This guide assumes you are using , and requires that you use or the equivalent. It also assumes you install shims/polyfills in your app with or the equivalent.
const process = (user) => {
if (user) {
if (user.isActive) {
sendEmail(user);
}
}
}
✅ Good:
const process = (user) => {
if (!user || !user.isActive) return;
sendEmail(user);
}
Avoid deeply nested code
❌ Bad:
if (a) {
if (b) {
if (c) {
doSomething();
}
}
}
✅ Good:
if (!a || !b || !c) return;
doSomething();
Types
string
number
boolean
null
undefined
symbol
bigint
const foo = 1;
let bar = foo;
bar = 9;
console.log(foo, bar); // => 1, 9
Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.
Polyfill is a term that originated from combining "poly," meaning many, and "fill," implying filling in the gaps. In the context of web development and programming with JavaScript, a polyfill is a piece of code that provides modern functionality on older browsers that do not naturally support it.
When a new web standard or API is introduced, it may take some time for all browsers to fully implement and support it. During this transition period, developers may use polyfills to ensure that their code works consistently across different browsers.
Why? This ensures that you can’t reassign your references, which can lead to bugs and difficulty comprehending code.
// bad
var a = 1;
var b = 2;
// good
const a = 1;
const b = 2;
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;
}
// const and let only exist in the blocks defined in.
{
let a = 1;
const b = 1;
var c = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError
console.log(c); // Prints 1
The above code shows that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block-scoped, while c is scoped to the containing function.
Objects
// bad
const item = new Object();
// good
const item = {};
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,
};
// bad
const atom = {
value: 1,
addValue: function (value) {
return atom.value + value;
},
};
// good
const atom = {
value: 1,
addValue(value) {
return atom.value + value;
},
};
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,
};
// bad
console.log(object.hasOwnProperty(key));
// good
console.log(Object.prototype.hasOwnProperty.call(object, key));
// better
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));
// best
console.log(Object.hasOwn(object, key)); // only supported in browsers that support ES2022
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));
/* or */
console.log(Object.hasOwn(object, key)); // https://www.npmjs.com/package/object.hasown
// 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 }
Arrays
// bad
const items = new Array();
// good
const items = [];
const someStack = [];
// bad
someStack[someStack.length] = 'abracadabra';
// good
someStack.push('abracadabra');
// bad
const len = items.length;
const itemsCopy = [];
let i;
for (i = 0; i < len; i += 1) {
itemsCopy[i] = items[i];
}
// good
const itemsCopy = [...items];
const foo = document.querySelectorAll('.foo');
// good
const nodes = Array.from(foo);
// best
const nodes = [...foo];
Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
// bad
function getFullName(user) {
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}`;
}
const arr = [1, 2, 3, 4];
// bad
const first = arr[0];
const second = arr[1];
// good
const [first, second] = arr;
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);
Strings
// bad
const name = "Capt. Janeway";
// bad - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;
// good
const name = 'Capt. Janeway';
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.';
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, ${name}?`;
}
Why? Backslashes harm readability, thus they should only be present when necessary.
// bad
const foo = '\'this\' \i\s \"quoted\"';
// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
Functions
// bad
function foo() {
// ...
}
// bad
const foo = function () {
// ...
};
// good
// lexical name distinguished from the variable-referenced invocation(s)
const short = function longUniqueMoreDescriptiveLexicalFoo() {
// ...
};
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.');
}());
// bad
if (currentUser) {
function test() {
console.log('Nope.');
}
}
// good
let test;
if (currentUser) {
test = () => {
console.log('Yup.');
};
}
// bad
function foo(name, options, arguments) {
// ...
}
// good
function foo(name, options, args) {
// ...
}
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('');
}
// 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 = {}) {
// ...
}
Why? They are confusing to reason about.
let b = 1;
// bad
function count(a = b++) {
console.log(a);
}
count(); // 1
count(); // 2
count(3); // 3
count(); // 3
// bad
function handleThings(opts = {}, name) {
// ...
