A mostly reasonable approach to JavaScript
- Types
- References
- Objects
- Arrays
- Strings
- Functions
- Arrow Functions
- Modules
- Properties
- Variables
- Comparison Operators & Equality
- Blocks
- Comments
- Commas
- Semicolons
- Type Casting & Coercion
- License
-
1.1 Primitives: When you access a primitive type you work directly on its value.
string
number
boolean
null
undefined
const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 9
-
1.2 Complex: When you access a complex type you work on a reference to its value.
object
array
function
const foo = [1, 2]; const bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9
-
2.1 Use
const
for all of your references; avoid usingvar
. 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 ofvar
. eslint:no-var
jscs:disallowVar
Why?
let
is block-scoped rather than function-scoped likevar
.// bad var count = 1; if (true) { count += 1; } // good, use the let. let count = 1; if (true) { count += 1; }
-
2.3 Note that both
let
andconst
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
-
3.1 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; }, };
-
3.2 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, };
-
3.3 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, };
-
3.4 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, };
-
4.1 Use the literal syntax for array creation. eslint:
no-array-constructor
// bad const items = new Array(); // good const items = [];
-
4.2 Use Array#push instead of direct assignment to add items to an array.
const someStack = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra');
-
4.3 Use array spreads
...
to copy arrays.// bad const len = items.length; const itemsCopy = []; let i; for (i = 0; i < len; i += 1) { itemsCopy[i] = items[i]; } // good const itemsCopy = [...items];
-
5.1 Use single quotes
''
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';
-
5.2 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.';
-
5.3 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}?`; }
- 5.4 Never use
eval()
on a string, it opens too many vulnerabilities.
-
6.1 Use named function expressions instead of function declarations. eslint:
func-style
jscs:requireFunctionDeclarations
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 const foo = function () { }; // bad function foo() { } // good const foo = function bar() { };
-
6.2 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.'); }());
- 6.3 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
-
6.4 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 = {}) { // ... }
-
6.5 Always put default parameters last.
// bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... }
-
6.6 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() {};
-
6.7 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; };
-
6.8 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) { }
-
7.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint:
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; });
-
7.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a
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 }));
-
7.3 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. 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; });
-
8.1 Always use modules (
import
/export
) over a non-standard module system. You can always transpile to your preferred module system.Why? Modules are the future, let's start using the future now.
// bad const AirbnbStyleGuide = require('./AirbnbStyleGuide'); module.exports = AirbnbStyleGuide.es6; // ok import AirbnbStyleGuide from './AirbnbStyleGuide'; export default AirbnbStyleGuide.es6; // best import { es6 } from './AirbnbStyleGuide'; export default es6;
-
8.2 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';
-
8.3 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;
-
8.4 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';
-
8.5 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 }
-
8.6 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() {}
-
8.7 Put all
import
s above non-import statements. eslint:import/first
Why? Since
import
s 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();
-
8.8 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';
-
8.9 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';
-
9.1 Use dot notation when accessing properties. eslint:
dot-notation
jscs:requireDotNotation
const luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi;
-
9.2 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');
-
10.1 Always use
const
to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint:no-undef
prefer-const
// bad superPower = new SuperPower(); // good const superPower = new SuperPower();
-
10.2 Use one
const
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';
-
10.3 Group all your
const
s and then group all yourlet
s.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;
-
10.4 Assign variables where you need them, but place them in a reasonable place.
Why?
let
andconst
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; }
-
10.5 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); // undefined console.log(b); // 1 console.log(c); // 1 // good (function example() { let a = 1; let b = a; let c = a; }()); console.log(a); // undefined console.log(b); // undefined console.log(c); // undefined // the same applies for `const`
-
10.6 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 ofnum++
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 let 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 let 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;
-
11.2 Conditional statements such as the
if
statement evaluate their expression using coercion with theToBoolean
abstract method and always follow these simple rules:- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string
''
, otherwise true
if ([0] && []) { // true // an array (even an empty one) is an object, objects will evaluate to true }
-
11.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.
// bad if (isValid === true) { // ...stuff... } // good if (isValid) { // ...stuff... } // bad if (name) { // ...stuff... } // good if (name !== '') { // ...stuff... } // bad if (collection.length) { // ...stuff... } // good if (collection.length > 0) { // ...stuff... }
- 11.4 For more information see Truth Equality and JavaScript by Angus Croll.
- 11.5 Use braces to create blocks in
case
anddefault
clauses that contain lexical declarations (e.g.let
,const
,function
, andclass
).
Why? Lexical declarations are visible in the entire
switch
block but only get initialized when assigned, which only happens when itscase
is reached. This causes problems when multiplecase
clauses attempt to define the same thing.
eslint rules: no-case-declarations
.
```javascript
// bad
switch (foo) {
case 1:
let x = 1;
break;
case 2:
const y = 2;
break;
case 3:
function f() {}
break;
default:
class C {}
}
// good
switch (foo) {
case 1: {
let x = 1;
break;
}
case 2: {
const y = 2;
break;
}
case 3: {
function f() {}
break;
}
case 4:
bar();
break;
default: {
class C {}
}
}
```
-
11.6 Ternaries should not be nested and generally be single line expressions.
eslint rules:
no-nested-ternary
.// bad const foo = maybe1 > maybe2 ? "bar" : value1 > value2 ? "baz" : null; // better const maybeNull = value1 > value2 ? 'baz' : null; const foo = maybe1 > maybe2 ? 'bar' : maybeNull; // best const maybeNull = value1 > value2 ? 'baz' : null; const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
-
11.7 Avoid unneeded ternary statements.
eslint rules:
no-unneeded-ternary
.// bad const foo = a ? a : b; const bar = c ? true : false; const baz = c ? false : true; // good const foo = a || b; const bar = !!c; const baz = !c;
-
12.1 Use braces with all multi-line blocks.
// bad if (test) return false; // good if (test) return false; // good if (test) { return false; } // bad function foo() { return false; } // good function bar() { return false; }
-
12.2 If you're using multi-line blocks with
if
andelse
, putelse
on the same line as yourif
block's closing brace. eslint:brace-style
jscs:disallowNewlineBeforeBlockStatements
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
-
13.1 Use
// FIXME:
to annotate problems.class Calculator extends Abacus { constructor() { super(); // FIXME: shouldn't use a global here total = 0; } }
-
13.2 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; } }
-
14.1 Leading commas: Nope. eslint:
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', };
-
14.2 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 )
-
15.1 Yup. eslint:
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; }());
- 16.1 Perform type coercion at the beginning of the statement.
-
16.2 Strings:
// => 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);
-
16.3 Numbers: Use
Number
for type casting andparseInt
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);
-
16.4 Booleans:
const age = 0; // bad const hasAge = new Boolean(age); // good const hasAge = Boolean(age); // best const hasAge = !!age;
-
17.1 Avoid single letter names. Be descriptive with your naming. eslint:
id-length
// bad function q() { // ...stuff... } // good function query() { // ..stuff.. }
-
17.2 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() {}
-
17.3 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', });
Inspired by AirBnB :)
(The MIT License)
Copyright (c) 2014-2016 Airbnb
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
We encourage you to fork this guide and change the rules to fit your team's style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.