@ariporad's JavaScript Style Guide

My Personal React/JSX Style Guide. ~~Stolen from~~ Inspired By [AirBnB's][airbnb]

Other Style Guides

• React
• CSS-in-JavaScript

Usage

$ npm i -D eslint-config-ariporad eslint-plugin-import

# React Only:
$ npm i -D eslint-plugin-react eslint-plugin-jsx-a11y

# Then, create a eslintrc:

# ES5
$ echo "{ \"extends\": \"ariporad/es5\" }" > .eslintrc.json

# ES6
$ echo "{ \"extends\": \"ariporad\" }" > .eslintrc.json

# React
$ echo "{ \"extends\": \"ariporad/react\" }" > .eslintrc.json

Table of Contents

1. [Types](#types)
1. [References](#references)
1. [Objects](#objects)
1. [Arrays](#arrays)
1. [Destructuring](#destructuring)
1. [Strings](#strings)
1. [Functions](#functions)
1. [Arrow Functions](#arrow-functions)
1. [Classes & Constructors](#classes--constructors)
1. [Modules](#modules)
1. [Iterators and Generators](#iterators-and-generators)
1. [Properties](#properties)
1. [Variables](#variables)
1. [Hoisting](#hoisting)
1. [Comparison Operators & Equality](#comparison-operators--equality)
1. [Blocks](#blocks)
1. [Comments](#comments)
1. [Whitespace](#whitespace)
1. [Commas](#commas)
1. [Semicolons](#semicolons)
1. [Type Casting & Coercion](#type-casting--coercion)
1. [Naming Conventions](#naming-conventions)
1. [Accessors](#accessors)
1. [Events](#events)
1. [jQuery](#jquery)
1. [ECMAScript 5 Compatibility](#ecmascript-5-compatibility)
1. [ECMAScript 6+ (ES 2015+) Styles](#ecmascript-6-es-2015-styles)
1. [Testing](#testing)
1. [Performance](#performance)
1. [Resources](#resources)
1. [In the Wild](#in-the-wild)
1. [Translation](#translation)
1. [The JavaScript Style Guide Guide](#the-javascript-style-guide-guide)
1. [Chat With Us About JavaScript](#chat-with-us-about-javascript)
1. [Contributors](#contributors)
1. [License](#license)

Types

<a name="types--primitives"></a><a name="1.1"></a>
- [1.1](#types--primitives) **Primitives**: When you access a primitive type you work directly on its value.

	+ `string`
	+ `number`
	+ `boolean`
	+ `null`
	+ `undefined`

	```javascript
	const foo = 1;
	let bar = foo;

	bar = 9;

	console.log(foo, bar); // => 1, 9
	```

<a name="types--complex"></a><a name="1.2"></a>
- [1.2](#types--complex)  **Complex**: When you access a complex type you work on a reference to its value.

	+ `object`
	+ `array`
	+ `function`

	```javascript
	const foo = [1, 2];
	const bar = foo;

	bar[0] = 9;

	console.log(foo[0], bar[0]); // => 9, 9
	```

⬆ back to top

References

<a name="references--prefer-const"></a><a name="2.1"></a>
- [2.1](#references--prefer-const) Use `const` for all of your references; avoid using `var`. eslint: [`prefer-const`](http://eslint.org/docs/rules/prefer-const.html), [`no-const-assign`](http://eslint.org/docs/rules/no-const-assign.html)

	> Why? This ensures that you can't reassign your references, which can lead to bugs and difficult to comprehend code.

	```javascript
	// bad
	var a = 1;
	var b = 2;

	// good
	const a = 1;
	const b = 2;
	```

<a name="references--disallow-var"></a><a name="2.2"></a>
- [2.2](#references--disallow-var) If you must reassign references, use `let` instead of `var`. eslint: [`no-var`](http://eslint.org/docs/rules/no-var.html) jscs: [`disallowVar`](http://jscs.info/rule/disallowVar)

	> Why? `let` is block-scoped rather than function-scoped like `var`.

	```javascript
	// bad
	var count = 1;
	if (true) {
		count += 1;
	}

	// good, use the let.
	let count = 1;
	if (true) {
		count += 1;
	}
	```

<a name="references--block-scope"></a><a name="2.3"></a>
- [2.3](#references--block-scope) Note that both `let` and `const` are block-scoped.

	```javascript
	// const and let only exist in the blocks they are defined in.
	{
		let a = 1;
		const b = 1;
	}
	console.log(a); // ReferenceError
	console.log(b); // ReferenceError
	```

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Objects

<a name="objects--no-new"></a><a name="3.1"></a>
- [3.1](#objects--no-new) Use the literal syntax for object creation. eslint: [`no-new-object`](http://eslint.org/docs/rules/no-new-object.html)

	```javascript
	// bad
	const item = new Object();

	// good
	const item = {};
	```

<a name="es6-computed-properties"></a><a name="3.4"></a>
- [3.2](#es6-computed-properties) Use computed property names when creating objects with dynamic property names.

	> Why? They allow you to define all the properties of an object in one place.

	```javascript

	function getKey(k) {
		return `a key named ${k}`;
	}

	// bad
	const obj = {
		id: 5,
		name: 'San Francisco',
	};
	obj[getKey('enabled')] = true;

	// good
	const obj = {
		id: 5,
		name: 'San Francisco',
		[getKey('enabled')]: true,
	};
	```

<a name="es6-object-shorthand"></a><a name="3.5"></a>
- [3.3](#es6-object-shorthand) Use object method shorthand. eslint: [`object-shorthand`](http://eslint.org/docs/rules/object-shorthand.html) jscs: [`requireEnhancedObjectLiterals`](http://jscs.info/rule/requireEnhancedObjectLiterals)

	```javascript
	// bad
	const atom = {
		value: 1,

		addValue: function (value) {
			return atom.value + value;
		},
	};

	// good
	const atom = {
		value: 1,

		addValue(value) {
			return atom.value + value;
		},
	};
	```

<a name="es6-object-concise"></a><a name="3.6"></a>
- [3.4](#es6-object-concise) Use property value shorthand. eslint: [`object-shorthand`](http://eslint.org/docs/rules/object-shorthand.html) jscs: [`requireEnhancedObjectLiterals`](http://jscs.info/rule/requireEnhancedObjectLiterals)

	> Why? It is shorter to write and descriptive.

	```javascript
	const lukeSkywalker = 'Luke Skywalker';

	// bad
	const obj = {
		lukeSkywalker: lukeSkywalker,
	};

	// good
	const obj = {
		lukeSkywalker,
	};
	```

<a name="objects--grouped-shorthand"></a><a name="3.7"></a>
- [3.5](#objects--grouped-shorthand) Group your shorthand properties at the beginning of your object declaration.

	> Why? It's easier to tell which properties are using the shorthand.

	```javascript
	const anakinSkywalker = 'Anakin Skywalker';
	const lukeSkywalker = 'Luke Skywalker';

	// bad
	const obj = {
		episodeOne: 1,
		twoJediWalkIntoACantina: 2,
		lukeSkywalker,
		episodeThree: 3,
		mayTheFourth: 4,
		anakinSkywalker,
	};

	// good
	const obj = {
		lukeSkywalker,
		anakinSkywalker,
		episodeOne: 1,
		twoJediWalkIntoACantina: 2,
		episodeThree: 3,
		mayTheFourth: 4,
	};
	```

<a name="objects--quoted-props"></a><a name="3.8"></a>
- [3.6](#objects--quoted-props) Only quote properties that are invalid identifiers. eslint: [`quote-props`](http://eslint.org/docs/rules/quote-props.html) jscs: [`disallowQuotedKeysInObjects`](http://jscs.info/rule/disallowQuotedKeysInObjects)

	> Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

	```javascript
	// bad
	const bad = {
		'foo': 3,
		'bar': 4,
		'data-blah': 5,
	};

	// good
	const good = {
		foo: 3,
		bar: 4,
		'data-blah': 5,
	};
	```

<a name="objects--prototype-builtins"></a>
- [3.7](#objects--prototype-builtins) Do not call `Object.prototype` methods directly, such as `hasOwnProperty`, `propertyIsEnumerable`, and `isPrototypeOf`.

