https://github.com/meteor/javascript
JavaScript Style Guide
https://github.com/meteor/javascript
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JavaScript Style Guide
- Host: GitHub
- URL: https://github.com/meteor/javascript
- Owner: meteor
- Fork: true (airbnb/javascript)
- Created: 2015-06-25T03:11:54.000Z (over 10 years ago)
- Default Branch: master
- Last Pushed: 2017-11-12T14:59:54.000Z (about 8 years ago)
- Last Synced: 2025-09-19T16:12:25.962Z (3 months ago)
- Language: JavaScript
- Size: 655 KB
- Stars: 25
- Watchers: 19
- Forks: 14
- Open Issues: 2
-
Metadata Files:
- Readme: README.md
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# Airbnb JavaScript Style Guide, Meteor edition
*A mostly reasonable approach to JavaScript, with some Meteor flavor*
All rules that are automatically linted have the ESLint logo, with a link to the rule, like so: 
## 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. [Constructors](#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 Styles](#ecmascript-6-styles)
1. [Testing (in AirBnB guide)](https://github.com/airbnb/javascript#testing)
1. [Performance (in AirBnB guide)](https://github.com/airbnb/javascript#performance)
1. [Resources (in AirBnB guide)](https://github.com/airbnb/javascript#resources)
1. [In the Wild (in AirBnB guide)](https://github.com/airbnb/javascript#in-the-wild)
1. [Translation (in AirBnB guide)](https://github.com/airbnb/javascript#translation)
1. [The JavaScript Style Guide Guide (in AirBnB guide)](https://github.com/airbnb/javascript#the-javascript-style-guide-guide)
1. [Chat With Us About Javascript (in AirBnB guide)](https://github.com/airbnb/javascript#chat-with-us-about-javascript)
1. [Contributors](#contributors)
1. [License](#license)
## Types
This section has been eliminated in the Meteor version, because it does not specify any style rules.
## References
- [2.1](#2.1)
Use `const` for all of your references; avoid using `var`.
> Why? This ensures that you can't reassign your references (mutation), 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;
```
- [2.2](#2.2)
If you must mutate references, use `let` instead of `var`.
> 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;
}
```
- [2.3](#2.3) This section removed in the Meteor edition, because it does not specify any style rules.
**[⬆ back to top](#table-of-contents)**
## Objects
- [3.1](#3.1)
Use the literal syntax for object creation.
```javascript
// bad
const item = new Object();
// good
const item = {};
```
- [3.2](#3.2) and [3.3](#3.3) eliminated in the Meteor edition because Babel can compile reserved words to work fine in old browsers.
- [3.4](#3.4) 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,
};
```
- [3.5](#3.5) Use object method shorthand.
```javascript
// bad
const atom = {
value: 1,
addValue: function (value) {
return atom.value + value;
},
};
// good
const atom = {
value: 1,
addValue(value) {
return atom.value + value;
},
};
```
- [3.6](#3.6) Use property value shorthand.
> Why? It is shorter to write and descriptive.
```javascript
const lukeSkywalker = 'Luke Skywalker';
// bad
const obj = {
lukeSkywalker: lukeSkywalker,
};
// good
const obj = {
lukeSkywalker,
};
```
- [3.7](#3.7) 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,
twoJedisWalkIntoACantina: 2,
lukeSkywalker,
episodeThree: 3,
mayTheFourth: 4,
anakinSkywalker,
};
// good
const obj = {
lukeSkywalker,
anakinSkywalker,
episodeOne: 1,
twoJedisWalkIntoACantina: 2,
episodeThree: 3,
mayTheFourth: 4,
};
```
- [3.8](#3.8) Use object spreads `...` to copy objects.
```javascript
// bad
const copy = {};
Object.keys(obj).forEach((key) => {
copy[key] = obj[key];
});
// bad
const copy = {};
for (var key in obj) {
if (Object.prototype.hasOwnProperty.call(obj, key)) {
copy[key] = obj[key];
}
}
// good
const copy = { ...obj };
```
**[⬆ back to top](#table-of-contents)**
## Arrays
- [4.1](#4.1)
Use the literal syntax for array creation.
```javascript
// bad
const items = new Array();
// good
const items = [];
```
- [4.2](#4.2) Use Array#push instead of direct assignment to add items to an array.
