A mostly reasonable approach to JavaScript
Other Style Guides
- Types
- References
- Objects
- Arrays
- Destructuring
- Strings
- Functions
- Arrow Functions
- Constructors
- Modules
- Iterators and Generators
- Properties
- Variables
- Hoisting
- Comparison Operators & Equality
- Blocks
- Comments
- Whitespace
- Commas
- Semicolons
- Type Casting & Coercion
- Naming Conventions
- Accessors
- Events
- jQuery
- ECMAScript 5 Compatibility
- ECMAScript 6 Styles
- Testing
- Performance
- Resources
- In the Wild
- Translation
- The JavaScript Style Guide Guide
- Chat With Us About JavaScript
- Contributors
- License
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1.1 Primitives: When you access a primitive type you work directly on its value.
stringnumberbooleannullundefined
const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 9
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1.2 Complex: When you access a complex type you work on a reference to its value.
objectarrayfunction
const foo = [1, 2]; const bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9
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2.1 Use
constfor all of your references; avoid usingvar. eslint:prefer-const,no-const-assignWhy? 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;
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2.2 If you must reassign references, use
letinstead ofvar. eslint:no-varjscs:disallowVarWhy?
letis block-scoped rather than function-scoped likevar.// bad var count = 1; if (true) { count += 1; } // good, use the let. let count = 1; if (true) { count += 1; }
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2.3 Note that both
letandconstare 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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3.1 Use the literal syntax for object creation. eslint:
no-new-object// bad const item = new Object(); // good const item = {};
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3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs:
disallowIdentiferNames// bad const superman = { default: { clark: 'kent' }, private: true, }; // good const superman = { defaults: { clark: 'kent' }, hidden: true, };
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3.3 Use readable synonyms in place of reserved words. jscs:
disallowIdentiferNames// bad const superman = { class: 'alien', }; // bad const superman = { klass: 'alien', }; // good const superman = { type: 'alien', };
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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.
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, };
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3.5 Use object method shorthand. eslint:
object-shorthandjscs:requireEnhancedObjectLiterals// bad const atom = { value: 1, addValue: function (value) { return atom.value + value; }, }; // good const atom = { value: 1, addValue(value) { return atom.value + value; }, };
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3.6 Use property value shorthand. eslint:
object-shorthandjscs:requireEnhancedObjectLiteralsWhy? It is shorter to write and descriptive.
const lukeSkywalker = 'Luke Skywalker'; // bad const obj = { lukeSkywalker: lukeSkywalker, }; // good const obj = { lukeSkywalker, };
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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.
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, };
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3.8 Only quote properties that are invalid identifiers. eslint:
quote-propsjscs:disallowQuotedKeysInObjects
Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
// bad
const bad = {
'foo': 3,
'bar': 4,
'data-blah': 5,
};
// good
const good = {
foo: 3,
bar: 4,
'data-blah': 5,
};-
4.1 Use the literal syntax for array creation. eslint:
no-array-constructor// bad const items = new Array(); // good const items = [];
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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');
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4.3 Use array spreads
...to copy arrays.// bad const len = items.length; const itemsCopy = []; let i; for (i = 0; i < len; i++) { itemsCopy[i] = items[i]; } // good const itemsCopy = [...items];
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4.4 To convert an array-like object to an array, use Array#from.
const foo = document.querySelectorAll('.foo'); const nodes = Array.from(foo);
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4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. 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 const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = memo.concat(item); }); // good const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = flatten; return flatten; }); // bad inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } else { return false; } }); // good inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } return false; });
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5.1 Use object destructuring when accessing and using multiple properties of an object. jscs:
requireObjectDestructuringWhy? Destructuring saves you from creating temporary references for those properties.
