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Note: this guide assumes you are using TypeScript v3.7 or later.

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Types
References
Objects
Arrays
Destructuring
Strings
Functions
Arrow Functions
Classes & Constructors
Modules
Iterators and Generators
Properties
Variables
Hoisting
Comparison Operators & Equality
Blocks
Control Statements
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Events
ECMAScript 5 Compatibility
ECMAScript 6+ (ES 2015+) Styles
Standard Library
Testing
Performance
Resources
In the Wild
Translation
The JavaScript Style Guide Guide
Chat With Us About JavaScript
Contributors
License
Amendments

Types

1.1 Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
- symbol
- bigint
```
 const foo = 1;
 let bar = foo;

 bar = 9;

 console.log(foo, bar); // => 1, 9
```
- Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

1.2 Complex: When you access a complex type you work on a reference to its value.

object
array

function

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

 bar[0] = 9;

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

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References

2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
```
 // bad
 var a = 1;
 var b = 2;

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

2.2 If you must reassign references, use let instead of var. eslint: no-var

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

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

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

2.3 Note that both let and const are block-scoped.

 // 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

3.1 Use the literal syntax for object creation. eslint: no-new-object

 // bad
 const item = new Object();

 // good
 const item = {};

3.2 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.

 function getKey(k: string) {
 	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,
 };

3.3 Use object method shorthand. eslint: object-shorthand

 // bad
 const atom = {
 	value: 1,

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

 // good
 const atom = {
 	value: 1,

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

3.4 Use property value shorthand. eslint: object-shorthand

Why? It is shorter and descriptive.

 const lukeSkywalker = 'Luke Skywalker';

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

 // good
 const obj = {
 	lukeSkywalker,
 };

3.5 Group your shorthand properties at the beginning of your object declaration.

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

 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,
 };

3.6 Only quote properties that are invalid identifiers. eslint: quote-props

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
 // bad
 const bad = {
 	'foo': 3,
 	'bar': 4,
 	'data-blah': 5,
 };

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

3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

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)).

 // 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.
 console.log(has.call(object, key));
 /* or */
 import has from 'has'; // https://www.npmjs.com/package/has
 console.log(has(object, key));

3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted. eslint: prefer-object-spread

 // 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 }

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Arrays

4.1 Use the literal syntax for array creation. eslint: no-array-constructor

 // bad
 const items = new Array();

 // good
 const items = [];

4.2 Use Array#push instead of direct assignment to add items to an array.

 const someStack = [];

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

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

4.3 Use array spreads ... to copy arrays.

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

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

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

4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

 const foo = document.querySelectorAll('.foo');

 // good
 const nodes = Array.from(foo);

 // best
 const nodes = [...foo];

4.5 Use Array.from for converting an array-like object to an array.

 const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };

 // bad
 const arr = Array.prototype.slice.call(arrLike);

 // good
 const arr = Array.from(arrLike);

4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
```
 // bad
 const baz = [...foo].map(bar);

 // good
 const baz = Array.from(foo, bar);
```

4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

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

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

 // bad - no returned value means `acc` becomes undefined after the first iteration
 [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
 	const flatten = acc.concat(item);
 });

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

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

 // 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,
 ];

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Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
```
 // bad
 function getFullName(user: User) {
 	const firstName = user.firstName;
 	const lastName = user.lastName;

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

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

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

5.2 Use array destructuring. eslint: prefer-destructuring

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

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

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

5.3 Use object destructuring for multiple return values, not array destructuring.

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

 // 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);

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Strings

6.1 Use single quotes '' for strings. eslint: quotes

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

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

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

6.2 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.

 // 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.';

6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

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

 // 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}?`;
 }

6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

Why? Backslashes harm readability, thus they should only be present when necessary.
```
 // bad
 const foo = '\'this\' \i\s \"quoted\"';

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

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Functions

7.1 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

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

7.2 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

7.3 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

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

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

7.4 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

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

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

7.5 Never use arguments, opt to use rest syntax ... instead. eslint: 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.

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

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

7.6 Use default parameter syntax rather than mutating function arguments.

 // 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 = {}) {
 	// ...
 }

7.7 Avoid side effects with default parameters.

Why? They are confusing to reason about.

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

7.8 Always put default parameters last. eslint: default-param-last

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

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

7.9 Never use the Function constructor to create a new function. eslint: no-new-func

Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
```
 // bad
 var add = new Function('a', 'b', 'return a + b');