```javascript
const someStack = [];
// bad
someStack[someStack.length] = 'abracadabra';
// good
someStack.push('abracadabra');
```
- [4.3](#4.3) Use array spreads `...` to copy arrays.
```javascript
// bad
const len = items.length;
const itemsCopy = [];
let i;
for (i = 0; i < len; i++) {
itemsCopy[i] = items[i];
}
// good
const itemsCopy = [...items];
```
- [4.4](#4.4) To convert an array-like object to an array, use Array#from.
```javascript
const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);
```
**[⬆ back to top](#table-of-contents)**
## Destructuring
- [5.1](#5.1) Use object destructuring when accessing and using multiple properties of an object.
> 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} ${lastName}`;
}
// good
function getFullName(obj) {
const { firstName, lastName } = obj;
return `${firstName} ${lastName}`;
}
// best
function getFullName({ firstName, lastName }) {
return `${firstName} ${lastName}`;
}
```
- [5.2](#5.2) Use array destructuring.
```javascript
const arr = [1, 2, 3, 4];
// bad
const first = arr[0];
const second = arr[1];
// good
const [first, second] = arr;
```
- [5.3](#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.
```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, right } = processInput(input);
```
**[⬆ back to top](#table-of-contents)**
## Strings
- [6.1](#6.1)
Use single quotes `''` for strings.
```javascript
// bad
const name = "Capt. Janeway";
// good
const name = 'Capt. Janeway';
```
- [6.2](#6.2)
Single-line strings longer than 80 characters should be written across multiple lines using string concatenation. If you are writing a multiline string, use template strings as in [6.4](#6.4). This is a change in the Meteor edition. Note: If overused, long strings with concatenation could impact performance. [jsPerf](http://jsperf.com/ya-string-concat) & [Discussion](https://github.com/airbnb/javascript/issues/40).
```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.';
```
- [6.3](#6.3) This non-rule merged as a note into the previous rule in the Meteor edition.
- [6.4](#6.4) When programmatically building up strings, use template strings instead of concatenation.
> 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();
}
// good
function sayHi(name) {
return `How are you, ${name}?`;
}
```
**[⬆ back to top](#table-of-contents)**
## Functions
- [7.1](#7.1)
Use function declarations instead of function expressions.
> Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use [Arrow Functions](#arrow-functions) in place of function expressions.
```javascript
// bad
const foo = function () {
};
// good
function foo() {
}
```
- [7.2](#7.2) Removed in the Meteor edition.
- [7.3](#7.3)
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. [7.4](#7.4) **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). This note merged into the previous rule in the Meteor edition.
```javascript
// bad
if (currentUser) {
function test() {
console.log('Nope.');
}
}
// good
let test;
if (currentUser) {
test = () => {
console.log('Yup.');
};
}
```
- [7.5](#7.5)
Never name a parameter `arguments`. This will take precedence over the `arguments` object that is given to every function scope.
```javascript
// bad
function nope(name, options, arguments) {
// ...stuff...
}
// good
function yup(name, options, args) {
// ...stuff...
}
```
- [7.6](#7.6) Never use `arguments`, opt to use rest syntax `...` instead.
> Why? `...` is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not 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('');
}
```
- [7.7](#7.7) 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 = {}) {
// ...
}
```
- [7.8](#7.8) 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
```
- [7.9](#7.9) Use argument spreads to interpolate
arguments in function calls.
```javascript
const prefix = [a, b];
const suffix = [c, d, e];
// bad
prefix.push.apply(prefix, suffix);
// good
prefix.push(...suffix);
// bad
someFunction.apply(null, prefix.concat(suffix));
// good
someFunction(...prefix, ...suffix);
```
The exception to this advice is when you really need to use a
different, non-`null` value of `this`. Then `.apply` (or `.call`) is
probably a better option.
The same goes for `new` expressions.
```javascript
class A {
constructor(...args) { ... }
}
// really bad
const instance = Object.create(A.prototype);
A.prototype.constructor.apply(instance, prefix.concat(suffix));
// good
const instance = new A(...prefix, ...suffix);
```
**[⬆ back to top](#table-of-contents)**
## Arrow Functions
- [8.1](#8.1) When you must use function expressions (as when passing an anonymous function), use arrow function notation.