// bad function getFullName(user) { const firstName = user.firstName; const lastName = user.lastName; return `${firstName} ${lastName}`; } // good function getFullName(user) { const { firstName, lastName } = user; return `${firstName} ${lastName}`; } // best function getFullName({ firstName, lastName }) { return `${firstName} ${lastName}`; }
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5.2 Use array destructuring. jscs:
requireArrayDestructuringconst arr = [1, 2, 3, 4]; // bad const first = arr[0]; const second = arr[1]; // good const [first, second] = arr;
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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, right } = processInput(input);
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6.1 Use single quotes
''for strings. eslint:quotesjscs:validateQuoteMarks// bad const name = "Capt. Janeway"; // good const name = 'Capt. Janeway';
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6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.
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6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.
// 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.';
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6.4 When programmatically building up strings, use template strings instead of concatenation. eslint:
prefer-templatetemplate-curly-spacingjscs:requireTemplateStringsWhy? 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}?`; }
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6.5 Never use
eval()on a string, it opens too many vulnerabilities.
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7.1 Use function declarations instead of function expressions. jscs:
requireFunctionDeclarationsWhy? 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 in place of function expressions.
// bad const foo = function () { }; // good function foo() { }
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7.2 Immediately invoked function expressions: eslint:
wrap-iifejscs:requireParenthesesAroundIIFEWhy? 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.'); }());
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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. eslint:
no-loop-func -
7.4 Note: ECMA-262 defines a
blockas a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.// bad if (currentUser) { function test() { console.log('Nope.'); } } // good let test; if (currentUser) { test = () => { console.log('Yup.'); }; }
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7.5 Never name a parameter
arguments. This will take precedence over theargumentsobject that is given to every function scope.// bad function nope(name, options, arguments) { // ...stuff... } // good function yup(name, options, args) { // ...stuff... }
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7.6 Never use
arguments, opt to use rest syntax...instead.prefer-rest-paramsWhy?
...is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like likearguments.// bad function concatenateAll() { const args = Array.prototype.slice.call(arguments); return args.join(''); } // good function concatenateAll(...args) { return args.join(''); }
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7.7 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 = {}) { // ... }
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7.8 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
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7.9 Always put default parameters last.
// bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... }
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7.10 Never use the Function constructor to create a new function.
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');
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7.11 Spacing in a function signature.
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() {};
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7.12 Never mutate parameters. eslint:
no-param-reassignWhy? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
// bad function f1(obj) { obj.key = 1; }; // good function f2(obj) { const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1; };
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7.13 Never reassign parameters. eslint:
no-param-reassignWhy? Reassigning parameters can lead to unexpected behavior, especially when accessing the
argumentsobject. 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) { }
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8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint:
prefer-arrow-callback,arrow-spacingjscs:requireArrowFunctionsWhy? 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; });
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8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a
returnstatement. eslint:arrow-parens,arrow-body-stylejscs:disallowParenthesesAroundArrowParam,requireShorthandArrowFunctionsWhy? Syntactic sugar. It reads well when multiple functions are chained together.
Why not? If you plan on returning an object.
// good [1, 2, 3].map(number => `A string containing the ${number}.`); // bad [1, 2, 3].map(number => { const nextNumber = number + 1; `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map((number) => { const nextNumber = number + 1; return `A string containing the ${nextNumber}.`; });
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8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
Why? It shows clearly where the function starts and ends.
// bad [1, 2, 3].map(number => 'As time went by, the string containing the ' + `${number} became much longer. So we needed to break it over multiple ` + 'lines.' ); // good [1, 2, 3].map(number => ( `As time went by, the string containing the ${number} became much ` + 'longer. So we needed to break it over multiple lines.' ));
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8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint:
arrow-parensjscs:disallowParenthesesAroundArrowParamWhy? Less visual clutter.
// bad [1, 2, 3].map((x) => x * x); // good [1, 2, 3].map(x => x * x); // good [1, 2, 3].map(number => ( `A long string with the ${number}. It’s so long that we’ve broken it ` + 'over multiple lines!' )); // 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; });
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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 => { return item.height > 256 ? item.largeSize : item.smallSize; }
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9.1 Always use
class. Avoid manipulatingprototypedirectly.Why?
classsyntax 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; } }
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9.2 Use
extendsfor 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]; } }
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9.3 Methods can return
thisto help with method chaining.// 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);
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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()}`; } }
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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.