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

7.10 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.
```
 // bad
 const f = function(){};
 const g = function (){};
 const h = function() {};

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

7.11 Never mutate parameters. eslint: no-param-reassign

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

Why not? When using a library like immer--such as in Redux Toolkit--objects aren't actually mutated and will not cause side effects.
```
 // bad
 function f1(obj) {
 	obj.key = 1;
 }

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

7.12 Never reassign parameters. eslint: no-param-reassign

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

 // 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) {
 	// ...
 }

7.13 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

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

 // 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]);

7.14 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. eslint: function-paren-newline

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

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

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

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

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Arrow Functions

8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

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.
```
 // 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;
 });
```

8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

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

 // 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 + 1}.`);

 // 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,
 }));

 // No implicit return with side effects
 function foo(callback) {
 	const val = callback();
 	if (val === true) {
 		// Do something if callback returns true
 	}
 }

 let bool = false;

 // bad
 foo(() => bool = true);

 // good
 foo(() => {
 	bool = true;
 });

8.3 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.

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

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

8.4 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens

Why? Minimizes diff churn when adding or removing arguments.

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

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

 // bad
 [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!`
 ));

 // 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;
 });

8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

 // 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;
 };

8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

 // bad
 (foo) =>
 	bar;

 (foo) =>
 	(bar);

 // good
 (foo) => bar;
 (foo) => (bar);
 (foo) => (
 		bar
 )

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Classes & Constructors

9.1 Always use class. Avoid manipulating prototype directly.

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

 // 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;
 	}
 }

9.2 Use extends for inheritance.

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

 // 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];
 	}
 }

9.3 Methods can return this to help with method chaining.

In functional applications, consider returning a new instance of the class for chainable methods to avoid mutating the original instance.

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

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

 const luke = new Jedi();

 luke.jump()
 	.setHeight(20);

 // good
 class Counter {
 	constructor(private count = 0);

 	add(amt = 1) {
 		return new Counter(this.count);
 	}

 	toString() {
 		return String(this.count);
 	}
 }

 const counter1 = new Counter();
 const counter2 = counter1.add();
 const counter3 = counter2.add().add();
 console.log(counter1, counter2, counter3); // => 0 1 3

9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

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

 	getName() {
 		return this.name;
 	}

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

9.5 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

 // 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';
 	}
 }

9.6 Avoid duplicate class members. eslint: no-dupe-class-members

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

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

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

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

9.7 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

 // bad
 class Foo {
 	bar() {
 		console.log('bar');
 	}
 }

 // good - this is used
 class Foo {
 	bar() {
 		console.log(this.bar);
 	}
 }

 // good - constructor is exempt
 class Foo {
 	constructor() {
 		// ...
 	}
 }

 // good - static methods aren't expected to use this
 class Foo {
 	static bar() {
 		console.log('bar');
 	}
 }

⬆ back to top

Modules

10.1 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.

 // 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;

10.2 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.

 // 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';

10.3 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.
```
 // bad
 let foo = 3;
 export { foo };

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

10.4 Put all imports above non-import statements. eslint: import/first

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

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

 import bar from 'bar';

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

 foo.init();

10.5 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.
```
 // bad
 import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

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

10.6 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.
```
 // 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';
```

10.7 Do not include JavaScript or TypeScript filename extensions eslint: import/extensions

Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

 // bad
 import foo from './foo.js';
 import bar from './bar.jsx';
 import baz from './baz/index.jsx';
 import qux from './qux.ts';
 import quux from './quux.tsx';

 // good
 import foo from './foo';
 import bar from './bar';
 import baz from './baz';
 import qux from './qux';
 import quux from './quux';

⬆ back to top

Iterators and Generators

11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator 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.

 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);

11.2 Don’t use generators for now.

Why? They don’t transpile well to ES5.

11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: 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.