}
// good
function handleThings(name, opts = {}) {
// ...
}
Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
// bad
const add = new Function('a', 'b', 'return a + b');
// still bad
const subtract = Function('a', 'b', 'return a - b');
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() {};
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;
}
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) {
// ...
}
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]);
// bad
function foo(bar,
baz,
quux) {
// ...
}
// good
function foo(
bar,
baz,
quux,
) {
// ...
}
// bad
console.log(foo,
bar,
baz);
// good
console.log(
foo,
bar,
baz,
);
Arrow Functions
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 named function expression.
// 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;
});
Why? Syntactic sugar. It reads well when multiple functions are chained together.
// bad
[1, 2, 3].map((number) => {
const nextNumber = number + 1;
`A string containing the ${nextNumber}.`;
});
// good
[1, 2, 3].map((number) => `A string containing the ${number + 1}.`);
// good
[1, 2, 3].map((number) => {
const nextNumber = number + 1;
return `A string containing the ${nextNumber}.`;
});
// good
[1, 2, 3].map((number, index) => ({
[index]: number,
}));
// No implicit return with side effects
function foo(callback) {
const val = callback();
if (val === true) {
// Do something if callback returns true
}
}
let bool = false;
// bad
foo(() => bool = true);
// good
foo(() => {
bool = true;
});
Why? It shows clearly where the function starts and ends.
Why? Minimizes diff churn when adding or removing arguments.
// bad
[1, 2, 3].map(x => x * x);
// good
[1, 2, 3].map((x) => x * x);
// bad
[1, 2, 3].map(number => (
`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));
// good
[1, 2, 3].map((number) => (
`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));
// bad
[1, 2, 3].map(x => {
const y = x + 1;
return x * y;
});
// good
[1, 2, 3].map((x) => {
const y = x + 1;
return x * y;
});
// 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';
}
}
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; }
}
// bad
class Foo {
bar() {
console.log('bar');
}
}
// good - this is used
class Foo {
bar() {
console.log(this.bar);
}
}
// good - constructor is exempt
class Foo {
constructor() {
// ...
}
}
// good - static methods aren't expected to use this
class Foo {
static bar() {
console.log('bar');
}
}
The # notation provides true encapsulation by making the variables truly private to the class. They are not accessible from outside the class, not even through inheritance or by using reflection techniques.
// bad
class Foo {
constructor(bar) {
this._bar = bar; // Using _ for "private" variable
}
}
// good
class Foo {
#bar; // Using # for private variable
constructor(bar) {
this.#bar = bar; // Defining private variable using #
}
}
Modules
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;
Why? This makes sure you have a single default export.
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';
// good
import AirbnbStyleGuide from './AirbnbStyleGuide';
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;
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';
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 };
Why? To encourage more files that only ever export one thing, which is better for readability and maintainability.
// bad
export function foo() {}
// good
export default function foo() {}
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();
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';
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';
Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
// bad
import foo from './foo.js';
import bar from './bar.jsx';
import baz from './baz/index.jsx';
// good
import foo from './foo';
import bar from './bar';
import baz from './baz';
Iterators and Generators
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);
Why? They don’t transpile well to ES5.
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* () {
// ...
};
Properties
const luke = {
jedi: true,
age: 28,
};
// bad
const isJedi = luke['jedi'];
// good
const isJedi = luke.jedi;
const luke = {
jedi: true,
age: 28,
};
function getProp(prop) {
return luke[prop];
}
const isJedi = getProp('jedi');
// bad
const binary = Math.pow(2, 10);
// good
const binary = 2 ** 10;
Variables
// bad
superPower = new SuperPower();
// good
const superPower = new SuperPower();
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';
Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.
// bad
let i, len, dragonball,
items = getItems(),
goSportsTeam = true;
// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;
// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;
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;
}
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`
Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
// bad
const array = [1, 2, 3];
let num = 1;
num++;
--num;
let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
let value = array[i];
sum += value;
if (value) {
truthyCount++;
}
}
// good
const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;
const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
Why? Linebreaks surrounding = can obfuscate the value of an assignment.