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'; } }
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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 Airb 10000 nbStyleGuide = require('./AirbnbStyleGuide'); module.exports = AirbnbStyleGuide.es6; // ok import AirbnbStyleGuide from './AirbnbStyleGuide'; export default AirbnbStyleGuide.es6; // best import { es6 } from './AirbnbStyleGuide'; export default es6;
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10.2 Do not use wildcard imports.
Why? This makes sure you have a single default export.
// bad import * as AirbnbStyleGuide from './AirbnbStyleGuide'; // good import AirbnbStyleGuide from './AirbnbStyleGuide';
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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.
// bad // filename es6.js export { es6 as default } from './airbnbStyleGuide'; // good // filename es6.js import { es6 } from './AirbnbStyleGuide'; export default es6;
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11.1 Don't use iterators. Prefer JavaScript's higher-order functions like
map()andreduce()instead of loops likefor-of. eslint:no-iteratorWhy? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.
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;
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11.2 Don't use generators for now.
Why? They don't transpile well to ES5.
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12.1 Use dot notation when accessing properties. eslint:
dot-notationjscs:requireDotNotationconst luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi;
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12.2 Use subscript notation
[]when accessing properties with a variable.const luke = { jedi: true, age: 28, }; function getProp(prop) { return luke[prop]; } const isJedi = getProp('jedi');
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13.1 Always use
constto declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.// bad superPower = new SuperPower(); // good const superPower = new SuperPower();
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13.2 Use one
constdeclaration per variable. eslint:one-varjscs:disallowMultipleVarDeclWhy? 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.// 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';
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13.3 Group all your
consts and then group all yourlets.Why? This is helpful when later on you might need to assign a variable depending on one of the previous 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;
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13.4 Assign variables where you need them, but place them in a reasonable place.
Why?
letandconstare 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; }
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14.1
vardeclarations get hoisted to the top of their scope, their assignment does not.constandletdeclarations 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; }
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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'); }; }
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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'); } }
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14.4 Function declarations hoist their name and the function body.
function example() { superPower(); // => Flying function superPower() { console.log('Flying'); } }
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For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.
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15.2 Conditional statements such as the
ifstatement evaluate their expression using coercion with theToBooleanabstract 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 }
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15.3 Use shortcuts.
// bad if (name !== '') { // ...stuff... } // good if (name) { // ...stuff... } // bad if (collection.length > 0) { // ...stuff... } // good if (collection.length) { // ...stuff... }
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15.4 For more information see Truth Equality and JavaScript by Angus Croll.
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15.5 Use braces to create blocks in
caseanddefaultclauses that contain lexical declarations (e.g.let,const,function, andclass).
Why? Lexical declarations are visible in the entire
switchblock but only get initialized when assigned, which only happens when itscaseis reached. This causes problems when multiplecaseclauses attempt to define the same thing.
eslint rules: no-case-declarations.
```javascript
// bad
switch (foo) {
case 1:
let x = 1;
break;
case 2:
const y = 2;
break;
case 3:
function f() {}
break;
default:
class C {}
}
// good
switch (foo) {
case 1: {
let x = 1;
break;
}
case 2: {
const y = 2;
break;
}
case 3: {
function f() {}
break;
}
case 4:
bar();
break;
default: {
class C {}
}
}
```
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15.6 Ternaries should not be nested and generally be single line expressions.
eslint rules:
no-nested-ternary.// bad const foo = maybe1 > maybe2 ? "bar" : value1 > value2 ? "baz" : null; // better const maybeNull = value1 > value2 ? 'baz' : null; const foo = maybe1 > maybe2 ? 'bar' : maybeNull; // best const maybeNull = value1 > value2 ? 'baz' : null; const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
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15.7 Avoid unneeded ternary statements.
eslint rules:
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;
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16.1 Use braces with all multi-line blocks.