 // 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

12.1 Use dot notation when accessing properties. eslint: dot-notation

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

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

 // good
 const isJedi = luke.jedi;

12.2 Use bracket notation [] when accessing properties with a variable.

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

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

 const isJedi = getProp('jedi');

12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.
```
 // bad
 const binary = Math.pow(2, 10);

 // good
 const binary = 2 ** 10;
```

⬆ back to top

Variables

13.1 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 prefer-const
```
 // bad
 superPower = new SuperPower();

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

13.2 Use one const or let declaration per variable or assignment. eslint: one-var

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.

 // 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';

13.3 Group all your consts and then group all your lets.

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

 // 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;

13.4 Assign variables where you need them, but place them in a reasonable place.

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

 // 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;
 }

13.5 Don’t chain variable assignments. eslint: no-multi-assign

Why? Chaining variable assignments creates implicit global variables.

 // 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); // throws ReferenceError
 console.log(b); // 1
 console.log(c); // 1

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

 console.log(a); // throws ReferenceError
 console.log(b); // throws ReferenceError
 console.log(c); // throws ReferenceError

 // the same applies for `const`

13.6 Avoid using unary increments and decrements (++, --). eslint 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.
```
 // 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;
```

13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

Why? Linebreaks surrounding = can obfuscate the value of an assignment.

 // bad
 const foo =
 	superLongLongLongLongLongLongLongLongFunctionName();

 // bad
 const foo
 	= 'superLongLongLongLongLongLongLongLongString';

 // good
 const foo = (
 	superLongLongLongLongLongLongLongLongFunctionName()
 );

 // good
 const foo = 'superLongLongLongLongLongLongLongLongString';

13.8 Disallow unused variables. eslint: no-unused-vars

Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

 // bad

 var some_unused_var = 42;

 // Write-only variables are not considered as used.
 var y = 10;
 y = 5;

 // A read for a modification of itself is not considered as used.
 var z = 0;
 z = z + 1;

 // Unused function arguments.
 function getX(x, y) {
 		return x;
 }

 // good

 function getXPlusY(x, y) {
 	return x + y;
 }

 var x = 1;
 var y = a + 2;

 alert(getXPlusY(x, y));

 // 'type' is ignored even if unused because it has a rest property sibling.
 // This is a form of extracting an object that omits the specified keys.
 var { type, ...coords } = data;
 // 'coords' is now the 'data' object without its 'type' property.

⬆ back to top

Hoisting

14.1 var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

 // 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;
 }

14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

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

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

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

14.3 Named function expressions hoist the variable name, not the function name or the function body.

 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');
 	};
 }

14.4 Function declarations hoist their name and the function body.

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

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

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

⬆ back to top

Comparison Operators & Equality

15.1 Use === and !== over == and !=. eslint: eqeqeq

15.2 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
```
 if ([0] && []) {
 	// true
 	// an array (even an empty one) is an object, objects will evaluate to true
 }
```

15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

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

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

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

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

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

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

15.4 For more information see Truth Equality and JavaScript by Angus Croll.

15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

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.

 // 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 {}
 	}
 }

15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

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

 // split into 2 separated ternary expressions
 const maybeNull = value1 > value2 ? 'baz' : null;

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

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

15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

 // 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;

15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

Why? This improves readability and clarifies the developer’s intention.

 // bad
 const foo = a && b < 0 || c > 0 || d + 1 === 0;

 // bad
 const bar = a ** b - 5 % d;

 // bad
 // one may be confused into thinking (a || b) && c
 if (a || b && c) {
 	return d;
 }

 // bad
 const bar = a + b / c * d;

 // good
 const foo = (a && b < 0) || c > 0 || (d + 1 === 0);

 // good
 const bar = a ** b - (5 % d);

 // good
 if (a || (b && c)) {
 	return d;
 }

 // good
 const bar = a + (b / c) * d;

⬆ back to top

Blocks

16.1 Use braces with all multiline blocks. eslint: nonblock-statement-body-position

 // 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;
 }

16.2 If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

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

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

16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

 // bad
 function foo() {
 	if (x) {
 		return x;
 	} else {
 		return y;
 	}
 }

 // bad
 function cats() {
 	if (x) {
 		return x;
 	} else if (y) {
 		return y;
 	}
 }

 // bad
 function dogs() {
 	if (x) {
 		return x;
 	} else {
 		if (y) {
 			return y;
 		}
 	}
 }

 // good
 function foo() {
 	if (x) {
 		return x;
 	}

 	return y;
 }

 // good
 function cats() {
 	if (x) {
 		return x;
 	}

 	if (y) {
 		return y;
 	}
 }

 // good
 function dogs(x) {
 	if (x) {
 		if (z) {
 			return y;
 		}
 	} else {
 		return z;
 	}
 }

⬆ back to top

Control Statements

17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should be at end of the line.

 // bad
 if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
 	thing1();
 }

 // bad
 if (foo === 123 &&
 	bar === 'abc') {
 	thing1();
 }

 // bad
 if (foo === 123
 	&& bar === 'abc') {
 	thing1();
 }

 // bad
 if (
 	foo === 123
 	&& bar === 'abc'
 ) {
 	thing1();
 }

 // good
 if (
 	foo === 123 &&
 	bar === 'abc'
 ) {
 	thing1();
 }

 // good
 if (
 	(foo === 123 || bar === 'abc') &&
 	doesItLookGoodWhenItBecomesThatLong() &&
 	isThisReallyHappening()
 ) {
 	thing1();
 }

 // good
 if (foo === 123 && bar === 'abc') {
 	thing1();
 }

17.2 Don't use selection operators in place of control statements.

 // bad
 !isRunning && startRunning();

 // good
 if (!isRunning) {
 	startRunning();
 }

⬆ back to top

Comments

18.1 Use /** ... */ for multiline comments.

 // 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;
 }

18.2 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.

 // 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;
 }

18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

 // 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;
 }

18.4 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.

18.5 Use // FIXME: to annotate problems.

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

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

18.6 Use // TODO: to annotate solutions to problems.

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

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

⬆ back to top

Whitespace

19.1 Use hard tabs (tab character). eslint: indent

Why? Hard tabs allow the individual to specify the level of indentation to fit their preference within their IDE. Using soft tabs (space character) to force everyone to use the same level of indentation is an accessibility concern for those with visual impairment that may require larger or smaller indentation (see this pull request in the Rome repository, inspired by this reddit thread).
```
 // bad
 function foo() {
 ∙∙∙∙let name;
 }