// bad
const foo =
superLongLongLongLongLongLongLongLongFunctionName();
// bad
const foo
= 'superLongLongLongLongLongLongLongLongString';
// good
const foo = (
superLongLongLongLongLongLongLongLongFunctionName()
);
// good
const foo = 'superLongLongLongLongLongLongLongLongString';
Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.
// bad
const some_unused_var = 42;
// Write-only variables are not considered as used.
let y = 10;
y = 5;
// A read for a modification of itself is not considered as used.
let z = 0;
z = z + 1;
// Unused function arguments.
function getX(x, y) {
return x;
}
// good
function getXPlusY(x, y) {
return x + y;
}
const x = 1;
const y = a + 2;
alert(getXPlusY(x, y));
// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
const { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.
Hoisting
// 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;
}
function example() {
console.log(anonymous); // => undefined
anonymous(); // => TypeError anonymous is not a function
var anonymous = function () {
console.log('anonymous function expression');
};
}
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');
};
}
function example() {
superPower(); // => Flying
function superPower() {
console.log('Flying');
}
}
Why? When variables, classes, or functions are declared after being used, it can harm readability since a reader won't know what a thing that's referenced is. It's much clearer for a reader to first encounter the source of a thing (whether imported from another module, or defined in the file) before encountering a use of the thing.
// bad
// Variable a is being used before it is being defined.
console.log(a); // this will be undefined, since while the declaration is hoisted, the initialization is not
var a = 10;
// Function fun is being called before being defined.
fun();
function fun() {}
// Class A is being used before being defined.
new A(); // ReferenceError: Cannot access 'A' before initialization
class A {
}
// `let` and `const` are hoisted, but they don't have a default initialization.
// The variables 'a' and 'b' are in a Temporal Dead Zone where JavaScript
// knows they exist (declaration is hoisted) but they are not accessible
// (as they are not yet initialized).
console.log(a); // ReferenceError: Cannot access 'a' before initialization
console.log(b); // ReferenceError: Cannot access 'b' before initialization
let a = 10;
const b = 5;
// good
var a = 10;
console.log(a); // 10
function fun() {}
fun();
class A {
}
new A();
let a = 10;
const b = 5;
console.log(a); // 10
console.log(b); // 5
Comparison Operators and Equality
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
}
// bad
if (isValid === true) {
// ...
}
// good
if (isValid) {
// ...
}
// bad
if (name) {
// ...
}
// good
if (name !== '') {
// ...
}
// bad
if (collection.length) {
// ...
}
// good
if (collection.length > 0) {
// ...
}
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 {}
}
}
// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;
const quux = a != null ? a : b;
// good
const foo = a || b;
const bar = !!c;
const baz = !c;
const quux = a ?? b;
Why? This improves readability and clarifies the developer’s intention.
// bad
const foo = a && b < 0 || c > 0 || d + 1 === 0;
// bad
const bar = a ** b - 5 % d;
// bad
// one may be confused into thinking (a || b) && c
if (a || b && c) {
return d;
}
// bad
const bar = a + b / c * d;
// good
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
// good
const bar = a ** b - (5 % d);
// good
if (a || (b && c)) {
return d;
}
// good
const bar = a + (b / c) * d;
Why? It provides precision by distinguishing null/undefined from other falsy values, enhancing code clarity and predictability.
// bad
const value = 0 ?? 'default';
// returns 0, not 'default'
// bad
const value = '' ?? 'default';
// returns '', not 'default'
// good
const value = null ?? 'default';
// returns 'default'
// good
const user = {
name: 'John',
age: null
};
const age = user.age ?? 18;
// returns 18
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;
}
// bad
if (test) {
thing1();
thing2();
}
else {
thing3();
}
// good
if (test) {
thing1();
thing2();
} else {
thing3();
}
// bad
function foo() {
if (x) {
return x;
} else {
return y;
}
}
// bad
function cats() {
if (x) {
return x;
} else if (y) {
return y;
}
}
// bad
function dogs() {
if (x) {
return x;
} else {
if (y) {
return y;
}
}
}
// good
function foo() {
if (x) {
return x;
}
return y;
}
// good
function cats() {
if (x) {
return x;
}
if (y) {
return y;
}
}
// good
function dogs(x) {
if (x) {
if (z) {
return y;
}
} else {
return z;
}
}
Control Statements
Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.
// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
thing1();
}
// bad
if (foo === 123 &&
bar === 'abc') {
thing1();
}
// bad
if (foo === 123
&& bar === 'abc') {
thing1();
}
// bad
if (
foo === 123 &&
bar === 'abc'
) {
thing1();
}
// good
if (
foo === 123
&& bar === 'abc'
) {
thing1();
}
// good
if (
(foo === 123 || bar === 'abc')
&& doesItLookGoodWhenItBecomesThatLong()
&& isThisReallyHappening()
) {
thing1();
}
// good
if (foo === 123 && bar === 'abc') {
thing1();
}
// bad
!isRunning && startRunning();
// good
if (!isRunning) {
startRunning();
}
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;
}
// 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;
}
// 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;
}
class Calculator extends Abacus {
constructor() {
super();
// FIXME: shouldn’t use a global here
total = 0;
}
}
class Calculator extends Abacus {
constructor() {
super();
// TODO: total should be configurable by an options param
this.total = 0;
}
}
Whitespace
// bad
function foo() {
∙∙∙∙let name;
}
// bad
function bar() {
∙let name;
}
// good
function baz() {
∙∙let name;
}
// 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',
});
// bad
if(isJedi) {
fight ();
}
// good
if (isJedi) {
fight();
}
// bad
function fight () {
console.log ('Swooosh!');
}
// good
function fight() {
console.log('Swooosh!');
}
// bad
const x=y+5;
// good
const x = y + 5;
// bad
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;
// bad
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;↵
↵
// good
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;↵
// 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;
// bad
function bar() {
console.log(foo);
}
// bad
if (baz) {
console.log(quux);
} else {
console.log(foo);
}
// bad
class Foo {
constructor(bar) {
this.bar = bar;
}
}
// good
function bar() {
console.log(foo);
}
// good
if (baz) {
console.log(quux);
} else {
console.log(foo);
}
// bad
class Person {
constructor(fullName, email, birthday) {
this.fullName = fullName;
this.email = email;
this.setAge(birthday);
}
setAge(birthday) {
const today = new Date();
const age = this.getAge(today, birthday);
this.age = age;
}
getAge(today, birthday) {
// ..
}
}
// good
class Person {
constructor(fullName, email, birthday) {
this.fullName = fullName;
this.email = email;
this.setAge(birthday);
}
setAge(birthday) {
const today = new Date();
const age = getAge(today, birthday);
this.age = age;
}
getAge(today, birthday) {
// ..
}
}
// bad
function bar( foo ) {
return foo;
}
// good
function bar(foo) {
return foo;
}
// bad
if ( foo ) {
console.log(foo);
}
// good
if (foo) {
console.log(foo);
}
// bad - multiple empty lines
const x = 1;
const y = 2;
// bad - 2+ newlines at end of file
const x = 1;
const y = 2;
// bad - 1+ newline(s) at beginning of file
const x = 1;
const y = 2;
// good
const x = 1;
const y = 2;
Commas
// 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',
};
// 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
);
Semicolons
// bad - raises exception
const luke = {}
const leia = {}
[luke, leia].forEach((jedi) => jedi.father = 'vader')
// bad - raises exception
const reaction = "No! That’s impossible!"
(async function meanwhileOnTheFalcon() {
// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
// ...
}())
// bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
function foo() {
return
'search your feelings, you know it to be foo'
}
// good
const luke = {};
const leia = {};
[luke, leia].forEach((jedi) => {
jedi.father = 'vader';
});
// good
const reaction = 'No! That’s impossible!';
(async function meanwhileOnTheFalcon() {
// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
// ...