// 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; }
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16.2 If you're using multi-line blocks with
ifandelse, putelseon the same line as yourifblock's closing brace. eslint:brace-stylejscs:disallowNewlineBeforeBlockStatements// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
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17.1 Use
/** ... */for multi-line comments. Include a description, specify types and values for all parameters and return values.// 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; }
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17.2 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; }
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17.3 Prefixing your comments with
FIXMEorTODOhelps 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 areFIXME -- need to figure this outorTODO -- need to implement. -
17.4 Use
// FIXME:to annotate problems.class Calculator extends Abacus { constructor() { super(); // FIXME: shouldn't use a global here total = 0; } }
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17.5 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; } }
-
18.1 Use soft tabs set to 2 spaces. eslint:
indentjscs:validateIndentation// bad function foo() { ∙∙∙∙const name; } // bad function bar() { ∙const name; } // good function baz() { ∙∙const name; }
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18.2 Place 1 space before the leading brace. 10000 eslint:
space-before-blocksjscs:requireSpaceBeforeBlockStatements// 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', });
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18.3 Place 1 space before the opening parenthesis in control statements (
if,whileetc.). Place no space between the argument list and the function name in function calls and declarations. eslint:space-after-keywords,space-before-keywordsjscs:requireSpaceAfterKeywords// bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); }
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18.4 Set off operators with spaces. eslint:
space-infix-opsjscs:requireSpaceBeforeBinaryOperators,requireSpaceAfterBinaryOperators// bad const x=y+5; // good const x = y + 5;
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18.5 End files with a single newline character.
// bad (function (global) { // ...stuff... })(this);
// bad (function (global) { // ...stuff... })(this);↵ ↵
// good (function (global) { // ...stuff... })(this);↵
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18.6 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-callno-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);
-
18.7 Leave a blank line after blocks and before the next statement. jscs:
requirePaddingNewLinesAfterBlocks// 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;
-
18.8 Do not pad your blocks with blank lines. eslint:
padded-blocksjscs:disallowPaddingNewlinesInBlocks// bad function bar() { console.log(foo); } // also bad if (baz) { console.log(qux); } else { console.log(foo); } // good function bar() { console.log(foo); } // good if (baz) { console.log(qux); } else { console.log(foo); }
-
18.9 Do not add spaces inside parentheses. eslint:
space-in-parensjscs:disallowSpacesInsideParentheses// bad function bar( foo ) { return foo; } // good function bar(foo) { return foo; } // bad if ( foo ) { console.log(foo); } // good if (foo) { console.log(foo); }
-
18.10 Do not add spaces inside brackets. eslint:
array-bracket-spacingjscs:disallowSpacesInsideArrayBrackets// bad const foo = [ 1, 2, 3 ]; console.log(foo[ 0 ]); // good const foo = [1, 2, 3]; console.log(foo[0]);
-
18.11 Add spaces inside curly braces. eslint:
object-curly-spacingjscs:disallowSpacesInsideObjectBrackets// bad const foo = {clark: 'kent'}; // good const foo = { clark: 'kent' };
-
18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint:
max-lenjscs:maximumLineLengthWhy? This ensures readability and maintainability.
// bad const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!'; // 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 = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' + 'Whatever wizard constrains a helpful ally. The counterpart ascends!'; // good $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' }, }) .done(() => console.log('Congratulations!')) .fail(() => console.log('You have failed this city.'));
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19.1 Leading commas: Nope. eslint:
comma-stylejscs:requireCommaBeforeLineBreak// 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 Additional trailing comma: Yup. eslint:
comma-danglejscs:requireTrailingCommaWhy? 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 graph', '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', ];
-
20.1 Yup. eslint:
semijscs:requireSemicolons// 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; }());
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21.1 Perform type coercion at the beginning of the statement.