	> Why? These methods may be shadowed by properties on the object in question - consider `{ hasOwnProperty: false }` - or, the object may be a null object (`Object.create(null)`).

	```javascript
	// bad
	console.log(object.hasOwnProperty(key));

	// good
	console.log(Object.prototype.hasOwnProperty.call(object, key));

	// best
	const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
	/* or */
	import has from 'has';
	// ...
	console.log(has.call(object, key));
	```

<a name="objects--rest-spread"></a>
- [3.8](#objects--rest-spread) Prefer the object spread operator over [`Object.assign`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

	```javascript
	// very bad
	const original = { a: 1, b: 2 };
	const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
	delete copy.a; // so does this

	// bad
	const original = { a: 1, b: 2 };
	const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

	// good
	const original = { a: 1, b: 2 };
	const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

	const { a, ...noA } = copy; // noA => { b: 2, c: 3 }
	```

⬆ back to top

Arrays

<a name="arrays--literals"></a><a name="4.1"></a>
- [4.1](#arrays--literals) Use the literal syntax for array creation. eslint: [`no-array-constructor`](http://eslint.org/docs/rules/no-array-constructor.html)

	```javascript
	// bad
	const items = new Array();

	// good
	const items = [];
	```

<a name="arrays--push"></a><a name="4.2"></a>
- [4.2](#arrays--push) Use [Array#push](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/push) instead of direct assignment to add items to an array.

	```javascript
	const someStack = [];

	// bad
	someStack[someStack.length] = 'abracadabra';

	// good
	someStack.push('abracadabra');
	```

<a name="es6-array-spreads"></a><a name="4.3"></a>
- [4.3](#es6-array-spreads) Use array spreads `...` to copy arrays.

	```javascript
	// bad
	const len = items.length;
	const itemsCopy = [];
	let i;

	for (i = 0; i < len; i += 1) {
		itemsCopy[i] = items[i];
	}

	// good
	const itemsCopy = [...items];
	```

<a name="arrays--from"></a><a name="4.4"></a>
- [4.4](#arrays--from) To convert an array-like object to an array, use [Array.from](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from).

	```javascript
	const foo = document.querySelectorAll('.foo');
	const nodes = Array.from(foo);
	```

<a name="arrays--callback-return"></a><a name="4.5"></a>
- [4.5](#arrays--callback-return) Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following [8.2](#8.2). eslint: [`array-callback-return`](http://eslint.org/docs/rules/array-callback-return)

	```javascript
	// good
	[1, 2, 3].map((x) => {
		const y = x + 1;
		return x * y;
	});

	// good
	[1, 2, 3].map(x => x + 1);

	// bad
	const flat = {};
	[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
		const flatten = memo.concat(item);
		flat[index] = flatten;
	});

	// good
	const flat = {};
	[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
		const flatten = memo.concat(item);
		flat[index] = flatten;
		return flatten;
	});

	// bad
	inbox.filter((msg) => {
		const { subject, author } = msg;
		if (subject === 'Mockingbird') {
			return author === 'Harper Lee';
		} else {
			return false;
		}
	});

	// good
	inbox.filter((msg) => {
		const { subject, author } = msg;
		if (subject === 'Mockingbird') {
			return author === 'Harper Lee';
		}

		return false;
	});
	```

⬆ back to top

- 4.6 Use line breaks after open and before close array brackets if an array has multiple lines

```javascript
// bad
const arr = [
	[0, 1], [2, 3], [4, 5],
];

const objectInArray = [{
	id: 1,
}, {
	id: 2,
}];

const numberInArray = [
	1, 2,
];

// good
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [
	{
		id: 1,
	},
	{
		id: 2,
	},
];

const numberInArray = [
	1,
	2,
];
```

⬆ back to top

Destructuring

<a name="destructuring--object"></a><a name="5.1"></a>
- [5.1](#destructuring--object) Use object destructuring when accessing and using multiple properties of an object. jscs: [`requireObjectDestructuring`](http://jscs.info/rule/requireObjectDestructuring)

	> Why? Destructuring saves you from creating temporary references for those properties.

	```javascript
	// bad
	function getFullName(user) {
		const firstName = user.firstName;
		const lastName = user.lastName;

		return `${firstName} ${lastName}`;
	}

	// good
	function getFullName(user) {
		const { firstName, lastName } = user;
		return `${firstName} ${lastName}`;
	}

	// best
	function getFullName({ firstName, lastName }) {
		return `${firstName} ${lastName}`;
	}
	```

<a name="destructuring--array"></a><a name="5.2"></a>
- [5.2](#destructuring--array) Use array destructuring. jscs: [`requireArrayDestructuring`](http://jscs.info/rule/requireArrayDestructuring)

	```javascript
	const arr = [1, 2, 3, 4];

	// bad
	const first = arr[0];
	const second = arr[1];

	// good
	const [first, second] = arr;
	```

<a name="destructuring--object-over-array"></a><a name="5.3"></a>
- [5.3](#destructuring--object-over-array) Use object destructuring for multiple return values, not array destructuring. jscs: [`disallowArrayDestructuringReturn`](http://jscs.info/rule/disallowArrayDestructuringReturn)

	> Why? You can add new properties over time or change the order of things without breaking call sites.

	```javascript
	// bad
	function processInput(input) {
		// then a miracle occurs
		return [left, right, top, bottom];
	}

	// the caller needs to think about the order of return data
	const [left, __, top] = processInput(input);

	// good
	function processInput(input) {
		// then a miracle occurs
		return { left, right, top, bottom };
	}

	// the caller selects only the data they need
	const { left, top } = processInput(input);
	```

⬆ back to top

Strings

<a name="strings--quotes"></a><a name="6.1"></a>
- [6.1](#strings--quotes) Use single quotes `''` for strings. eslint: [`quotes`](http://eslint.org/docs/rules/quotes.html) jscs: [`validateQuoteMarks`](http://jscs.info/rule/validateQuoteMarks)

	```javascript
	// bad
	const name = "Capt. Janeway";

	// bad - template literals should contain interpolation or newlines
	const name = `Capt. Janeway`;

	// good
	const name = 'Capt. Janeway';
	```

<a name="strings--line-length"></a><a name="6.2"></a>
- [6.2](#strings--line-length) Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

	> Why? Broken strings are painful to work with and make code less searchable.

	```javascript
	// bad
	const errorMessage = 'This is a super long error that was thrown because \
	of Batman. When you stop to think about how Batman had anything to do \
	with this, you would get nowhere \
	fast.';

	// bad
	const errorMessage = 'This is a super long error that was thrown because ' +
		'of Batman. When you stop to think about how Batman had anything to do ' +
		'with this, you would get nowhere fast.';

	// good
	const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
	```

<a name="es6-template-literals"></a><a name="6.4"></a>
- [6.3](#es6-template-literals) When programmatically building up strings, use template strings instead of concatenation. eslint: [`prefer-template`](http://eslint.org/docs/rules/prefer-template.html) [`template-curly-spacing`](http://eslint.org/docs/rules/template-curly-spacing) jscs: [`requireTemplateStrings`](http://jscs.info/rule/requireTemplateStrings)

	> Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

	```javascript
	// bad
	function sayHi(name) {
		return 'How are you, ' + name + '?';
	}

	// bad
	function sayHi(name) {
		return ['How are you, ', name, '?'].join();
	}

	// bad
	function sayHi(name) {
		return `How are you, ${ name }?`;
	}

	// good
	function sayHi(name) {
		return `How are you, ${name}?`;
	}
	```

<a name="strings--eval"></a><a name="6.5"></a>
- [6.4](#strings--eval) Never use `eval()` on a string, it opens too many vulnerabilities.