}());
// good
function foo() {
return 'search your feelings, you know it to be foo';
}
Type Casting and Coercion
// => this.reviewScore = 9;
// bad
const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
// bad
const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
// bad
const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string
// good
const totalScore = String(this.reviewScore);
Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.
const inputValue = '4';
// bad
const val = new Number(inputValue);
// bad
const val = +inputValue;
// bad
const val = inputValue >> 0;
// bad
const val = parseInt(inputValue);
// good
const val = Number(inputValue);
// good
const val = parseInt(inputValue, 10);
// 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;
const age = 0;
// bad
const hasAge = new Boolean(age);
// good
const hasAge = Boolean(age);
// best
const hasAge = !!age;
Naming Conventions
// bad
function q() {
// ...
}
// good
function query() {
// ...
}
// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}
// good
const thisIsMyObject = {};
function thisIsMyFunction() {}
// 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',
});
Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';
// good
this.firstName = 'Panda';
// good, in environments where WeakMaps are available
// see https://kangax.github.io/compat-table/es6/#test-WeakMap
const firstNames = new WeakMap();
firstNames.set(this, 'Panda');
// 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);
};
}
// file 1 contents
class CheckBox {
// ...
}
export default CheckBox;
// file 2 contents
export default function fortyTwo() { return 42; }
// file 3 contents
export default function insideDirectory() {}
// in some other file
// bad
import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
// bad
import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
import forty_two from './forty_two'; // snake_case import/filename, camelCase export
import inside_directory from './inside_directory'; // snake_case import, camelCase export
import index from './inside_directory/index'; // requiring the index file explicitly
import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
// good
import CheckBox from './CheckBox'; // PascalCase export/import/filename
import fortyTwo from './fortyTwo'; // camelCase export/import/filename
import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
// ^ supports both insideDirectory.js and insideDirectory/index.js
function makeStyleGuide() {
// ...
}
export default makeStyleGuide;
Why? Names are for readability, not to appease a computer algorithm.
// bad
import SmsContainer from './containers/SmsContainer';
// bad
const HttpRequests = [
// ...
];
// good
import SMSContainer from './containers/SMSContainer';
// good
const HTTPRequests = [
// ...
];
// also good
const httpRequests = [
// ...
];
// best
import TextMessageContainer from './containers/TextMessageContainer';
// best
const requests = [
// ...
];
Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.
What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
// bad
const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';
// bad
export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';
// bad
export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';
// ---
// allowed but does not supply semantic value
export const apiKey = 'SOMEKEY';
// better in most cases
export const API_KEY = 'SOMEKEY';
// ---
// bad - unnecessarily uppercases key while adding no semantic value
export const MAPPING = {
KEY: 'value'
};
// good
export const MAPPING = {
key: 'value',
};
Accessors
// bad
class Dragon {
get age() {
// ...
}
set age(value) {
// ...
}
}
// good
class Dragon {
getAge() {
// ...
}
setAge(value) {
// ...
}
}
// bad
if (!dragon.age()) {
return false;
}
// good
if (!dragon.hasAge()) {
return false;
}
// 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
});
jQuery
// bad
const sidebar = $('.sidebar');
// good
const $sidebar = $('.sidebar');
// good
const $sidebarBtn = $('.sidebar-btn');
// bad
function setSidebar() {
$('.sidebar').hide();
// ...
$('.sidebar').css({
'background-color': 'pink',
});
}
// good
function setSidebar() {
const $sidebar = $('.sidebar');
$sidebar.hide();
// ...
$sidebar.css({
'background-color': 'pink',
});
}
// bad
$('ul', '.sidebar').hide();
// bad
$('.sidebar').find('ul').hide();
// good
$('.sidebar ul').hide();
// good
$('.sidebar > ul').hide();
// good
$sidebar.find('ul').hide();
ECMAScript 5 Compatibility
ECMAScript 6+ (ES 2015+) Styles
Standard Library
Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.