-
21.2 Strings:
// => this.reviewScore = 9; // bad const totalScore = this.reviewScore + ''; // good const totalScore = String(this.reviewScore);
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21.3 Numbers: Use
Numberfor type casting andparseIntalways with a radix for parsing strings. eslint:radixconst 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);
-
21.4 If for whatever reason you are doing something wild and
parseIntis 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;
-
21.5 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
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21.6 Booleans:
const age = 0; // bad const hasAge = new Boolean(age); // good const hasAge = Boolean(age); // good const hasAge = !!age;
-
22.1 Avoid single letter names. Be descriptive with your naming.
// bad function q() { // ...stuff... } // good function query() { // ..stuff.. }
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22.2 Use camelCase when naming objects, functions, and instances. eslint:
camelcasejscs:requireCamelCaseOrUpperCaseIdentifiers// bad const OBJEcttsssss = {}; const this_is_my_object = {}; function c() {} // good const thisIsMyObject = {}; function thisIsMyFunction() {}
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22.3 Use PascalCase when naming constructors or classes. eslint:
new-capjscs:requireCapitalizedConstructors// 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 Use a leading underscore
_when naming private properties. eslint:no-underscore-danglejscs:disallowDanglingUnderscores// bad this.__firstName__ = 'Panda'; this.firstName_ = 'Panda'; // good this._firstName = 'Panda';
-
22.5 Don't save references to
this. Use arrow functions or Function#bind. jscs:disallowNodeTypes// 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); }; }
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22.6 If your file exports a single class, your filename should be exactly the name of the class.
// file contents class CheckBox { // ... } export default CheckBox; // in some other file // bad import CheckBox from './checkBox'; // bad import CheckBox from './check_box'; // good import CheckBox from './CheckBox';
-
22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
function makeStyleGuide() { } export default makeStyleGuide;
-
22.8 Use PascalCase when you export a singleton / function library / bare object.
const AirbnbStyleGuide = { es6: { } }; export default AirbnbStyleGuide;
-
23.1 Accessor functions for properties are not required.
-
23.2 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 dragon.age(); // good dragon.getAge(); // bad dragon.age(25); // good dragon.setAge(25);
-
23.3 If the property is a
boolean, useisVal()orhasVal().// bad if (!dragon.age()) { return false; } // good if (!dragon.hasAge()) { return false; }
-
23.4 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]; } }
-
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:
// 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 });
-
25.1 Prefix jQuery object variables with a
$. jscs:requireDollarBeforejQueryAssignment// bad const sidebar = $('.sidebar'); // good const $sidebar = $('.sidebar'); // good const $sidebarBtn = $('.sidebar-btn');
-
25.2 Cache jQuery lookups.
// 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 For DOM queries use Cascading
$('.sidebar ul')or parent > child$('.sidebar > ul'). jsPerf -
25.4 Use
findwith scoped jQuery object queries.// bad $('ul', '.sidebar').hide(); // bad $('.sidebar').find('ul').hide(); // good $('.sidebar ul').hide(); // good $('.sidebar > ul').hide(); // good $sidebar.find('ul').hide();
- 26.1 Refer to Kangax's ES5 compatibility table.
- 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
- Iterators and Generators
- Modules
-
28.1 Yup.
function foo() { return true; }
-
28.2 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
mochaat Airbnb.tapeis 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.
- On Layout & Web Performance
- String vs Array Concat
- Try/Catch Cost In a Loop
- Bang Function
- jQuery Find vs Context, Selector
- innerHTML vs textContent for script text
- Long String Concatenation
- Loading...
Learning ES6
- Draft ECMA 2015 (ES6) Spec
- ExploringJS
- ES6 Compatibility Table
- Comprehensive Overview of ES6 Features
Read This
Tools
- Code Style Linters
Other Style Guides
- Google JavaScript Style Guide
- jQuery Core Style Guidelines
- Principles of Writing Consistent, Idiomatic JavaScript
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
- DailyJS
- JavaScript Weekly
- JavaScript, JavaScript...