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

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

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

19.2 Place 1 space before the leading brace. eslint: space-before-blocks

 // 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',
 });

19.3 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

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

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

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

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

19.4 Set off operators with spaces. eslint: space-infix-ops

 // bad
 const x=y+5;

 // good
 const x = y + 5;

19.5 End files with a single newline character. eslint: eol-last

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

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

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

19.6 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 no-whitespace-before-property

 // 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);

19.7 Leave a blank line after blocks and before the next statement.

 // 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;

19.8 Do not pad your blocks with blank lines. eslint: padded-blocks

 // bad
 function bar() {

 	console.log(foo);

 }

 // bad
 if (baz) {

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

 }

 // bad
 class Foo {

 	constructor(bar) {
 		this.bar = bar;
 	}
 }

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

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

19.9 Do not use multiple blank lines to pad your code. eslint: no-multiple-empty-lines

 // bad
 class Person {
 	constructor(fullName, email, birthday) {
 		this.fullName = fullName;


 		this.email = email;


 		this.setAge(birthday);
 	}


 	setAge(birthday) {
 		const today = new Date();


 		const age = this.getAge(today, birthday);


 		this.age = age;
 	}


 	getAge(today, birthday) {
 		// ..
 	}
 }

 // good
 class Person {
 	constructor(fullName, email, birthday) {
 		this.fullName = fullName;
 		this.email = email;
 		this.setAge(birthday);
 	}

 	setAge(birthday) {
 		const today = new Date();
 		const age = getAge(today, birthday);
 		this.age = age;
 	}

 	getAge(today, birthday) {
 		// ..
 	}
 }

19.10 Do not add spaces inside parentheses. eslint: space-in-parens

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

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

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

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

19.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

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

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

19.12 Add spaces inside curly braces. eslint: object-curly-spacing

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

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

19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

Why? This ensures readability and maintainability.

 // 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: 'John' },
 })
 	.done(() => console.log('Congratulations!'))
 	.fail(() => console.log('You have failed this city.'));

19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing
```
 // bad
 function foo() {return true;}
 if (foo) { bar = 0;}

 // good
 function foo() { return true; }
 if (foo) { bar = 0; }
```

19.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

 // bad
 var foo = 1,bar = 2;
 var arr = [1 , 2];

 // good
 var foo = 1, bar = 2;
 var arr = [1, 2];

19.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

 // bad
 obj[foo ]
 obj[ 'foo']
 var x = {[ b ]: a}
 obj[foo[ bar ]]

 // good
 obj[foo]
 obj['foo']
 var x = { [b]: a }
 obj[foo[bar]]

19.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing
```
 // bad
 func ();

 func
 ();

 // good
 func();
```

