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COLLECTED BY
Collection: Save Page Now
Other Style Guides
●ES5 (Deprecated)
●React
●CSS-in-JavaScript
●CSS & Sass
●Ruby
string
●number
●boolean
●null
●undefined
const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 91.2 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
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 jscs: disallowVar
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
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) { 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 jscs: requireEnhancedObjectLiterals
// bad const atom = { value: 1, addValue: function (value) { return atom.value + value; }, }; // good const atom = { value: 1, addValue(value) { return atom.value + value; }, };3.4 Use property value shorthand. eslint:
object-shorthand jscs: requireEnhancedObjectLiterals
Why? It is shorter to write 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 jscs: 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, };3.7 Do not call
Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.
Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
// bad console.log(object.hasOwnProperty(key)); // good console.log(Object.prototype.hasOwnProperty.call(object, key)); // best const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope. /* or */ import has from 'has'; // ... console.log(has.call(object, key));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.
// 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 }
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 array-like object to an array, use Array.from.
const foo = document.querySelectorAll('.foo'); const nodes = Array.from(foo);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 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 const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = flatten; }); // good const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = flatten; return flatten; }); // bad inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } else { return false; } }); // good inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } return false; });●4.6 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, ];
requireObjectDestructuring
Why? 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}`; }5.2 Use array destructuring. jscs:
requireArrayDestructuring
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. jscs:
disallowArrayDestructuringReturn
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);
'' for strings. eslint: quotes jscs: validateQuoteMarks
// 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 jscs: requireTemplateStrings
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}'`;
func-style jscs: disallowFunctionDeclarations
Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to name the expression - anonymous functions can make it harder to locate the problem in an Error’s call stack. (Discussion)
// bad function foo() { // ... } // bad const foo = function () { // ... }; // good const foo = function bar() { // ... };7.2 Wrap immediately invoked function expressions in parentheses. eslint:
wrap-iife jscs: requireParenthesesAroundIIFE
Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
// immediately-invoked function expression (IIFE) (function () { console.log('Welcome to the Internet. Please follow me.'); }());●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 block as 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.'); }; }7.5 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) { // ... }7.6 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(''); }7.7 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 = {}) { // ... }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(); // 37.9 Always put default parameters last.
// bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... }7.10 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');7.11 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() {};7.12 Never mutate parameters. eslint:
no-param-reassign
Why? 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; }7.13 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) { // ... }7.14 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]);7.15 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.
// bad function foo(bar, baz, quux) { // ... } // good function foo( bar, baz, quux, ) { // ... } // bad console.log(foo, bar, baz); // good console.log( foo, bar, baz, );
prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions
Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.
Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.
// 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 jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions
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}.`); // 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 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint or do not include
disallowParenthesesAroundArrowParam for jscs. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam
Why? 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 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; };
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.
// 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 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; } }
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 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';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;10.4 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';10.5 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 };10.6 In modules with a single export, prefer default export over named export. eslint:
import/prefer-default-export
// bad export function foo() {} // good export default function foo() {}10.7 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();10.8 Multiline imports should be indented just like multiline array and object literals. Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.
// bad import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path'; // good import { longNameA, longNameB, longNameC, longNameD, longNameE, } from 'path';10.9 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';
for-inorfor-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* () { // ... };
dot-notation jscs: requireDotNotation
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;
constorlet 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
constorlet declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl
Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.
// 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 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;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. 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++ornum ++. 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;
var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). 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 & HoistingbyBen Cherry.
=== and !== over == and !=. eslint: eqeqeq
15.2 Conditional statements such as the ifstatement 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 falseif+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; // 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;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;
// 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 multi-line blocks with
ifand else, put else on the same line as your ifblock’s closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. It’s up to you whether the logical operator should begin or end 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(); } // good if ( (foo === 123 || bar === "abc") && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening() ) { thing1(); } // good if (foo === 123 && bar === 'abc') { thing1(); } // good if ( foo === 123 && bar === 'abc' ) { thing1(); } // good if ( foo === 123 && bar === 'abc' ) { thing1(); }
/** ... */ for multi-line 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
FIXMEorTODO 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 outorTODO: -- 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; } }
indent jscs: validateIndentation
// bad function foo() { ∙∙∙∙let name; } // bad function bar() { ∙let name; } // good function baz() { ∙∙let name; }19.2 Place 1 space before the leading brace. eslint:
space-before-blocks jscs: 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', });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 jscs: requireSpaceAfterKeywords
// 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 jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators
// 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. 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;19.8 Do not pad your blocks with blank lines. eslint:
padded-blocks jscs: 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); }19.9 Do not add spaces inside parentheses. eslint:
space-in-parens jscs: 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); }19.10 Do not add spaces inside brackets. eslint:
array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets
// bad const foo = [ 1, 2, 3 ]; console.log(foo[ 0 ]); // good const foo = [1, 2, 3]; console.log(foo[0]);19.11 Add spaces inside curly braces. eslint:
object-curly-spacing jscs: requireSpacesInsideObjectBrackets
// bad const foo = {clark: 'kent'}; // good const foo = { clark: 'kent' };19.12 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 jscs: maximumLineLength
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.'));
comma-style jscs: 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', };20.2 Additional trailing comma: Yup. eslint:
comma-dangle jscs: requireTrailingComma
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 );
semi jscs: requireSemicolons
// bad (function () { const name = 'Skywalker' return name })() // good (function () { const name = 'Skywalker'; return name; }()); // good, but legacy (guards against the function becoming an argument when two files with IIFEs are concatenated) ;((() => { const name = 'Skywalker'; return name; })());Read more.
// => this.reviewScore = 9; // 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
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; // => -214748364722.6 Booleans:
const age = 0; // bad const hasAge = new Boolean(age); // good const hasAge = Boolean(age); // best const hasAge =!!age;
id-length
// bad function q() { // ... } // good function query() { // ... }23.2 Use camelCase when naming objects, functions, and instances. eslint:
camelcase jscs: requireCamelCaseOrUpperCaseIdentifiers
// bad const OBJEcttsssss = {}; const this_is_my_object = {}; function c() {} // good const thisIsMyObject = {}; function thisIsMyFunction() {}23.3 Use PascalCase only when naming constructors or classes. eslint:
new-cap jscs: 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', });23.4 Do not use trailing or leading underscores. eslint:
no-underscore-dangle jscs: disallowDanglingUnderscores
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';23.5 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); }; }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.js23.7 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.
const AirbnbStyleGuide = { es6: { }, }; export default AirbnbStyleGuide;23.9 Acronyms and initialisms should always be all capitalized, 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 = [ // ... ]; // best import TextMessageContainer from './containers/TextMessageContainer'; // best const Requests = [ // ... ];
// 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()orhasVal().
// 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]; } }
// 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 });
$. jscs: requireDollarBeforejQueryAssignment
// bad const sidebar = $('.sidebar'); // good const $sidebar = $('.sidebar'); // good const $sidebarBtn = $('.sidebar-btn');26.2 Cache jQuery lookups.
// bad function setSidebar() { $('.sidebar').hide(); // ... $('.sidebar').css({ 'background-color': 'pink', }); } // good function setSidebar() { const $sidebar = $('.sidebar'); $sidebar.hide(); // ... $sidebar.css({ 'background-color': 'pink', }); }●26.3 For DOM queries use Cascading
$('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf
26.4 Use find with 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();
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 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.
map(), reduce(), and filter() optimized for traversing arrays?
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