// bad
isNaN('1.2'); // false
isNaN('1.2.3'); // true
// good
Number.isNaN('1.2.3'); // false
Number.isNaN(Number('1.2.3')); // true
Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.
// bad
isFinite('2e3'); // true
// good
Number.isFinite('2e3'); // false
Number.isFinite(parseInt('2e3', 10)); // true
Testing
function foo() {
return true;
}
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.
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.
Performance
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Resources
Learning ES6+
Read This
Tools
Code Style Linters
Other Style Guides
Other Styles
Primitives: When you access a primitive type, you work directly on its value. (Pass by Value)
Complex: When you access a complex type you work on a reference to its value. (Pass by Reference)
Use const for all of your references; avoid using var. eslint: ,
If you must reassign references, use let instead of var. eslint:
Note that both let and const are block-scoped, whereas var is function-scoped.
Use the literal syntax for object creation. eslint:
Use computed property names when creating objects with dynamic property names.
Use object method shorthand. eslint:
Use property value shorthand. eslint:
Group your shorthand properties at the beginning of your object declaration.
Only quote properties that are invalid identifiers. eslint:
Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint:
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)). In modern browsers that support ES2022, or with a polyfill such as , Object.hasOwn can also be used as an alternative to Object.prototype.hasOwnProperty.call.
Prefer the object spread syntax over to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint:
Use the literal syntax for array creation. eslint:
Use instead of direct assignment to add items to an array.
Use array spreads ... to copy arrays.
To convert an iterable object to an array, use spreads ... instead of
Use for converting an array-like object to an array.
Use instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
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 . eslint:
Use line breaks after opening array brackets and before closing array brackets, if an array has multiple lines
Use object destructuring when accessing and using multiple properties of an object. eslint:
Use array destructuring. eslint:
Use object destructuring for multiple return values, not array destructuring.
Use single quotes '' for strings. eslint:
Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.
When programmatically building up strings, use template strings instead of concatenation. eslint:
Never use eval() on a string; it opens too many vulnerabilities. eslint:
Do not unnecessarily escape characters in strings. eslint:
Use named function expressions instead of function declarations. eslint: ,
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 explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. ()
Wrap immediately invoked function expressions in parentheses. eslint:
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:
Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.
Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.
Never use arguments, opt to use rest syntax ... instead. eslint:
Use default parameter syntax rather than mutating function arguments.
Avoid side effects with default parameters.
Always put default parameters last. eslint:
Never use the Function constructor to create a new function. eslint:
Spacing in a function signature. eslint:
Never mutate parameters. eslint:
Never reassign parameters. eslint:
Prefer the use of the spread syntax ... to call variadic functions. eslint:
Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint:
When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: ,
If the function body consists of a single statement returning an without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: ,
In case the expression spans over multiple lines, wrap it in parentheses for better readability.
Always include parentheses around arguments for clarity and consistency. eslint:
Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint:
Enforce the location of arrow function bodies with implicit returns. eslint:
Always use class. Avoid manipulating prototype directly.
Use extends for inheritance.
Methods can return this to help with method chaining.
It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
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:
Avoid duplicate class members. eslint:
Class methods should use this or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint:
Use the # notation and syntax for private properties in a class instead of _.
Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.
Do not use wildcard imports.
And do not export directly from an import.
Only import from a path in one place. eslint:
Do not export mutable bindings. eslint:
In modules with a single export, prefer default export over named export. eslint:
Put all imports above non-import statements. eslint:
Multiline imports should be indented just like multiline array and object literals. eslint:
Disallow Webpack loader syntax in module import statements. eslint:
Do not include JavaScript filename extensions eslint:
Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint:
Don’t use generators for now.
If you must use generators, or if you disregard , make sure their function signature is spaced properly. eslint:
Use dot notation when accessing properties. eslint:
Use bracket notation [] when accessing properties with a variable.