19.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

 // bad
 var obj = { foo : 42 };
 var obj2 = { foo:42 };

 // good
 var obj = { foo: 42 };

19.19 Avoid trailing spaces at the end of lines. eslint: no-trailing-spaces

19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

 // bad - multiple empty lines
 var x = 1;


 var y = 2;

 // bad - 2+ newlines at end of file
 var x = 1;
 var y = 2;


 // bad - 1+ newline(s) at beginning of file

 var x = 1;
 var y = 2;

 // good
 var x = 1;
 var y = 2;

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Commas

20.1 Leading commas: Nope. eslint: comma-style

 // bad
 const story = [
 		once
 	, upon
 	, aTime
 ];

 // good
 const story = [
 	once,
 	upon,
 	aTime,
 ];

 // bad
 const hero = {
 		firstName: 'Ada'
 	, lastName: 'Lovelace'
 	, birthYear: 1815
 	, superPower: 'computers'
 };

 // good
 const hero = {
 	firstName: 'Ada',
 	lastName: 'Lovelace',
 	birthYear: 1815,
 	superPower: 'computers',
 };

20.2 Additional trailing comma: Yup. eslint: comma-dangle

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

 // bad - git diff without trailing comma
 const hero = {
 			firstName: 'Florence',
 -    lastName: 'Nightingale'
 +    lastName: 'Nightingale',
 +    inventorOf: ['coxcomb chart', 'modern nursing']
 };

 // good - git diff with trailing comma
 const hero = {
 			firstName: 'Florence',
 			lastName: 'Nightingale',
 +    inventorOf: ['coxcomb chart', 'modern nursing'],
 };

 // bad
 const hero = {
 	firstName: 'Dana',
 	lastName: 'Scully'
 };

 const heroes = [
 	'Batman',
 	'Superman'
 ];

 // good
 const hero = {
 	firstName: 'Dana',
 	lastName: 'Scully',
 };

 const heroes = [
 	'Batman',
 	'Superman',
 ];

 // bad
 function createHero(
 	firstName,
 	lastName,
 	inventorOf
 ) {
 	// does nothing
 }

 // good
 function createHero(
 	firstName,
 	lastName,
 	inventorOf,
 ) {
 	// does nothing
 }

 // good (note that a comma must not appear after a "rest" element)
 function createHero(
 	firstName,
 	lastName,
 	inventorOf,
 	...heroArgs
 ) {
 	// does nothing
 }

 // bad
 createHero(
 	firstName,
 	lastName,
 	inventorOf
 );

 // good
 createHero(
 	firstName,
 	lastName,
 	inventorOf,
 );

 // good (note that a comma must not appear after a "rest" element)
 createHero(
 	firstName,
 	lastName,
 	inventorOf,
 	...heroArgs
 );

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Semicolons

21.1 Yup. eslint: semi

Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

 // bad - raises exception
 const luke = {}
 const leia = {}
 [luke, leia].forEach((jedi) => jedi.father = 'vader')

 // bad - raises exception
 const reaction = "No! That’s impossible!"
 (async function meanwhileOnTheFalcon() {
 	// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
 	// ...
 }())

 // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
 function foo() {
 	return
 		'search your feelings, you know it to be foo'
 }

 // good
 const luke = {};
 const leia = {};
 [luke, leia].forEach((jedi) => {
 	jedi.father = 'vader';
 });

 // good
 const reaction = "No! That’s impossible!";
 (async function meanwhileOnTheFalcon() {
 	// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
 	// ...
 }());

 // good
 function foo() {
 	return 'search your feelings, you know it to be foo';
 }

Type Casting & Coercion

22.1 Perform type coercion at the beginning of the statement.

22.2 Strings: eslint: no-new-wrappers

 // => this.reviewScore = 9;

 // bad
 const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"

 // bad
 const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()

 // bad
 const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string

 // good
 const totalScore = String(this.reviewScore);

22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.
```
 const inputValue = '4';

 // bad
 const val = new Number(inputValue);

 // bad
 const val = +inputValue;

 // bad
 const val = inputValue >> 0;

 // bad
 const val = parseInt(inputValue);

 // good
 const val = Number(inputValue);

 // good
 const val = parseInt(inputValue, 10);
```

22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.

 // good
 /**
  * parseInt was the reason my code was slow.
  * Bitshifting the String to coerce it to a
  * Number made it a lot faster.
  */
 const val = inputValue >> 0;

22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
 2147483647 >> 0; // => 2147483647
 2147483648 >> 0; // => -2147483648
 2147483649 >> 0; // => -2147483647
```

22.6 Booleans: eslint: no-new-wrappers

 const age = 0;

 // bad
 const hasAge = new Boolean(age);

 // good
 const hasAge = Boolean(age);

 // best
 const hasAge = !!age;

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Naming Conventions

23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

 // bad
 function q() {
 	// ...
 }

 // good
 function query() {
 	// ...
 }

23.2 Use camelCase when naming objects, functions, and instances. eslint: @typescript-eslint/naming-convention

 // bad
 const OBJEcttsssss = {};
 const this_is_my_object = {};
 function c() {}

 // good
 const thisIsMyObject = {};
 function thisIsMyFunction() {}

23.3 Use PascalCase only when naming constructors, classes, or enums. eslint: @typescript-eslint/naming-convention

 // bad
 function user(options) {
 	this.name = options.name;
 }

 const bad = new user({
 	name: 'nope',
 });

 // bad
 enum userType {
 	admin,
 	user,
 	anonymous
 }

 // good
 class User {
 	constructor(options) {
 		this.name = options.name;
 	}
 }

 const good = new User({
 	name: 'yup',
 });

 // good
 enum UserType {
 	Admin,
 	User,
 	Anonymous
 }

23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
 // bad
 this.__firstName__ = 'Panda';
 this.firstName_ = 'Panda';
 this._firstName = 'Panda';

 // good
 this.firstName = 'Panda';

 // good, in environments where WeakMaps are available
 // see https://kangax.github.io/compat-table/es6/#test-WeakMap
 const firstNames = new WeakMap();
 firstNames.set(this, 'Panda');
```

23.5 Don’t save references to this. Use arrow functions or Function#bind.

 // bad
 function foo() {
 	const self = this;
 	return function () {
 		console.log(self);
 	};
 }

 // bad
 function foo() {
 	const that = this;
 	return function () {
 		console.log(that);
 	};
 }

 // good
 function foo() {
 	return () => {
 		console.log(this);
 	};
 }

23.6 A base filename should exactly match the name of its default export.

 // file 1 contents
 class CheckBox {
 	// ...
 }
 export default CheckBox;

 // file 2 contents
 export default function fortyTwo() { return 42; }

 // file 3 contents
 export default function insideDirectory() {}

 // in some other file
 // bad
 import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
 import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
 import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

 // bad
 import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
 import forty_two from './forty_two'; // snake_case import/filename, camelCase export
 import inside_directory from './inside_directory'; // snake_case import, camelCase export
 import index from './inside_directory/index'; // requiring the index file explicitly
 import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

 // good
 import CheckBox from './CheckBox'; // PascalCase export/import/filename
 import fortyTwo from './fortyTwo'; // camelCase export/import/filename
 import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
 // ^ supports both insideDirectory.js and insideDirectory/index.js

23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
```
 function makeStyleGuide() {
 	// ...
 }

 export default makeStyleGuide;
```

23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object / enum.
```
 const AirbnbStyleGuide = {
 	es6: {
 	},
 };

 export default AirbnbStyleGuide;
```

23.9 Acronyms and initialisms should always be all uppercased, or all lowercased.

Why? Names are for readability, not to appease a computer algorithm.

 // bad
 import SmsContainer from './containers/SmsContainer';

 // bad
 const HttpRequests = [
 	// ...
 ];

 // good
 import SMSContainer from './containers/SMSContainer';

 // good
 const HTTPRequests = [
 	// ...
 ];

 // also good
 const httpRequests = [
 	// ...
 ];

 // best
 import TextMessageContainer from './containers/TextMessageContainer';

 // best
 const requests = [
 	// ...
 ];

23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.

Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.
- What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
- What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
```
 // bad
 const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';

 // bad
 export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';

 // bad
 export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';

 // ---

 // allowed but does not supply semantic value
 export const apiKey = 'SOMEKEY';

 // better in most cases
 export const API_KEY = 'SOMEKEY';

 // ---

 // bad - unnecessarily uppercases key while adding no semantic value
 export const MAPPING = {
 	KEY: 'value'
 };

 // good
 export const MAPPING = {
 	key: 'value'
 };
```

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Accessors

24.1 Accessor functions for properties are not required.

24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

 // bad
 class Dragon {
 	get age() {
 		// ...
 	}

 	set age(value) {
 		// ...
 	}
 }

 // good
 class Dragon {
 	getAge() {
 		// ...
 	}

 	setAge(value) {
 		// ...
 	}
 }

24.3 If the property/method is a boolean, use isVal() or hasVal().

 // bad
 if (!dragon.age()) {
 	return false;
 }

 // good
 if (!dragon.hasAge()) {
 	return false;
 }

24.4 It’s okay to create get() and set() functions, but be consistent.

 class Jedi {
 	constructor(options = {}) {
 		const lightsaber = options.lightsaber || 'blue';
 		this.set('lightsaber', lightsaber);
 	}

 	set(key, val) {
 		this[key] = val;
 	}

 	get(key) {
 		return this[key];
 	}
 }

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Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

 // bad
 $(this).trigger('listingUpdated', listing.id);

 // ...

 $(this).on('listingUpdated', (e, listingID) => {
 	// do something with listingID
 });

prefer:

 // good
 $(this).trigger('listingUpdated', { listingID: listing.id });

 // ...

 $(this).on('listingUpdated', (e, data) => {
 	// do something with data.listingID
 });

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ECMAScript 5 Compatibility

26.1 Refer to Kangax’s ES5 compatibility table.

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ECMAScript 6+ (ES 2015+) Styles

27.1 This is a collection of links to the various ES6+ features.

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
Exponentiation Operator
Iterators and Generators
Modules

27.2 Do not use TC39 proposals that have not reached stage 3.

Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

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Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

28.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.
```
 // bad
 isNaN('1.2'); // false
 isNaN('1.2.3'); // true

 // good
 Number.isNaN('1.2.3'); // false
 Number.isNaN(Number('1.2.3')); // true
```

28.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.
```
 // bad
 isFinite('2e3'); // true

 // good
 Number.isFinite('2e3'); // false
 Number.isFinite(parseInt('2e3', 10)); // true
```

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Testing

29.1 Yup.
```
 function foo() {
 	return true;
 }
```

29.2 No, but seriously:
- Whichever testing framework you use, you should be writing tests!
- Strive to write many small pure functions, and minimize where mutations occur.
- Be cautious about stubs and mocks - they can make your tests more brittle.
- We primarily use mocha and jest at Airbnb. tape is also used occasionally for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
- Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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Resources

Learning ES6+

Read This

Standard ECMA-262

Tools

Code Style Linters
- ESlint - Airbnb Style .eslintrc
- JSHint - Airbnb Style .jshintrc
Neutrino Preset - @neutrinojs/airbnb

Other Style Guides

Other Styles

Naming this in nested functions - Christian Johansen
Conditional Callbacks - Ross Allen
Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

Understanding JavaScript Closures - Angus Croll
Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
You Might Not Need jQuery - Zack Bloom & Adam Schwartz
ES6 Features - Luke Hoban
Frontend Guidelines - Benjamin De Cock

Books

JavaScript: The Good Parts - Douglas Crockford
JavaScript Patterns - Stoyan Stefanov
Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
Maintainable JavaScript - Nicholas C. Zakas
JavaScript Web Applications - Alex MacCaw
Pro JavaScript Techniques - John Resig
Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
Human JavaScript - Henrik Joreteg
Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
JSBooks - Julien Bouquillon
Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
Eloquent JavaScript - Marijn Haverbeke
You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

Podcasts

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The JavaScript Style Guide Guide

Reference

Chat With Us About JavaScript

Find us on gitter.

Contributors

View Contributors

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

⬆ back to top

Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

Name		Name	Last commit message	Last commit date
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css-in-javascript		css-in-javascript
linters		linters
packages		packages
react		react
.editorconfig		.editorconfig
.gitignore		.gitignore
.npmrc		.npmrc
.travis.yml		.travis.yml
LICENSE.md		LICENSE.md
README.md		README.md
package.json		package.json

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Imagine Learning TypeScript Style Guide() {

Table of Contents

Types

References

Objects

Arrays

Destructuring

Strings

Functions

Arrow Functions

Classes & Constructors

Modules

Iterators and Generators

Properties

Variables

Hoisting

Comparison Operators & Equality

Blocks

Control Statements

Comments

Whitespace

Commas

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Events

ECMAScript 5 Compatibility

ECMAScript 6+ (ES 2015+) Styles

Standard Library

Testing

Performance

Resources

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

Amendments

};

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