<a name="strings--escaping"></a>
- [6.5](#strings--escaping) Do not unnecessarily escape characters in strings. eslint: [`no-useless-escape`](http://eslint.org/docs/rules/no-useless-escape)

	> Why? Backslashes harm readability, thus they should only be present when necessary.

	```javascript
	// bad
	const foo = '\'this\' \i\s \"quoted\"';

	// good
	const foo = '\'this\' is "quoted"';
	const foo = `my name is '${name}'`;
	```

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Functions

<a name="functions--declarations"></a><a name="7.1"></a>
- [7.1](#functions--declarations) Use named function expressions instead of function declarations. eslint: [`func-style`](http://eslint.org/docs/rules/func-style) jscs: [`disallowFunctionDeclarations`](http://jscs.info/rule/disallowFunctionDeclarations)

	> Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to name the expression - anonymous functions can make it harder to locate the problem in an Error's call stack. ([Discussion](https://github.com/airbnb/javascript/issues/794))

	```javascript
	// bad
	function foo() {
		// ...
	}

	// bad
	const foo = function () {
		// ...
	};

	// good
	const foo = function bar() {
		// ...
	};
	```

<a name="functions--iife"></a><a name="7.2"></a>
- [7.2](#functions--iife) Wrap immediately invoked function expressions in parentheses. eslint: [`wrap-iife`](http://eslint.org/docs/rules/wrap-iife.html) jscs: [`requireParenthesesAroundIIFE`](http://jscs.info/rule/requireParenthesesAroundIIFE)

	> Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

	```javascript
	// immediately-invoked function expression (IIFE)
	(function () {
		console.log('Welcome to the Internet. Please follow me.');
	}());
	```

<a name="functions--in-blocks"></a><a name="7.3"></a>
- [7.3](#functions--in-blocks) Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: [`no-loop-func`](http://eslint.org/docs/rules/no-loop-func.html)

<a name="functions--note-on-blocks"></a><a name="7.4"></a>
- [7.4](#functions--note-on-blocks) **Note:** ECMA-262 defines a `block` as a list of statements. A function declaration is not a statement. [Read ECMA-262's note on this issue](http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf#page=97).

	```javascript
	// bad
	if (currentUser) {
		function test() {
			console.log('Nope.');
		}
	}

	// good
	let test;
	if (currentUser) {
		test = () => {
			console.log('Yup.');
		};
	}
	```

<a name="functions--arguments-shadow"></a><a name="7.5"></a>
- [7.5](#functions--arguments-shadow) Never name a parameter `arguments`. This will take precedence over the `arguments` object that is given to every function scope.

	```javascript
	// bad
	function foo(name, options, arguments) {
		// ...
	}

	// good
	function foo(name, options, args) {
		// ...
	}
	```

<a name="es6-rest"></a><a name="7.6"></a>
- [7.6](#es6-rest) Never use `arguments`, opt to use rest syntax `...` instead. eslint: [`prefer-rest-params`](http://eslint.org/docs/rules/prefer-rest-params)

	> Why? `...` is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like `arguments`.

	```javascript
	// bad
	function concatenateAll() {
		const args = Array.prototype.slice.call(arguments);
		return args.join('');
	}

	// good
	function concatenateAll(...args) {
		return args.join('');
	}
	```

<a name="es6-default-parameters"></a><a name="7.7"></a>
- [7.7](#es6-default-parameters) Use default parameter syntax rather than mutating function arguments.

	```javascript
	// really bad
	function handleThings(opts) {
		// No! We shouldn't mutate function arguments.
		// Double bad: if opts is falsy it'll be set to an object which may
		// be what you want but it can introduce subtle bugs.
		opts = opts || {};
		// ...
	}

	// still bad
	function handleThings(opts) {
		if (opts === void 0) {
			opts = {};
		}
		// ...
	}

	// good
	function handleThings(opts = {}) {
		// ...
	}
	```

<a name="functions--default-side-effects"></a><a name="7.8"></a>
- [7.8](#functions--default-side-effects) Avoid side effects with default parameters.

	> Why? They are confusing to reason about.

	```javascript
	var b = 1;
	// bad
	function count(a = b++) {
		console.log(a);
	}
	count();  // 1
	count();  // 2
	count(3); // 3
	count();  // 3
	```

<a name="functions--defaults-last"></a><a name="7.9"></a>
- [7.9](#functions--defaults-last) Always put default parameters last.

	```javascript
	// bad
	function handleThings(opts = {}, name) {
		// ...
	}

	// good
	function handleThings(name, opts = {}) {
		// ...
	}
	```

<a name="functions--constructor"></a><a name="7.10"></a>
- [7.10](#functions--constructor) Never use the Function constructor to create a new function. eslint: [`no-new-func`](http://eslint.org/docs/rules/no-new-func)

	> Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

	```javascript
	// bad
	var add = new Function('a', 'b', 'return a + b');

	// still bad
	var subtract = Function('a', 'b', 'return a - b');
	```

<a name="functions--signature-spacing"></a><a name="7.11"></a>
- [7.11](#functions--signature-spacing) Spacing in a function signature. eslint: [`space-before-function-paren`](http://eslint.org/docs/rules/space-before-function-paren) [`space-before-blocks`](http://eslint.org/docs/rules/space-before-blocks)

	> Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

	```javascript
	// bad
	const f = function(){};
	const g = function (){};
	const h = function() {};

	// good
	const x = function () {};
	const y = function a() {};
	```

<a name="functions--mutate-params"></a><a name="7.12"></a>
- [7.12](#functions--mutate-params) Never mutate parameters. eslint: [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign.html)

	> Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

	```javascript
	// bad
	function f1(obj) {
		obj.key = 1;
	}

	// good
	function f2(obj) {
		const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
	}
	```

<a name="functions--reassign-params"></a><a name="7.13"></a>
- [7.13](#functions--reassign-params) Never reassign parameters. eslint: [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign.html)

	> Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the `arguments` object. It can also cause optimization issues, especially in V8.

	```javascript
	// bad
	function f1(a) {
		a = 1;
		// ...
	}

	function f2(a) {
		if (!a) { a = 1; }
		// ...
	}

	// good
	function f3(a) {
		const b = a || 1;
		// ...
	}

	function f4(a = 1) {
		// ...
	}
	```

<a name="functions--spread-vs-apply"></a><a name="7.14"></a>
- [7.14](#functions--spread-vs-apply) Prefer the use of the spread operator `...` to call variadic functions. eslint: [`prefer-spread`](http://eslint.org/docs/rules/prefer-spread)