Use exponentiation operator ** when calculating exponentiations. eslint: .
Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint:
Use one const or let declaration per variable or assignment. eslint:
Group all your consts and then group all your lets.
Assign variables where you need them, but place them in a reasonable place.
Don’t chain variable assignments. eslint:
Avoid using unary increments and decrements (++, --). eslint
Avoid linebreaks before or after = in an assignment. If your assignment violates , surround the value in parens. eslint .
Disallow unused variables. eslint:
var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called . It’s important to know why .
Anonymous function expressions hoist their variable name, but not the function assignment.
Named function expressions hoist the variable name, not the function name or the function body.
Function declarations hoist their name and the function body.
Variables, classes, and functions should be defined before they can be used. eslint:
For more information refer to by .
Use === and !== over == and !=. eslint:
Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
Use shortcuts for booleans, but explicit comparisons for strings and numbers.
For more information see by Angus Croll.
Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint:
Ternaries should not be nested and generally be single line expressions. eslint:
Avoid unneeded ternary statements. eslint:
When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint:
The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined. Otherwise, it returns the left-hand side operand.
Use braces with all multiline blocks. eslint:
If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint:
If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint:
In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.
Don't use selection operators in place of control statements.
Use /** ... */ for multiline comments.
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.
Start all comments with a space to make it easier to read. eslint:
Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.
Use // FIXME: to annotate problems.
Use // TODO: to annotate solutions to problems.
Use soft tabs (space character) set to 2 spaces. eslint:
Place 1 space before the leading brace. eslint:
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:
Set off operators with spaces. eslint:
End files with a single newline character. eslint:
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:
Leave a blank line after blocks and before the next statement.
Do not pad your blocks with blank lines. eslint:
Do not use multiple blank lines to pad your code. eslint:
Do not add spaces inside parentheses. eslint:
Do not add spaces inside brackets. eslint:
Add spaces inside curly braces. eslint:
Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per , long strings are exempt from this rule, and should not be broken up. eslint:
Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint:
Avoid spaces before commas and require a space after commas. eslint:
Enforce spacing inside of computed property brackets. eslint:
Avoid spaces between functions and their invocations. eslint:
Enforce spacing between keys and values in object literal properties. eslint:
Avoid trailing spaces at the end of lines. eslint:
Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint:
Leading commas: Nope. eslint:
Additional trailing comma: Yup. eslint:
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 in legacy browsers.
Yup. eslint:
Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called to determine whether it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.
.
Perform type coercion at the beginning of the statement.
Strings: eslint:
Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint:
If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for , leave a comment explaining why and what you’re doing.
Note: Be careful when using bitshift operations. Numbers are represented as , but bitshift operations always return a 32-bit integer (). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. . Largest signed 32-bit Int is 2,147,483,647:
Booleans: eslint:
Avoid single letter names. Be descriptive with your naming. eslint:
Use camelCase when naming objects, functions, and instances. eslint:
Use PascalCase only when naming constructors or classes. eslint:
Do not use trailing or leading underscores. eslint:
Don’t save references to this. Use arrow functions or .
A base filename should exactly match the name of its default export.
Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
Use PascalCase when you export a constructor / class / singleton / function library / bare object.
Acronyms and initialisms should always be all uppercased, or all lowercased.
You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.
Accessor functions for properties are not required.
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').
If the property/method is a boolean, use isVal() or hasVal().
It’s okay to create get() and set() functions, but be consistent.
When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
Prefix jQuery object variables with a $.
Cache jQuery lookups.
For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul').
Use find with scoped jQuery object queries.
Refer to ’s ES5 .
This is a collection of links to the various ES6+ features.
Do not use that have not reached stage 3.
Why? , and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.
The contains utilities that are functionally broken but remain for legacy reasons.
Use Number.isNaN instead of global isNaN. eslint:
Use Number.isFinite instead of global isFinite. eslint:
Yup.
No, but seriously:
We primarily use and at Airbnb. is also used occasionally for small, separate modules.