- Bocoup Weblog
- Adequately Good
- NCZOnline
- Perfection Kills
- Ben Alman
- Dmitry Baranovskiy
- Dustin Diaz
- nettuts
Podcasts
This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.
- Aan Zee: AanZee/javascript
- Adult Swim: adult-swim/javascript
- Airbnb: airbnb/javascript
- Apartmint: apartmint/javascript
- Avalara: avalara/javascript
- Avant: avantcredit/javascript
- Billabong: billabong/javascript
- Bisk: bisk/javascript
- Blendle: blendle/javascript
- Brainshark: brainshark/javascript
- ComparaOnline: comparaonline/javascript
- Compass Learning: compasslearning/javascript-style-guide
- DailyMotion: dailymotion/javascript
- Digitpaint digitpaint/javascript
- Ecosia: ecosia/javascript
- Evernote: evernote/javascript-style-guide
- Evolution Gaming: evolution-gaming/javascript
- ExactTarget: ExactTarget/javascript
- Expensify Expensify/Style-Guide
- Flexberry: Flexberry/javascript-style-guide
- Gawker Media: gawkermedia/javascript
- General Electric: GeneralElectric/javascript
- GoodData: gooddata/gdc-js-style
- Grooveshark: grooveshark/javascript
- How About We: howaboutwe/javascript
- Huballin: huballin/javascript
- HubSpot: HubSpot/javascript
- Hyper: hyperoslo/javascript-playbook
- InfoJobs: InfoJobs/JavaScript-Style-Guide
- Intent Media: intentmedia/javascript
- Jam3: Jam3/Javascript-Code-Conventions
- JeopardyBot: kesne/jeopardy-bot
- JSSolutions: JSSolutions/javascript
- Kinetica Solutions: kinetica/javascript
- Mighty Spring: mightyspring/javascript
- MinnPost: MinnPost/javascript
- MitocGroup: MitocGroup/javascript
- ModCloth: modcloth/javascript
- Money Advice Service: moneyadviceservice/javascript
- Muber: muber/javascript
- National Geographic: natgeo/javascript
- National Park Service: nationalparkservice/javascript
- Nimbl3: nimbl3/javascript
- Orion Health: orionhealth/javascript
- OutBoxSoft: OutBoxSoft/javascript
- Peerby: Peerby/javascript
- Razorfish: razorfish/javascript-style-guide
- reddit: reddit/styleguide/javascript
- React: /facebook/react/blob/master/CONTRIBUTING.md#style-guide
- REI: reidev/js-style-guide
- Ripple: ripple/javascript-style-guide
- SeekingAlpha: seekingalpha/javascript-style-guide
- Shutterfly: shutterfly/javascript
- Springload: springload/javascript
- StudentSphere: studentsphere/javascript
- Target: target/javascript
- TheLadders: TheLadders/javascript
- T4R Technology: T4R-Technology/javascript
- VoxFeed: VoxFeed/javascript-style-guide
- WeBox Studio: weboxstudio/javascript
- Weggo: Weggo/javascript
- Zillow: zillow/javascript
- ZocDoc: ZocDoc/javascript
This style guide is also available in other languages:
Brazilian Portuguese: armoucar/javascript-style-guide
Bulgarian: borislavvv/javascript
Catalan: fpmweb/javascript-style-guide
Chinese (Simplified): sivan/javascript-style-guide
Chinese (Traditional): jigsawye/javascript
French: nmussy/javascript-style-guide
German: timofurrer/javascript-style-guide
Italian: sinkswim/javascript-style-guide
Japanese: mitsuruog/javacript-style-guide
Korean: tipjs/javascript-style-guide
Polish: mjurczyk/javascript
Russian: uprock/javascript
Spanish: paolocarrasco/javascript-style-guide
Thai: lvarayut/javascript-style-guide
- Find us on gitter.
(The MIT License)
Copyright (c) 2014-2016 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.
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.