> 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) {
return x * x;
});
// good
[1, 2, 3].map((x) => {
return x * x;
});
```
- [8.2](#8.2) If the function body fits on one line and there is only a single argument, feel free to omit the braces and parentheses, and use the implicit return. Otherwise, add the parentheses, braces, and use a `return` statement.
> Why? Syntactic sugar. It reads well when multiple functions are chained together.
> Why not? If you plan on returning an object.
```javascript
// good
[1, 2, 3].map(x => x * x);
// good
[1, 2, 3].reduce((total, n) => {
return total + n;
}, 0);
```
**[⬆ back to top](#table-of-contents)**
## Constructors
- [9.1](#9.1) 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;
}
}
```
- [9.2](#9.2) 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];
}
}
```
- [9.3](#9.3) 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);
```
- [9.4](#9.4) It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
```javascript
class Jedi {
contructor(options = {}) {
this.name = options.name || 'no name';
}
getName() {
return this.name;
}
toString() {
return `Jedi - ${this.getName()}`;
}
}
```
**[⬆ back to top](#table-of-contents)**
## Modules
- [10.1](#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.
```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;
```
- [10.2](#10.2) 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';
```
- [10.3](#10.3) 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;
```
**[⬆ back to top](#table-of-contents)**
## Iterators and Generators
- [11.1](#11.1) Don't use iterators. Prefer JavaScript's higher-order functions like `map()` and `reduce()` instead of loops like `for-of`.
> Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side-effects.
```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;
```
- [11.2](#11.2) Don't use generators for now.
> Why? They don't transpile well to ES5.
**[⬆ back to top](#table-of-contents)**
## Properties
- [12.1](#12.1) Use dot notation when accessing properties.
```javascript
const luke = {
jedi: true,
age: 28,
};
// bad
const isJedi = luke['jedi'];
// good
const isJedi = luke.jedi;
```
- [12.2](#12.2) Use subscript 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](#table-of-contents)**
## Variables
- [13.1](#13.1) Always use `const` 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.
```javascript
// bad
superPower = new SuperPower();
// good
const superPower = new SuperPower();
```
- [13.2](#13.2) Use one `const` declaration per variable.
> 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.
```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';
```
- [13.3](#13.3) 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;
```
- [13.4](#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.
```javascript
// good
function() {
test();
console.log('doing stuff..');
//..other stuff..
const name = getName();
if (name === 'test') {
return false;
}
return name;
}
// bad - unnessary function call
function(hasName) {
const name = getName();
if (!hasName) {
return false;
}
this.setFirstName(name);
return true;
}
// good
function(hasName) {
if (!hasName) {
return false;
}
const name = getName();
this.setFirstName(name);
return true;
}
```
**[⬆ back to top](#table-of-contents)**
## Hoisting
- [14.1](#14.1) [14.2](#14.2) [14.3](#14.3) [14.4](#14.4) Non-rules removed in the Meteor edition.
- For more information refer to [JavaScript Scoping & Hoisting](http://www.adequatelygood.com/2010/2/JavaS cript-Scoping-and-Hoisting) by [Ben Cherry](http://www.adequatelygood.com/).
**[⬆ back to top](#table-of-contents)**
## Comparison Operators & Equality
- [15.1](#15.1) Use `===` and `!==` over `==` and `!=`.
- [15.2](#15.2)/[15.3](#15.3)/[15.4](#15.4)
Use shortcuts. Note: Conditional statements such as the `if` statement evaluate their expression using coercion with the `ToBoolean` abstract method and always follow the simple rules below. For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll. (15.2, 15.3, and 15.4 merged in Meteor edition):
+ **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
// bad
if (name !== '') {
// ...stuff...
}
// good
if (name) {
// ...stuff...
}
// bad
if (collection.length > 0) {
// ...stuff...
}
// good
if (collection.length) {
// ...stuff...
}
// note
if ([0]) {
// true
// An array is an object, objects evaluate to true
}
```
**[⬆ back to top](#table-of-contents)**
## Blocks
- [16.1](#16.1)
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() { return false; }
// good
function() {
return false;
}
```
- [16.2](#16.2)
If you're using multi-line blocks with `if` and `else`, put `else` on the same line as your
`if` block's closing brace.