	> Why? It's cleaner, you don't need to supply a context, and you can not easily compose `new` with `apply`.

	```javascript
	// bad
	const x = [1, 2, 3, 4, 5];
	console.log.apply(console, x);

	// good
	const x = [1, 2, 3, 4, 5];
	console.log(...x);

	// bad
	new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

	// good
	new Date(...[2016, 8, 5]);
	```

<a name="functions--signature-invocation-indentation"></a>
- [7.15](#functions--signature-invocation-indentation) Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item.

	```javascript
	// bad
	function foo(bar,
						baz,
						quux) {
		// ...
	}

	// good
	function foo(
		bar,
		baz,
		quux,
	) {
		// ...
	}

	// bad
	console.log(foo,
		bar,
		baz);

	// good
	console.log(
		foo,
		bar,
		baz,
	);
	```

⬆ back to top

Arrow Functions

<a name="arrows--use-them"></a><a name="8.1"></a>
- [8.1](#arrows--use-them) When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: [`prefer-arrow-callback`](http://eslint.org/docs/rules/prefer-arrow-callback.html), [`arrow-spacing`](http://eslint.org/docs/rules/arrow-spacing.html) jscs: [`requireArrowFunctions`](http://jscs.info/rule/requireArrowFunctions)

	> Why? It creates a version of the function that executes in the context of `this`, which is usually what you want, and is a more concise syntax.

	> Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.

	```javascript
	// bad
	[1, 2, 3].map(function (x) {
		const y = x + 1;
		return x * y;
	});

	// good
	[1, 2, 3].map((x) => {
		const y = x + 1;
		return x * y;
	});
	```

<a name="arrows--implicit-return"></a><a name="8.2"></a>
- [8.2](#arrows--implicit-return) If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a `return` statement. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html), [`arrow-body-style`](http://eslint.org/docs/rules/arrow-body-style.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam), [`requireShorthandArrowFunctions`](http://jscs.info/rule/requireShorthandArrowFunctions)

	> Why? Syntactic sugar. It reads well when multiple functions are chained together.

	```javascript
	// bad
	[1, 2, 3].map(number => {
		const nextNumber = number + 1;
		`A string containing the ${nextNumber}.`;
	});

	// good
	[1, 2, 3].map(number => `A string containing the ${number}.`);

	// good
	[1, 2, 3].map((number) => {
		const nextNumber = number + 1;
		return `A string containing the ${nextNumber}.`;
	});

	// good
	[1, 2, 3].map((number, index) => ({
		[index]: number,
	}));
	```

<a name="arrows--paren-wrap"></a><a name="8.3"></a>
- [8.3](#arrows--paren-wrap) In case the expression spans over multiple lines, wrap it in parentheses for better readability.

	> Why? It shows clearly where the function starts and ends.

	```javascript
	// bad
	['get', 'post', 'put'].map(httpMethod => Object.prototype.hasOwnProperty.call(
			httpMagicObjectWithAVeryLongName,
			httpMethod,
		)
	);

	// good
	['get', 'post', 'put'].map(httpMethod => (
		Object.prototype.hasOwnProperty.call(
			httpMagicObjectWithAVeryLongName,
			httpMethod,
		)
	));
	```

<a name="arrows--one-arg-parens"></a><a name="8.4"></a>
- [8.4](#arrows--one-arg-parens) If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the ["always" option](http://eslint.org/docs/rules/arrow-parens#always) for eslint or do not include [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam) for jscs. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam)

	> Why? Less visual clutter.

	```javascript
	// bad
	[1, 2, 3].map((x) => x * x);

	// good
	[1, 2, 3].map(x => x * x);

	// good
	[1, 2, 3].map(number => (
		`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
	));

	// bad
	[1, 2, 3].map(x => {
		const y = x + 1;
		return x * y;
	});

	// good
	[1, 2, 3].map((x) => {
		const y = x + 1;
		return x * y;
	});
	```

<a name="arrows--confusing"></a><a name="8.5"></a>
- [8.5](#arrows--confusing) Avoid confusing arrow function syntax (`=>`) with comparison operators (`<=`, `>=`). eslint: [`no-confusing-arrow`](http://eslint.org/docs/rules/no-confusing-arrow)

	```javascript
	// bad
	const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;

	// bad
	const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;

	// good
	const itemHeight = item => (item.height > 256 ? item.largeSize : item.smallSize);

	// good
	const itemHeight = (item) => {
		const { height, largeSize, smallSize } = item;
		return height > 256 ? largeSize : smallSize;
	};
	```

⬆ back to top

Classes & Constructors

<a name="constructors--use-class"></a><a name="9.1"></a>
- [9.1](#constructors--use-class) Always use `class`. Avoid manipulating `prototype` directly.

	> Why? `class` syntax is more concise and easier to reason about.

	```javascript
	// bad
	function Queue(contents = []) {
		this.queue = [...contents];
	}
	Queue.prototype.pop = function () {
		const value = this.queue[0];
		this.queue.splice(0, 1);
		return value;
	};


	// good
	class Queue {
		constructor(contents = []) {
			this.queue = [...contents];
		}
		pop() {
			const value = this.queue[0];
			this.queue.splice(0, 1);
			return value;
		}
	}
	```

<a name="constructors--extends"></a><a name="9.2"></a>
- [9.2](#constructors--extends) Use `extends` for inheritance.

	> Why? It is a built-in way to inherit prototype functionality without breaking `instanceof`.

	```javascript
	// bad
	const inherits = require('inherits');
	function PeekableQueue(contents) {
		Queue.apply(this, contents);
	}
	inherits(PeekableQueue, Queue);
	PeekableQueue.prototype.peek = function () {
		return this.queue[0];
	};

	// good
	class PeekableQueue extends Queue {
		peek() {
			return this.queue[0];
		}
	}
	```

<a name="constructors--chaining"></a><a name="9.3"></a>
- [9.3](#constructors--chaining) Methods can return `this` to help with method chaining.

	```javascript
	// bad
	Jedi.prototype.jump = function () {
		this.jumping = true;
		return true;
	};

	Jedi.prototype.setHeight = function (height) {
		this.height = height;
	};

	const luke = new Jedi();
	luke.jump(); // => true
	luke.setHeight(20); // => undefined

	// good
	class Jedi {
		jump() {
			this.jumping = true;
			return this;
		}

		setHeight(height) {
			this.height = height;
			return this;
		}
	}

	const luke = new Jedi();

	luke.jump()
		.setHeight(20);
	```


<a name="constructors--tostring"></a><a name="9.4"></a>
- [9.4](#constructors--tostring) It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

	```javascript
	class Jedi {
		constructor(options = {}) {
			this.name = options.name || 'no name';
		}

		getName() {
			return this.name;
		}

		toString() {
			return `Jedi - ${this.getName()}`;
		}
	}
	```

<a name="constructors--no-useless"></a><a name="9.5"></a>
- [9.5](#constructors--no-useless) Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: [`no-useless-constructor`](http://eslint.org/docs/rules/no-useless-constructor)

	```javascript
	// bad
	class Jedi {
		constructor() {}

		getName() {
			return this.name;
		}
	}

	// bad
	class Rey extends Jedi {
		constructor(...args) {
			super(...args);
		}
	}

	// good
	class Rey extends Jedi {
		constructor(...args) {
			super(...args);
			this.name = 'Rey';
		}
	}
	```

<a name="classes--no-duplicate-members"></a>
- [9.6](#classes--no-duplicate-members) Avoid duplicate class members. eslint: [`no-dupe-class-members`](http://eslint.org/docs/rules/no-dupe-class-members)

	> Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

	```javascript
	// bad
	class Foo {
		bar() { return 1; }
		bar() { return 2; }
	}

	// good
	class Foo {
		bar() { return 1; }
	}

	// good
	class Foo {
		bar() { return 2; }
	}
	```

⬆ back to top

Modules

<a name="modules--use-them"></a><a name="10.1"></a>
- [10.1](#modules--use-them) Always use modules (`import`/`export`) over a non-standard module system. You can always transpile to your preferred module system.

