ...
    }
}

should be rewritten as

if (foo) {
    // ...
} else if (bar) {
    // ...
}

Rule Details

This rule disallows if statements as the only statement in else blocks.

Examples of incorrect code for this rule:

/*eslint no-lonely-if: "error"*/

if (condition) {
    // ...
} else {
    if (anotherCondition) {
        // ...
    }
}

if (condition) {
    // ...
} else {
    if (anotherCondition) {
        // ...
    } else {
        // ...
    }
}

Examples of correct code for this rule:

/*eslint no-lonely-if: "error"*/

if (condition) {
    // ...
} else if (anotherCondition) {
    // ...
}

if (condition) {
    // ...
} else if (anotherCondition) {
    // ...
} else {
    // ...
}

if (condition) {
    // ...
} else {
    if (anotherCondition) {
        // ...
    }
    doSomething();
}

When Not To Use It

Disable this rule if the code is clearer without requiring the else if form. Source: http://eslint.org/docs/rules/