```javascript
// bad
if (test) {
thing1();
thing2();
}
else {
thing3();
}
// good
if (test) {
thing1();
thing2();
} else {
thing3();
}
```
**[⬆ back to top](#table-of-contents)**
## Comments
- [17.1](#17.1) Use `/** ... */` for method doc comments. Include a description, specify types and values for all parameters and return values. In the Meteor edition, this rule has been updated to only refer to doc comments.
```javascript
// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {
// ...stuff...
return element;
}
// good
/**
* make() returns a new element
* based on the passed in tag name
*
* @param {String} tag
* @return {Element} element
*/
function make(tag) {
// ...stuff...
return element;
}
```
- [17.2](#17.2) Use `//` for all other comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment. Has been updated in Meteor edition to refer to all non-doc comments.
```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;
}
```
- [17.3](#17.3) 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`.
- [17.4](#17.4) Use `// FIXME:` to annotate problems.
```javascript
class Calculator {
constructor() {
// FIXME: shouldn't use a global here
total = 0;
}
}
```
- [17.5](#17.5) Use `// TODO:` to annotate solutions to problems.
```javascript
class Calculator {
constructor() {
// TODO: total should be configurable by an options param
this.total = 0;
}
}
```
**[⬆ back to top](#table-of-contents)**
## Whitespace
- [18.1](#18.1)
Use soft tabs set to 2 spaces.
```javascript
// bad
function() {
∙∙∙∙const name;
}
// bad
function() {
∙const name;
}
// good
function() {
∙∙const name;
}
```
- [18.2](#18.2)
Place 1 space before the leading brace.
```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',
});
```
- [18.3](#18.3)
Place 1 space before the opening parenthesis in control statements (`if`, `while` etc.). Place no space before the argument list in function calls and declarations.
```javascript
// bad
if(isJedi) {
fight ();
}
// good
if (isJedi) {
fight();
}
// bad
function fight () {
console.log ('Swooosh!');
}
// good
function fight() {
console.log('Swooosh!');
}
```
- [18.4](#18.4)
Set off operators with spaces.
```javascript
// bad
const x=y+5;
// good
const x = y + 5;
```
- [18.5](#18.5) End files with a single newline character.
```javascript
// bad
(function(global) {
// ...stuff...
})(this);
```
```javascript
// bad
(function(global) {
// ...stuff...
})(this);↵
↵
```
```javascript
// good
(function(global) {
// ...stuff...
})(this);↵
```
- [18.5](#18.5) Use indentation when making long method chains. Use a leading dot, which
emphasizes that the line is a method call, not a new statement.
```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').class('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);
```
- [18.6](#18.6) Leave a blank line after blocks and before the next statement.
```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;
```
**[⬆ back to top](#table-of-contents)**
## Commas
- [19.1](#19.1)
Leading commas: **Nope.**
```javascript
// 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',
};
```
- [19.2](#19.2)
Additional trailing comma: **Yup.**
> 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](es5/README.md#commas) in legacy browsers.
```javascript
// bad - git diff without trailing comma
const hero = {
firstName: 'Florence',
- lastName: 'Nightingale'
+ lastName: 'Nightingale',
+ inventorOf: ['coxcomb graph', 'mordern nursing']
}
// good - git diff with trailing comma
const hero = {
firstName: 'Florence',
lastName: 'Nightingale',
+ inventorOf: ['coxcomb chart', 'mordern nursing'],
}
// bad
const hero = {
firstName: 'Dana',
lastName: 'Scully'
};
const heroes = [
'Batman',
'Superman'
];
// good
const hero = {
firstName: 'Dana',
lastName: 'Scully',
};
const heroes = [
'Batman',
'Superman',
];
```
**[⬆ back to top](#table-of-contents)**
## Semicolons
```javascript
// bad
(function() {
const name = 'Skywalker'
return name
})()
// good
(() => {
const name = 'Skywalker';
return name;
})();
// good (guards against the function becoming an argument when two files with IIFEs are concatenated)
;(() => {
const name = 'Skywalker';
return name;
})();
```
[Read more](http://stackoverflow.com/a/7365214/1712802).
**[⬆ back to top](#table-of-contents)**
## Type Casting & Coercion
- [21.1](#21.1) Perform type coercion at the beginning of the statement.