	> Why? Modules are the future, let's start using the future now.

	```javascript
	// bad
	const AirbnbStyleGuide = require('./AirbnbStyleGuide');
	module.exports = AirbnbStyleGuide.es6;

	// ok
	import AirbnbStyleGuide from './AirbnbStyleGuide';
	export default AirbnbStyleGuide.es6;

	// best
	import { es6 } from './AirbnbStyleGuide';
	export default es6;
	```

<a name="modules--no-wildcard"></a><a name="10.2"></a>
- [10.2](#modules--no-wildcard) Do not use wildcard imports.

	> Why? This makes sure you have a single default export.

	```javascript
	// bad
	import * as AirbnbStyleGuide from './AirbnbStyleGuide';

	// good
	import AirbnbStyleGuide from './AirbnbStyleGuide';
	```

<a name="modules--no-export-from-import"></a><a name="10.3"></a>
- [10.3](#modules--no-export-from-import) And do not export directly from an import.

	> Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

	```javascript
	// bad
	// filename es6.js
	export { es6 as default } from './AirbnbStyleGuide';

	// good
	// filename es6.js
	import { es6 } from './AirbnbStyleGuide';
	export default es6;
	```

<a name="modules--no-duplicate-imports"></a>
- [10.4](#modules--no-duplicate-imports) Only import from a path in one place.

eslint: no-duplicate-imports > Why? Having multiple lines that import from the same path can make code harder to maintain.

	```javascript
	// bad
	import foo from 'foo';
	// … some other imports … //
	import { named1, named2 } from 'foo';

	// good
	import foo, { named1, named2 } from 'foo';

	// good
	import foo, {
		named1,
		named2,
	} from 'foo';
	```

<a name="modules--no-mutable-exports"></a>
- [10.5](#modules--no-mutable-exports) Do not export mutable bindings.

eslint: import/no-mutable-exports > Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

	```javascript
	// bad
	let foo = 3;
	export { foo };

	// good
	const foo = 3;
	export { foo };
	```

<a name="modules--prefer-default-export"></a>
- [10.6](#modules--prefer-default-export) In modules with a single export, prefer default export over named export.

eslint: import/prefer-default-export

	```javascript
	// bad
	export function foo() {}

	// good
	export default function foo() {}
	```

<a name="modules--imports-first"></a>
- [10.7](#modules--imports-first) Put all `import`s above non-import statements.

eslint: import/first > Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

	```javascript
	// bad
	import foo from 'foo';
	foo.init();

	import bar from 'bar';

	// good
	import foo from 'foo';
	import bar from 'bar';

	foo.init();
	```

<a name="modules--multiline-imports-over-newlines"></a>
- [10.8](#modules--multiline-imports-over-newlines) Multiline imports should be indented just like multiline array and object literals.

	> Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

	```javascript
	// bad
	import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

	// good
	import {
		longNameA,
		longNameB,
		longNameC,
		longNameD,
		longNameE,
	} from 'path';
	```

<a name="modules--no-webpack-loader-syntax"></a>
- [10.9](#modules--no-webpack-loader-syntax) Disallow Webpack loader syntax in module import statements.

eslint: import/no-webpack-loader-syntax > Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.

	```javascript
	// bad
	import fooSass from 'css!sass!foo.scss';
	import barCss from 'style!css!bar.css';

	// good
	import fooSass from 'foo.scss';
	import barCss from 'bar.css';
	```

⬆ back to top

Iterators and Generators

<a name="iterators--nope"></a><a name="11.1"></a>
- [11.1](#iterators--nope) Don't use iterators. Prefer JavaScript's higher-order functions instead of loops like `for-in` or `for-of`. eslint: [`no-iterator`](http://eslint.org/docs/rules/no-iterator.html) [`no-restricted-syntax`](http://eslint.org/docs/rules/no-restricted-syntax)

	> Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

	> Use `map()` / `every()` / `filter()` / `find()` / `findIndex()` / `reduce()` / `some()` / ... to iterate over arrays, and `Object.keys()` / `Object.values()` / `Object.entries()` to produce arrays so you can iterate over objects.

	```javascript
	const numbers = [1, 2, 3, 4, 5];

	// bad
	let sum = 0;
	for (let num of numbers) {
		sum += num;
	}
	sum === 15;

	// good
	let sum = 0;
	numbers.forEach(num => sum += num);
	sum === 15;

	// best (use the functional force)
	const sum = numbers.reduce((total, num) => total + num, 0);
	sum === 15;

	// bad
	const increasedByOne = [];
	for (let i = 0; i < numbers.length; i++) {
		increasedByOne.push(numbers[i] + 1);
	}

	// good
	const increasedByOne = [];
	numbers.forEach(num => increasedByOne.push(num + 1));

	// best (keeping it functional)
	const increasedByOne = numbers.map(num => num + 1);
	```

<a name="generators--nope"></a><a name="11.2"></a>
- [11.2](#generators--nope) Don't use generators for now.

	> Why? They don't transpile well to ES5.

<a name="generators--spacing"></a>
- [11.3](#generators--spacing) If you must use generators, or if you disregard [our advice](#generators--nope), make sure their function signature is spaced properly. eslint: [`generator-star-spacing`](http://eslint.org/docs/rules/generator-star-spacing)

	> Why? `function` and `*` are part of the same conceptual keyword - `*` is not a modifier for `function`, `function*` is a unique construct, different from `function`.

	```javascript
	// bad
	function * foo() {
		// ...
	}

	// bad
	const bar = function * () {
		// ...
	};

	// bad
	const baz = function *() {
		// ...
	};

	// bad
	const quux = function*() {
		// ...
	};

	// bad
	function*foo() {
		// ...
	}

	// bad
	function *foo() {
		// ...
	}

	// very bad
	function
	*
	foo() {
		// ...
	}

	// very bad
	const wat = function
	*
	() {
		// ...
	};

	// good
	function* foo() {
		// ...
	}

	// good
	const foo = function* () {
		// ...
	};
	```

⬆ back to top

Properties

<a name="properties--dot"></a><a name="12.1"></a>
- [12.1](#properties--dot) Use dot notation when accessing properties. eslint: [`dot-notation`](http://eslint.org/docs/rules/dot-notation.html) jscs: [`requireDotNotation`](http://jscs.info/rule/requireDotNotation)

	```javascript
	const luke = {
		jedi: true,
		age: 28,
	};

	// bad
	const isJedi = luke['jedi'];

	// good
	const isJedi = luke.jedi;
	```

<a name="properties--bracket"></a><a name="12.2"></a>
- [12.2](#properties--bracket) Use bracket notation `[]` when accessing properties with a variable.

	```javascript
	const luke = {
		jedi: true,
		age: 28,
	};

	function getProp(prop) {
		return luke[prop];
	}

	const isJedi = getProp('jedi');
	```

⬆ back to top

Variables

<a name="variables--const"></a><a name="13.1"></a>
- [13.1](#variables--const) Always use `const` or `let` to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: [`no-undef`](http://eslint.org/docs/rules/no-undef) [`prefer-const`](http://eslint.org/docs/rules/prefer-const)

	```javascript
	// bad
	superPower = new SuperPower();

	// good
	const superPower = new SuperPower();
	```

<a name="variables--one-const"></a><a name="13.2"></a>
- [13.2](#variables--one-const) Use one `const` or `let` declaration per variable. eslint: [`one-var`](http://eslint.org/docs/rules/one-var.html) jscs: [`disallowMultipleVarDecl`](http://jscs.info/rule/disallowMultipleVarDecl)

	> Why? It's easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

	```javascript
	// bad
	const items = getItems(),
			goSportsTeam = true,
			dragonball = 'z';

	// bad
	// (compare to above, and try to spot the mistake)
	const items = getItems(),
			goSportsTeam = true;
			dragonball = 'z';

	// good
	const items = getItems();
	const goSportsTeam = true;
	const dragonball = 'z';
	```

<a name="variables--const-let-group"></a><a name="13.3"></a>
- [13.3](#variables--const-let-group) Group all your `const`s and then group all your `let`s.

	> Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

	```javascript
	// bad
	let i, len, dragonball,
			items = getItems(),
			goSportsTeam = true;

	// bad
	let i;
	const items = getItems();
	let dragonball;
	const goSportsTeam = true;
	let len;

	// good
	const goSportsTeam = true;
	const items = getItems();
	let dragonball;
	let i;
	let length;
	```

<a name="variables--define-where-used"></a><a name="13.4"></a>
- [13.4](#variables--define-where-used) Assign variables where you need them, but place them in a reasonable place.

	> Why? `let` and `const` are block scoped and not function scoped.

	```javascript
	// bad - unnecessary function call
	function checkName(hasName) {
		const name = getName();

		if (hasName === 'test') {
			return false;
		}

		if (name === 'test') {
			this.setName('');
			return false;
		}

		return name;
	}

	// good
	function checkName(hasName) {
		if (hasName === 'test') {
			return false;
		}

		const name = getName();

		if (name === 'test') {
			this.setName('');
			return false;
		}

		return name;
	}
	```
<a name="variables--no-chain-assignment"></a><a name="13.5"></a>
- [13.5](#variables--no-chain-assignment) Don't chain variable assignments.

	> Why? Chaining variable assignments creates implicit global variables.

	```javascript
	// bad
	(function example() {
		// JavaScript interprets this as
		// let a = ( b = ( c = 1 ) );
		// The let keyword only applies to variable a; variables b and c become
		// global variables.
		let a = b = c = 1;
	}());

	console.log(a); // undefined
	console.log(b); // 1
	console.log(c); // 1

	// good
	(function example() {
		let a = 1;
		let b = a;
		let c = a;
	}());

	console.log(a); // undefined
	console.log(b); // undefined
	console.log(c); // undefined

	// the same applies for `const`
	```

<a name="variables--unary-increment-decrement"></a><a name="13.6"></a>
- [13.6](#variables--unary-increment-decrement) Avoid using unary increments and decrements (++, --). eslint [`no-plusplus`](http://eslint.org/docs/rules/no-plusplus)

	> Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like `num += 1` instead of `num++` or `num ++`. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

	```javascript
	// bad

	const array = [1, 2, 3];
	let num = 1;
	num++;
	--num;

	let sum = 0;
	let truthyCount = 0;
	for (let i = 0; i < array.length; i++) {
		let value = array[i];
		sum += value;
		if (value) {
			truthyCount++;
		}
	}

	// good

	const array = [1, 2, 3];
	let num = 1;
	num += 1;
	num -= 1;

	const sum = array.reduce((a, b) => a + b, 0);
	const truthyCount = array.filter(Boolean).length;
	```

⬆ back to top

Hoisting

<a name="hoisting--about"></a><a name="14.1"></a>
- [14.1](#hoisting--about) `var` declarations get hoisted to the top of their scope, their assignment does not. `const` and `let` declarations are blessed with a new concept called [Temporal Dead Zones (TDZ)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Temporal_dead_zone_and_errors_with_let). It's important to know why [typeof is no longer safe](http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).

	```javascript
	// we know this wouldn't work (assuming there
	// is no notDefined global variable)
	function example() {
		console.log(notDefined); // => throws a ReferenceError
	}

	// creating a variable declaration after you
	// reference the variable will work due to
	// variable hoisting. Note: the assignment
	// value of `true` is not hoisted.
	function example() {
		console.log(declaredButNotAssigned); // => undefined
		var declaredButNotAssigned = true;
	}

	// the interpreter is hoisting the variable
	// declaration to the top of the scope,
	// which means our example could be rewritten as:
	function example() {
		let declaredButNotAssigned;
		console.log(declaredButNotAssigned); // => undefined
		declaredButNotAssigned = true;
	}

	// using const and let
	function example() {
		console.log(declaredButNotAssigned); // => throws a ReferenceError
		console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
		const declaredButNotAssigned = true;
	}
	```

<a name="hoisting--anon-expressions"></a><a name="14.2"></a>
- [14.2](#hoisting--anon-expressions) Anonymous function expressions hoist their variable name, but not the function assignment.

	```javascript
	function example() {
		console.log(anonymous); // => undefined

		anonymous(); // => TypeError anonymous is not a function

		var anonymous = function () {
			console.log('anonymous function expression');
		};
	}
	```

<a name="hoisting--named-expresions"></a><a name="14.3"></a>
- [14.3](#hoisting--named-expresions) Named function expressions hoist the variable name, not the function name or the function body.

	```javascript
	function example() {
		console.log(named); // => undefined

		named(); // => TypeError named is not a function

		superPower(); // => ReferenceError superPower is not defined

		var named = function superPower() {
			console.log('Flying');
		};
	}

	// the same is true when the function name
	// is the same as the variable name.
	function example() {
		console.log(named); // => undefined

		named(); // => TypeError named is not a function

		var named = function named() {
			console.log('named');
		};
	}
	```

<a name="hoisting--declarations"></a><a name="14.4"></a>
- [14.4](#hoisting--declarations) Function declarations hoist their name and the function body.

	```javascript
	function example() {
		superPower(); // => Flying

		function superPower() {
			console.log('Flying');
		}
	}
	```

- For more information refer to [JavaScript Scoping & Hoisting](http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting/) by [Ben Cherry](http://www.adequatelygood.com/).

⬆ back to top

Comparison Operators & Equality

<a name="comparison--eqeqeq"></a><a name="15.1"></a>
- [15.1](#comparison--eqeqeq) Use `===` and `!==` over `==` and `!=`. eslint: [`eqeqeq`](http://eslint.org/docs/rules/eqeqeq.html)

<a name="comparison--if"></a><a name="15.2"></a>
- [15.2](#comparison--if) Conditional statements such as the `if` statement evaluate their expression using coercion with the `ToBoolean` abstract method and always follow these simple rules:

	+ **Objects** evaluate to **true**
	+ **Undefined** evaluates to **false**
	+ **Null** evaluates to **false**
	+ **Booleans** evaluate to **the value of the boolean**
	+ **Numbers** evaluate to **false** if **+0, -0, or NaN**, otherwise **true**
	+ **Strings** evaluate to **false** if an empty string `''`, otherwise **true**

	```javascript
	if ([0] && []) {
		// true
		// an array (even an empty one) is an object, objects will evaluate to true
	}
	```

<a name="comparison--shortcuts"></a><a name="15.3"></a>
- [15.3](#comparison--shortcuts) Use shortcuts for booleans, but explicit comparisons for strings and numbers.

	```javascript
	// bad
	if (isValid === true) {
		// ...
	}

	// good
	if (isValid) {
		// ...
	}

	// bad
	if (name) {
		// ...
	}

	// good
	if (name !== '') {
		// ...
	}

	// bad
	if (collection.length) {
		// ...
	}

	// good
	if (collection.length > 0) {
		// ...
	}
	```

<a name="comparison--moreinfo"></a><a name="15.4"></a>
- [15.4](#comparison--moreinfo) For more information see [Truth Equality and JavaScript](https://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.