- [21.2](#21.2) Strings:
```javascript
// => this.reviewScore = 9;
// bad
const totalScore = this.reviewScore + '';
// good
const totalScore = String(this.reviewScore);
```
- [21.3](#21.3)
Use `parseInt` to convert strings to numbers, and always with a radix argument.
```javascript
const inputValue = '4';
// bad
const val = new Number(inputValue);
// bad
const val = inputValue >> 0;
// bad
const val = parseInt(inputValue);
// good
const val = parseInt(inputValue, 10);
```
Use the `Number` constructor (without `new`) to coerce values that are
not strings to numbers, as in `Number(new Date) + 1000`. The unary `+`
operator is an acceptable shorthand for `Number(...)`, but only if the
expression is not involved in a larger expression, as in `+new Date`.
Note that the `Number` constructor will return `NaN` if the value
cannot be converted to a number, including when the value is
undefined. If the output might be `NaN` (likely because the input
might be undefined), be sure to test for that possibility after
attempting the conversion:
```javascript
let val = Number(inputValue);
if (isNaN(val)) {
val = 0;
}
```
Better yet, avoid the possibility of `NaN` through other means, such
as providing default values for optional function parameters.
- [21.4](#21.4) If you have a very good reason for
using some other kind of coercion technique, be it for performance or
because you need a very specific output behavior, then you should
absolutely leave a comment justifying your choice.
```javascript
// 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;
// good
// Truthy inputs must always be coerced to 1, and falsy inputs must
// always be cocerced to 0.
const zeroOrOne = inputValue ? 1 : 0;
```
- [21.5](#21.5) **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109). Largest signed 32-bit Int is 2,147,483,647:
```javascript
2147483647 >> 0 //=> 2147483647
2147483648 >> 0 //=> -2147483648
2147483649 >> 0 //=> -2147483647
```
```javascript
const age = 0;
// bad
const hasAge = new Boolean(age);
// good
const hasAge = Boolean(age);
// good
const hasAge = !!age;
```
**[⬆ back to top](#table-of-contents)**
## Naming Conventions
- [22.1](#22.1) Avoid single letter names. Be descriptive with your naming.
```javascript
// bad
function q() {
// ...stuff...
}
// good
function query() {
// ..stuff..
}
```
- [22.2](#22.2)
Use camelCase when naming objects, functions, and instances.
```javascript
// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}
// good
const thisIsMyObject = {};
function thisIsMyFunction() {}
```
- [22.3](#22.3) Use PascalCase when naming constructors or classes.
```javascript
// bad
function user(options) {
this.name = options.name;
}
const bad = new user({
name: 'nope',
});
// good
class User {
constructor(options) {
this.name = options.name;
}
}
const good = new User({
name: 'yup',
});
```
- [22.4](#22.4) Use a leading underscore `_` when naming private properties.
```javascript
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
// good
this._firstName = 'Panda';
```
- [22.5](#22.5) Don't save references to `this`. Use arrow functions or Function#bind.
```javascript
// 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);
};
}
```
- [22.6](#22.6) If your file exports a single class, your filename should be exactly the name of the class.
```javascript
// file contents
export default class CheckBox {
// ...
}
// in some other file
// bad
import CheckBox from './checkBox';
// bad
import CheckBox from './check_box';
// good
import CheckBox from './CheckBox';
```
- [22.7](#22.7) Use camelCase when you export-default a function. Your filename should be identical to your function's name.
```javascript
function makeStyleGuide() {
}
export default makeStyleGuide;
```
- [22.8](#22.8) Use PascalCase when you export a singleton / function library / bare object.
```javascript
const AirbnbStyleGuide = {
es6: {
}
};
export default AirbnbStyleGuide;
```
- [22.9](#22.9) Prefer `export`ing declarations
where they are declared, rather than at the end of the file:
```javascript
// bad
function createUser(name) { ... }
function getOrCreateUser(name) { ... }
// ... rest of file ...
export {
createUser,
getOrCreateUser,
}
// good
export function createUser(name) { ... }
export function getOrCreateUser(name) { ... }
// ... rest of file ...
```
This style ensures that the set of `export`s remains up-to-date as
declarations are added or removed.
**[⬆ back to top](#table-of-contents)**
## Accessors
- [23.1](#23.1) Accessor functions for properties are not required.
- [23.2](#23.2) If you do make accessor functions use getVal() and setVal('hello').
```javascript
// bad
dragon.age();
// good
dragon.getAge();
// bad
dragon.age(25);
// good
dragon.setAge(25);
```
- [23.3](#23.3) If the property is a boolean, use isVal() or hasVal().
```javascript
// bad
if (!dragon.age()) {
return false;
}
// good
if (!dragon.hasAge()) {
return false;
}
```
- [23.4](#23.4) It's okay to create get() and set() functions, but be consistent.
```javascript
class Jedi {
constructor(options = {}) {
const lightsaber = options.lightsaber || 'blue';
this.set('lightsaber', lightsaber);
}
set(key, val) {
this[key] = val;
}
get(key) {
return this[key];
}
}
```
- [23.5](#23.5) Strongly prefer accessor methods to
defining ES5 getter and setter properties:
```javascript
class Jedi {
constructor({ lightsaber = blue }) {
this._lightsaber = lightsaber;
}
// bad; use a normal getter method instead
get lightsaber() {
return this._lightsaber;
}
// bad; use a normal setter method instead
set lightsaber(newLightSaber) {
return this._lightsaber = newLightSaber;
}
}
```
Not only is this style rarely more readable than the method
equivalent, performance can suffer dramatically when code relies on
specially-defined properties rather than just using normal properties.
**[⬆ back to top](#table-of-contents)**
## Events
- [24.1](#24.1) When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass 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:
```javascript
// bad
$(this).trigger('listingUpdated', listing.id);
...
$(this).on('listingUpdated', function(e, listingId) {
// do something with listingId
});
```
prefer:
```javascript
// good
$(this).trigger('listingUpdated', { listingId : listing.id });
...
$(this).on('listingUpdated', function(e, data) {
// do something with data.listingId
});
```
**[⬆ back to top](#table-of-contents)**
## jQuery
- [25.1](#25.1) Prefix jQuery object variables with a `$`.
```javascript
// bad
const sidebar = $('.sidebar');
// good
const $sidebar = $('.sidebar');
```
- [25.2](#25.2) Cache jQuery lookups.
```javascript
// bad
function setSidebar() {
$('.sidebar').hide();
// ...stuff...
$('.sidebar').css({
'background-color': 'pink'
});
}
// good
function setSidebar() {
const $sidebar = $('.sidebar');
$sidebar.hide();
// ...stuff...
$sidebar.css({
'background-color': 'pink'
});
}
```
- [25.3](#25.3) For DOM queries use Cascading `$('.sidebar ul')` or parent > child `$('.sidebar > ul')`. [jsPerf](http://jsperf.com/jquery-find-vs-context-sel/16)
- [25.4](#25.4) Use `find` with scoped jQuery object queries.
```javascript
// bad
$('ul', '.sidebar').hide();
// bad
$('.sidebar').find('ul').hide();
// good
$('.sidebar ul').hide();
// good
$('.sidebar > ul').hide();
// good
$sidebar.find('ul').hide();
```
**[⬆ back to top](#table-of-contents)**
## ECMAScript 5 Compatibility
- [26.1](#26.1) Refer to [Kangax](https://twitter.com/kangax/)'s ES5 [compatibility table](http://kangax.github.com/es5-compat-table/).
**[⬆ back to top](#table-of-contents)**
## ECMAScript 6 Styles
- [27.1](#27.1) This is a collection of links to the various es6 features.
1. [Arrow Functions](#arrow-functions)
1. [Classes](#constructors)
1. [Object Shorthand](#es6-object-shorthand)
1. [Object Concise](#es6-object-concise)
1. [Object Computed Properties](#es6-computed-properties)
1. [Template Strings](#es6-template-literals)
1. [Destructuring](#destructuring)
1. [Default Parameters](#es6-default-parameters)
1. [Rest](#es6-rest)
1. [Array Spreads](#es6-array-spreads)
1. [Let and Const](#references)
1. [Iterators and Generators](#iterators-and-generators)
1. [Modules](#modules)
**[⬆ back to top](#table-of-contents)**
## Contributors
- [View Contributors](https://github.com/airbnb/javascript/graphs/contributors)
## License
(The MIT License)
Copyright (c) 2014 Airbnb
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](#table-of-contents)**
# };