<a name="comparison--switch-blocks"></a><a name="15.5"></a>
- [15.5](#comparison--switch-blocks) Use braces to create blocks in `case` and `default` clauses that contain lexical declarations (e.g. `let`, `const`, `function`, and `class`).

	> Why? Lexical declarations are visible in the entire `switch` block but only get initialized when assigned, which only happens when its `case` is reached. This causes problems when multiple `case` clauses attempt to define the same thing.

	eslint rules: [`no-case-declarations`](http://eslint.org/docs/rules/no-case-declarations.html).

	```javascript
	// bad
	switch (foo) {
		case 1:
			let x = 1;
			break;
		case 2:
			const y = 2;
			break;
		case 3:
			function f() {
				// ...
			}
			break;
		default:
			class C {}
	}

	// good
	switch (foo) {
		case 1: {
			let x = 1;
			break;
		}
		case 2: {
			const y = 2;
			break;
		}
		case 3: {
			function f() {
				// ...
			}
			break;
		}
		case 4:
			bar();
			break;
		default: {
			class C {}
		}
	}
	```

<a name="comparison--nested-ternaries"></a><a name="15.6"></a>
- [15.6](#comparison--nested-ternaries) Ternaries should not be nested and generally be single line expressions.

	eslint rules: [`no-nested-ternary`](http://eslint.org/docs/rules/no-nested-ternary.html).

	```javascript
	// bad
	const foo = maybe1 > maybe2
		? "bar"
		: value1 > value2 ? "baz" : null;

	// better
	const maybeNull = value1 > value2 ? 'baz' : null;

	const foo = maybe1 > maybe2
		? 'bar'
		: maybeNull;

	// best
	const maybeNull = value1 > value2 ? 'baz' : null;

	const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
	```

<a name="comparison--unneeded-ternary"></a><a name="15.7"></a>
- [15.7](#comparison--unneeded-ternary) Avoid unneeded ternary statements.

	eslint rules: [`no-unneeded-ternary`](http://eslint.org/docs/rules/no-unneeded-ternary.html).

	```javascript
	// bad
	const foo = a ? a : b;
	const bar = c ? true : false;
	const baz = c ? false : true;

	// good
	const foo = a || b;
	const bar = !!c;
	const baz = !c;
	```

⬆ back to top

Blocks

<a name="blocks--braces"></a><a name="16.1"></a>
- [16.1](#blocks--braces) Use braces with all multi-line blocks.

	```javascript
	// bad
	if (test)
		return false;

	// good
	if (test) return false;

	// good
	if (test) {
		return false;
	}

	// bad
	function foo() { return false; }

	// good
	function bar() {
		return false;
	}
	```

<a name="blocks--cuddled-elses"></a><a name="16.2"></a>
- [16.2](#blocks--cuddled-elses) If you're using multi-line blocks with `if` and `else`, put `else` on the same line as your `if` block's closing brace. eslint: [`brace-style`](http://eslint.org/docs/rules/brace-style.html) jscs:  [`disallowNewlineBeforeBlockStatements`](http://jscs.info/rule/disallowNewlineBeforeBlockStatements)

	```javascript
	// bad
	if (test) {
		thing1();
		thing2();
	}
	else {
		thing3();
	}

	// good
	if (test) {
		thing1();
		thing2();
	} else {
		thing3();
	}
	```

⬆ back to top

Comments

<a name="comments--multiline"></a><a name="17.1"></a>
- [17.1](#comments--multiline) Use `/** ... */` for multi-line comments.

	```javascript
	// bad
	// make() returns a new element
	// based on the passed in tag name
	//
	// @param {String} tag
	// @return {Element} element
	function make(tag) {

		// ...

		return element;
	}

	// good
	/**
	* make() returns a new element
	* based on the passed-in tag name
	*/
	function make(tag) {

		// ...

		return element;
	}
	```

<a name="comments--singleline"></a><a name="17.2"></a>
- [17.2](#comments--singleline) Use `//` for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it's on the first line of a block.

	```javascript
	// bad
	const active = true;  // is current tab

	// good
	// is current tab
	const active = true;

	// bad
	function getType() {
		console.log('fetching type...');
		// set the default type to 'no type'
		const type = this.type || 'no type';

		return type;
	}

	// good
	function getType() {
		console.log('fetching type...');

		// set the default type to 'no type'
		const type = this.type || 'no type';

		return type;
	}

	// also good
	function getType() {
		// set the default type to 'no type'
		const type = this.type || 'no type';

		return type;
	}
	```

- [17.3](#comments--spaces) Start all comments with a space to make it easier to read. eslint: [`spaced-comment`](http://eslint.org/docs/rules/spaced-comment)

	```javascript
	// bad
	//is current tab
	const active = true;

	// good
	// is current tab
	const active = true;

	// bad
	/**
	*make() returns a new element
	*based on the passed-in tag name
	*/
	function make(tag) {

		// ...

		return element;
	}

	// good
	/**
	* make() returns a new element
	* based on the passed-in tag name
	*/
	function make(tag) {

		// ...

		return element;
	}
	```

<a name="comments--actionitems"></a><a name="17.3"></a>
- [17.4](#comments--actionitems) Prefixing your comments with `FIXME` or `TODO` helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are `FIXME: -- need to figure this out` or `TODO: -- need to implement`.

<a name="comments--fixme"></a><a name="17.4"></a>
- [17.5](#comments--fixme) Use `// FIXME:` to annotate problems.

	```javascript
	class Calculator extends Abacus {
		constructor() {
			super();

			// FIXME: shouldn't use a global here
			total = 0;
		}
	}
	```

<a name="comments--todo"></a><a name="17.5"></a>
- [17.6](#comments--todo) Use `// TODO:` to annotate solutions to problems.

	```javascript
	class Calculator extends Abacus {
		constructor() {
			super();

			// TODO: total should be configurable by an options param
			this.total = 0;
		}
	}
	```

⬆ back to top

Whitespace

<a name="whitespace--spaces"></a><a name="18.1"></a>
- [18.1](#whitespace--spaces) Use 1 tab character tabs. Tabs are the superior method of indentation, so use them. eslint: [`indent`](http://eslint.org/docs/rules/indent.html) jscs: [`validateIndentation`](http://jscs.info/rule/validateIndentation)

	```javascript
	// bad
	function foo() {
	∙∙let name;
	}

	// bad
	function bar() {
	∙∙∙∙let name;
	}

	// very bad
	function bar() {
	∙∙⟶let name;
	}

	// good
	function baz() {
	⟶let name;
	}
	```

<a name="whitespace--before-blocks"></a><a name="18.2"></a>
- [18.2](#whitespace--before-blocks) Place 1 space before the leading brace. eslint: [`space-before-blocks`](http://eslint.org/docs/rules/space-before-blocks.html) jscs: [`requireSpaceBeforeBlockStatements`](http://jscs.info/rule/requireSpaceBeforeBlockStatements)

	```javascript
	// bad
	function test(){
		console.log('test');
	}

	// good
	function test() {
		console.log('test');
	}

	// bad
	dog.set('attr',{
		age: '1 year',
		breed: 'Bernese Mountain Dog',
	});

	// good
	dog.set('attr', {
		age: '1 year',
		breed: 'Bernese Mountain Dog',
	});
	```

<a name="whitespace--around-keywords"></a><a name="18.3"></a>
- [18.3](#whitespace--around-keywords) Place 1 space before the opening parenthesis in control statements (`if`, `while` etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: [`keyword-spacing`](http://eslint.org/docs/rules/keyword-spacing.html) jscs: [`requireSpaceAfterKeywords`](http://jscs.info/rule/requireSpaceAfterKeywords)

	```javascript
	// bad
	if(isJedi) {
		fight ();
	}

	// good
	if (isJedi) {
		fight();
	}

	// bad
	function fight () {
		console.log ('Swooosh!');
	}

	// good
	function fight() {
		console.log('Swooosh!');
	}
	```

<a name="whitespace--infix-ops"></a><a name="18.4"></a>
- [18.4](#whitespace--infix-ops) Set off operators with spaces. eslint: [`space-infix-ops`](http://eslint.org/docs/rules/space-infix-ops.html) jscs: [`requireSpaceBeforeBinaryOperators`](http://jscs.info/rule/requireSpaceBeforeBinaryOperators), [`requireSpaceAfterBinaryOperators`](http://jscs.info/rule/requireSpaceAfterBinaryOperators)

	```javascript
	// bad
	const x=y+5;

	// good
	const x = y + 5;
	```

<a name="whitespace--newline-at-end"></a><a name="18.5"></a>
- [18.5](#whitespace--newline-at-end) End files with a single newline character. eslint: [`eol-last`](https://github.com/eslint/eslint/blob/master/docs/rules/eol-last.md)

	```javascript
	// bad
	import { es6 } from './AirbnbStyleGuide';
		// ...
	export default es6;
	```

	```javascript
	// bad
	import { es6 } from './AirbnbStyleGuide';
		// ...
	export default es6;↵
	↵
	```

	```javascript
	// good
	import { es6 } from './AirbnbStyleGuide';
		// ...
	export default es6;↵
	```

<a name="whitespace--chains"></a><a name="18.6"></a>
- [18.6](#whitespace--chains) Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which
	emphasizes that the line is a method call, not a new statement. eslint: [`newline-per-chained-call`](http://eslint.org/docs/rules/newline-per-chained-call) [`no-whitespace-before-property`](http://eslint.org/docs/rules/no-whitespace-before-property)

	```javascript
	// bad
	$('#items').find('.selected').highlight().end().find('.open').updateCount();

	// bad
	$('#items').
		find('.selected').
			highlight().
			end().
		find('.open').
			updateCount();

	// good
	$('#items')
		.find('.selected')
			.highlight()
			.end()
		.find('.open')
			.updateCount();

	// bad
	const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
			.attr('width', (radius + margin) * 2).append('svg:g')
			.attr('transform', `translate(${radius + margin},${radius + margin})`)
			.call(tron.led);

	// good
	const leds = stage.selectAll('.led')
			.data(data)
		.enter().append('svg:svg')
			.classed('led', true)
			.attr('width', (radius + margin) * 2)
		.append('svg:g')
			.attr('transform', `translate(${radius + margin},${radius + margin})`)
			.call(tron.led);

	// good
	const leds = stage.selectAll('.led').data(data);
	```

<a name="whitespace--after-blocks"></a><a name="18.7"></a>
- [18.7](#whitespace--after-blocks) Leave a blank line after blocks and before the next statement. jscs: [`requirePaddingNewLinesAfterBlocks`](http://jscs.info/rule/requirePaddingNewLinesAfterBlocks)

	```javascript
	// bad
	if (foo) {
		return bar;
	}
	return baz;

	// good
	if (foo) {
		return bar;
	}

	return baz;

	// bad
	const obj = {
		foo() {
		},
		bar() {
		},
	};
	return obj;

	// good
	const obj = {
		foo() {
		},

		bar() {
		},
	};

	return obj;

	// bad
	const arr = [
		function foo() {
		},
		function bar() {
		},
	];
	return arr;

	// good
	const arr = [
		function foo() {
		},

		function bar() {
		},
	];

	return arr;
	```

<a name="whitespace--padded-blocks"></a><a name="18.8"></a>
- [18.8](#whitespace--padded-blocks) Do not pad your blocks with blank lines. eslint: [`padded-blocks`](http://eslint.org/docs/rules/padded-blocks.html) jscs:  [`disallowPaddingNewlinesInBlocks`](http://jscs.info/rule/disallowPaddingNewlinesInBlocks)

	```javascript
	// bad
	function bar() {

		console.log(foo);

	}

	// also bad
	if (baz) {

		console.log(qux);
	} else {
		console.log(foo);

	}

	// good
	function bar() {
		console.log(foo);
	}

	// good
	if (baz) {
		console.log(qux);
	} else {
		console.log(foo);
	}
	```

<a name="whitespace--in-parens"></a><a name="18.9"></a>
- [18.9](#whitespace--in-parens) Do not add spaces inside parentheses. eslint: [`space-in-parens`](http://eslint.org/docs/rules/space-in-parens.html) jscs: [`disallowSpacesInsideParentheses`](http://jscs.info/rule/disallowSpacesInsideParentheses)

	```javascript
	// bad
	function bar( foo ) {
		return foo;
	}

	// good
	function bar(foo) {
		return foo;
	}

	// bad
	if ( foo ) {
		console.log(foo);
	}

	// good
	if (foo) {
		console.log(foo);
	}
	```

<a name="whitespace--in-brackets"></a><a name="18.10"></a>
- [18.10](#whitespace--in-brackets) Do not add spaces inside brackets. eslint: [`array-bracket-spacing`](http://eslint.org/docs/rules/array-bracket-spacing.html) jscs: [`disallowSpacesInsideArrayBrackets`](http://jscs.info/rule/disallowSpacesInsideArrayBrackets)

	```javascript
	// bad
	const foo = [ 1, 2, 3 ];
	console.log(foo[ 0 ]);

	// good
	const foo = [1, 2, 3];
	console.log(foo[0]);
	```

<a name="whitespace--in-braces"></a><a name="18.11"></a>
- [18.11](#whitespace--in-braces) Add spaces inside curly braces. eslint: [`object-curly-spacing`](http://eslint.org/docs/rules/object-curly-spacing.html) jscs: [`requireSpacesInsideObjectBrackets`](http://jscs.info/rule/requireSpacesInsideObjectBrackets)

	```javascript
	// bad
	const foo = {clark: 'kent'};

	// good
	const foo = { clark: 'kent' };
	```

<a name="whitespace--max-len"></a><a name="18.12"></a>
- [18.12](#whitespace--max-len) Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per [above](#strings--line-length), long strings are exempt from this rule, and should not be broken up. eslint: [`max-len`](http://eslint.org/docs/rules/max-len.html) jscs: [`maximumLineLength`](http://jscs.info/rule/maximumLineLength)

	> Why? This ensures readability and maintainability.

	```javascript
	// bad
	const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

	// bad
	$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

	// good
	const foo = jsonData
		&& jsonData.foo
		&& jsonData.foo.bar
		&& jsonData.foo.bar.baz
		&& jsonData.foo.bar.baz.quux
		&& jsonData.foo.bar.baz.quux.xyzzy;

	// good
	$.ajax({
		method: 'POST',
		url: 'https://airbnb.com/',
		data: { name: