Name: javascript-style-guide
Owner: SpatialDev
Description: JavaScript Style Guide
Forked from: airbnb/javascript
Created: 2016-02-24 22:14:39.0
Updated: 2016-02-25 16:23:31.0
Pushed: 2016-02-24 19:23:24.0
Homepage: null
Size: 2260
Language: JavaScript
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A mostly reasonable approach to JavaScript
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Other Style Guides
1.1 Primitives: When you access a primitive type you work directly on its value.
string
number
boolean
null
undefined
t foo = 1;
bar = foo;
= 9;
ole.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
t foo = [1, 2];
t bar = foo;
0] = 9;
ole.log(foo[0], bar[0]); // => 9, 9
2.1 Use 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.
ad
a = 1;
b = 2;
ood
t a = 1;
t 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 likevar
.
ad
count = 1;
true) {
unt += 1;
ood, use the let.
count = 1;
true) {
unt += 1;
2.3 Note that both let
and const
are block-scoped.
onst and let only exist in the blocks they are defined in.
t a = 1;
nst b = 1;
ole.log(a); // ReferenceError
ole.log(b); // ReferenceError
3.1 Use the literal syntax for object creation. eslint: no-new-object
ad
t item = new Object();
ood
t item = {};
3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It?s OK to use them in ES6 modules and server-side code. jscs: disallowIdentiferNames
ad
t superman = {
fault: { clark: 'kent' },
ivate: true,
ood
t superman = {
faults: { clark: 'kent' },
dden: true,
3.3 Use readable synonyms in place of reserved words. jscs: disallowIdentiferNames
ad
t superman = {
ass: 'alien',
ad
t superman = {
ass: 'alien',
ood
t superman = {
pe: 'alien',
3.4 Use computed property names when creating objects with dynamic property names.
Why? They allow you to define all the properties of an object in one place.
tion getKey(k) {
turn `a key named ${k}`;
ad
t obj = {
: 5,
me: 'San Francisco',
getKey('enabled')] = true;
ood
t obj = {
: 5,
me: 'San Francisco',
etKey('enabled')]: true,
3.5 Use object method shorthand. eslint: object-shorthand
jscs: requireEnhancedObjectLiterals
ad
t atom = {
lue: 1,
dValue: function (value) {
return atom.value + value;
ood
t atom = {
lue: 1,
dValue(value) {
return atom.value + value;
3.6 Use property value shorthand. eslint: object-shorthand
jscs: requireEnhancedObjectLiterals
Why? It is shorter to write and descriptive.
t lukeSkywalker = 'Luke Skywalker';
ad
t obj = {
keSkywalker: lukeSkywalker,
ood
t obj = {
keSkywalker,
3.7 Group your shorthand properties at the beginning of your object declaration.
Why? It's easier to tell which properties are using the shorthand.
t anakinSkywalker = 'Anakin Skywalker';
t lukeSkywalker = 'Luke Skywalker';
ad
t obj = {
isodeOne: 1,
oJediWalkIntoACantina: 2,
keSkywalker,
isodeThree: 3,
yTheFourth: 4,
akinSkywalker,
ood
t obj = {
keSkywalker,
akinSkywalker,
isodeOne: 1,
oJediWalkIntoACantina: 2,
isodeThree: 3,
yTheFourth: 4,
3.8 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.
ad
t bad = {
': 3,
': 4,
a-blah': 5,
ood
t good = {
3,
4,
a-blah': 5,
4.1 Use the literal syntax for array creation. eslint: no-array-constructor
ad
t items = new Array();
ood
t items = [];
4.2 Use Array#push instead of direct assignment to add items to an array.
t someStack = [];
ad
Stack[someStack.length] = 'abracadabra';
ood
Stack.push('abracadabra');
4.3 Use array spreads ...
to copy arrays.
ad
t len = items.length;
t itemsCopy = [];
i;
(i = 0; i < len; i++) {
emsCopy[i] = items[i];
ood
t itemsCopy = [...items];
4.4 To convert an array-like object to an array, use Array#from.
t foo = document.querySelectorAll('.foo');
t 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 following 8.2. eslint: array-callback-return
ood
2, 3].map((x) => {
nst y = x + 1;
turn x * y;
ood
2, 3].map(x => x + 1);
ad
t flat = {};
1], [2, 3], [4, 5]].reduce((memo, item, index) => {
nst flatten = memo.concat(item);
at[index] = memo.concat(item);
ood
t flat = {};
1], [2, 3], [4, 5]].reduce((memo, item, index) => {
nst flatten = memo.concat(item);
at[index] = flatten;
turn flatten;
ad
x.filter((msg) => {
nst { subject, author } = msg;
(subject === 'Mockingbird') {
return author === 'Harper Lee';
else {
return false;
ood
x.filter((msg) => {
nst { subject, author } = msg;
(subject === 'Mockingbird') {
return author === 'Harper Lee';
turn false;
5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring
Why? Destructuring saves you from creating temporary references for those properties.
ad
tion getFullName(user) {
nst firstName = user.firstName;
nst lastName = user.lastName;
turn `${firstName} ${lastName}`;
ood
tion getFullName(user) {
nst { firstName, lastName } = user;
turn `${firstName} ${lastName}`;
est
tion getFullName({ firstName, lastName }) {
turn `${firstName} ${lastName}`;
5.2 Use array destructuring. jscs: requireArrayDestructuring
t arr = [1, 2, 3, 4];
ad
t first = arr[0];
t second = arr[1];
ood
t [first, second] = arr;
5.3 Use object destructuring for multiple return values, not array destructuring.
Why? You can add new properties over time or change the order of things without breaking call sites.
ad
tion processInput(input) {
then a miracle occurs
turn [left, right, top, bottom];
he caller needs to think about the order of return data
t [left, __, top] = processInput(input);
ood
tion processInput(input) {
then a miracle occurs
turn { left, right, top, bottom };
he caller selects only the data they need
t { left, right } = processInput(input);
6.1 Use single quotes ''
for strings. eslint: quotes
jscs: validateQuoteMarks
ad
t name = "Capt. Janeway";
ood
t name = 'Capt. Janeway';
6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.
6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.
ad
t 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.';
ad
t errorMessage = 'This is a super long error that was thrown because \
atman. When you stop to think about how Batman had anything to do \
this, you would get nowhere \
.';
ood
t errorMessage = 'This is a super long error that was thrown because ' +
f Batman. When you stop to think about how Batman had anything to do ' +
ith this, you would get nowhere fast.';
6.4 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.
ad
tion sayHi(name) {
turn 'How are you, ' + name + '?';
ad
tion sayHi(name) {
turn ['How are you, ', name, '?'].join();
ad
tion sayHi(name) {
turn `How are you, ${ name }?`;
ood
tion sayHi(name) {
turn `How are you, ${name}?`;
6.5 Never use eval()
on a string, it opens too many vulnerabilities.
7.1 Use function declarations instead of function expressions. jscs: requireFunctionDeclarations
Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.
ad
t foo = function () {
ood
tion foo() {
7.2 Immediately invoked function expressions: 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.
mmediately-invoked function expression (IIFE)
ction () {
nsole.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.
ad
currentUser) {
nction test() {
console.log('Nope.');
ood
test;
currentUser) {
st = () => {
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.
ad
tion nope(name, options, arguments) {
...stuff...
ood
tion yup(name, options, args) {
...stuff...
7.6 Never use arguments
, opt to use rest syntax ...
instead. prefer-rest-params
Why?
...
is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like likearguments
.
ad
tion concatenateAll() {
nst args = Array.prototype.slice.call(arguments);
turn args.join('');
ood
tion concatenateAll(...args) {
turn args.join('');
7.7 Use default parameter syntax rather than mutating function arguments.
eally bad
tion 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.
ts = opts || {};
...
till bad
tion handleThings(opts) {
(opts === void 0) {
opts = {};
...
ood
tion handleThings(opts = {}) {
...
7.8 Avoid side effects with default parameters.
Why? They are confusing to reason about.
b = 1;
ad
tion count(a = b++) {
nsole.log(a);
t(); // 1
t(); // 2
t(3); // 3
t(); // 3
7.9 Always put default parameters last.
ad
tion handleThings(opts = {}, name) {
...
ood
tion handleThings(name, opts = {}) {
...
7.10 Never use the Function constructor to create a new function.
Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
ad
add = new Function('a', 'b', 'return a + b');
till bad
subtract = Function('a', 'b', 'return a - b');
7.11 Spacing in a function signature.
Why? Consistency is good, and you shouldn?t have to add or remove a space when adding or removing a name.
ad
t f = function(){};
t g = function (){};
t h = function() {};
ood
t x = function () {};
t 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.
ad
tion f1(obj) {
j.key = 1;
ood
tion f2(obj) {
nst 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.
ad
tion f1(a) {
= 1;
tion f2(a) {
(!a) { a = 1; }
ood
tion f3(a) {
nst b = a || 1;
tion f4(a = 1) {
8.1 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.
ad
2, 3].map(function (x) {
nst y = x + 1;
turn x * y;
ood
2, 3].map((x) => {
nst y = x + 1;
turn x * y;
8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return
statement. eslint: arrow-parens
, arrow-body-style
jscs: disallowParenthesesAroundArrowParam
, requireShorthandArrowFunctions
Why? Syntactic sugar. It reads well when multiple functions are chained together.
Why not? If you plan on returning an object.
ood
2, 3].map(number => `A string containing the ${number}.`);
ad
2, 3].map(number => {
nst nextNumber = number + 1;
string containing the ${nextNumber}.`;
ood
2, 3].map((number) => {
nst nextNumber = number + 1;
turn `A string containing the ${nextNumber}.`;
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.
ad
2, 3].map(number => 'As time went by, the string containing the ' +
{number} became much longer. So we needed to break it over multiple ` +
ines.'
ood
2, 3].map(number => (
s time went by, the string containing the ${number} became much ` +
onger. So we needed to break it over multiple lines.'
8.4 If your function takes a single argument and doesn?t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens
jscs: disallowParenthesesAroundArrowParam
Why? Less visual clutter.
ad
2, 3].map((x) => x * x);
ood
2, 3].map(x => x * x);
ood
2, 3].map(number => (
long string with the ${number}. It?s so long that we?ve broken it ` +
ver multiple lines!'
ad
2, 3].map(x => {
nst y = x + 1;
turn x * y;
ood
2, 3].map((x) => {
nst y = x + 1;
turn x * y;
8.5 Avoid confusing arrow function syntax (=>
) with comparison operators (<=
, >=
). eslint: no-confusing-arrow
ad
t itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;
ad
t itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;
ood
t itemHeight = item => { return item.height > 256 ? item.largeSize : item.smallSize; }
9.1 Always use class
. Avoid manipulating prototype
directly.
Why?
class
syntax is more concise and easier to reason about.
ad
tion Queue(contents = []) {
is._queue = [...contents];
e.prototype.pop = function () {
nst value = this._queue[0];
is._queue.splice(0, 1);
turn value;
ood
s Queue {
nstructor(contents = []) {
this._queue = [...contents];
p() {
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
.
ad
t inherits = require('inherits');
tion PeekableQueue(contents) {
eue.apply(this, contents);
rits(PeekableQueue, Queue);
ableQueue.prototype.peek = function () {
turn this._queue[0];
ood
s PeekableQueue extends Queue {
ek() {
return this._queue[0];
9.3 Methods can return this
to help with method chaining.
ad
.prototype.jump = function () {
is.jumping = true;
turn true;
.prototype.setHeight = function (height) {
is.height = height;
t luke = new Jedi();
.jump(); // => true
.setHeight(20); // => undefined
ood
s Jedi {
mp() {
this.jumping = true;
return this;
tHeight(height) {
this.height = height;
return this;
t luke = new Jedi();
.jump()
etHeight(20);
9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
s Jedi {
nstructor(options = {}) {
this.name = options.name || 'no name';
tName() {
return this.name;
String() {
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. no-useless-constructor
ad
s Jedi {
nstructor() {}
tName() {
return this.name;
ad
s Rey extends Jedi {
nstructor(...args) {
super(...args);
ood
s Rey extends Jedi {
nstructor(...args) {
super(...args);
this.name = 'Rey';
10.1 Always use modules (import
/export
) over a non-standard module system. You can always transpile to your preferred module system.
Why? Modules are the future, let's start using the future now.
ad
t AirbnbStyleGuide = require('./AirbnbStyleGuide');
le.exports = AirbnbStyleGuide.es6;
k
rt AirbnbStyleGuide from './AirbnbStyleGuide';
rt default AirbnbStyleGuide.es6;
est
rt { es6 } from './AirbnbStyleGuide';
rt default es6;
10.2 Do not use wildcard imports.
Why? This makes sure you have a single default export.
ad
rt * as AirbnbStyleGuide from './AirbnbStyleGuide';
ood
rt 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.
ad
ilename es6.js
rt { es6 as default } from './airbnbStyleGuide';
ood
ilename es6.js
rt { es6 } from './AirbnbStyleGuide';
rt default es6;
11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map()
and reduce()
instead of loops like for-of
. eslint: no-iterator
Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.
t numbers = [1, 2, 3, 4, 5];
ad
sum = 0;
(let num of numbers) {
m += num;
=== 15;
ood
sum = 0;
ers.forEach(num => sum += num);
=== 15;
est (use the functional force)
t sum = numbers.reduce((total, num) => total + num, 0);
=== 15;
11.2 Don't use generators for now.
Why? They don't transpile well to ES5.
12.1 Use dot notation when accessing properties. eslint: dot-notation
jscs: requireDotNotation
t luke = {
di: true,
e: 28,
ad
t isJedi = luke['jedi'];
ood
t isJedi = luke.jedi;
12.2 Use subscript notation []
when accessing properties with a variable.
t luke = {
di: true,
e: 28,
tion getProp(prop) {
turn luke[prop];
t isJedi = getProp('jedi');
13.1 Always use const
to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.
ad
rPower = new SuperPower();
ood
t superPower = new SuperPower();
13.2 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.
ad
t items = getItems(),
goSportsTeam = true,
dragonball = 'z';
ad
compare to above, and try to spot the mistake)
t items = getItems(),
goSportsTeam = true;
dragonball = 'z';
ood
t items = getItems();
t goSportsTeam = true;
t dragonball = 'z';
13.3 Group all your const
s and then group all your let
s.
Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.
ad
i, len, dragonball,
items = getItems(),
goSportsTeam = true;
ad
i;
t items = getItems();
dragonball;
t goSportsTeam = true;
len;
ood
t goSportsTeam = true;
t items = getItems();
dragonball;
i;
length;
13.4 Assign variables where you need them, but place them in a reasonable place.
Why?
let
andconst
are block scoped and not function scoped.
ad - unnecessary function call
tion checkName(hasName) {
nst name = getName();
(hasName === 'test') {
return false;
(name === 'test') {
this.setName('');
return false;
turn name;
ood
tion checkName(hasName) {
(hasName === 'test') {
return false;
nst name = getName();
(name === 'test') {
this.setName('');
return false;
turn name;
14.1 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.
e know this wouldn't work (assuming there
s no notDefined global variable)
tion example() {
nsole.log(notDefined); // => throws a ReferenceError
reating a variable declaration after you
eference the variable will work due to
ariable hoisting. Note: the assignment
alue of `true` is not hoisted.
tion example() {
nsole.log(declaredButNotAssigned); // => undefined
r declaredButNotAssigned = true;
he interpreter is hoisting the variable
eclaration to the top of the scope,
hich means our example could be rewritten as:
tion example() {
t declaredButNotAssigned;
nsole.log(declaredButNotAssigned); // => undefined
claredButNotAssigned = true;
sing const and let
tion example() {
nsole.log(declaredButNotAssigned); // => throws a ReferenceError
nsole.log(typeof declaredButNotAssigned); // => throws a ReferenceError
nst declaredButNotAssigned = true;
14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
tion example() {
nsole.log(anonymous); // => undefined
onymous(); // => TypeError anonymous is not a function
r anonymous = function () {
console.log('anonymous function expression');
14.3 Named function expressions hoist the variable name, not the function name or the function body.
tion example() {
nsole.log(named); // => undefined
med(); // => TypeError named is not a function
perPower(); // => ReferenceError superPower is not defined
r named = function superPower() {
console.log('Flying');
he same is true when the function name
s the same as the variable name.
tion example() {
nsole.log(named); // => undefined
med(); // => TypeError named is not a function
r named = function named() {
console.log('named');
14.4 Function declarations hoist their name and the function body.
tion example() {
perPower(); // => Flying
nction superPower() {
console.log('Flying');
For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.
15.2 Conditional statements such as the if
statement evaluate their expression using coercion with the ToBoolean
abstract method and always follow these simple rules:
''
, otherwise true[0] && []) {
true
an array (even an empty one) is an object, objects will evaluate to true
15.3 Use shortcuts.
ad
name !== '') {
...stuff...
ood
name) {
...stuff...
ad
collection.length > 0) {
...stuff...
ood
collection.length) {
...stuff...
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
).
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
.
ad
ch (foo) {
se 1:
let x = 1;
break;
se 2:
const y = 2;
break;
se 3:
function f() {}
break;
fault:
class C {}
ood
ch (foo) {
se 1: {
let x = 1;
break;
se 2: {
const y = 2;
break;
se 3: {
function f() {}
break;
se 4:
bar();
break;
fault: {
class C {}
15.6 Ternaries should not be nested and generally be single line expressions.
eslint rules: no-nested-ternary
.
ad
t foo = maybe1 > maybe2
"bar"
value1 > value2 ? "baz" : null;
etter
t maybeNull = value1 > value2 ? 'baz' : null;
t foo = maybe1 > maybe2
'bar'
maybeNull;
est
t maybeNull = value1 > value2 ? 'baz' : null;
t foo = maybe1 > maybe2 ? 'bar' : maybeNull;
15.7 Avoid unneeded ternary statements.
eslint rules: no-unneeded-ternary
.
ad
t foo = a ? a : b;
t bar = c ? true : false;
t baz = c ? false : true;
ood
t foo = a || b;
t bar = !!c;
t baz = !c;
16.1 Use braces with all multi-line blocks.
ad
test)
turn false;
ood
test) return false;
ood
test) {
turn false;
ad
tion foo() { return false; }
ood
tion bar() {
turn false;
16.2 If you're using multi-line blocks with if
and else
, put else
on the same line as your
if
block's closing brace. eslint: brace-style
jscs: disallowNewlineBeforeBlockStatements
ad
test) {
ing1();
ing2();
{
ing3();
ood
test) {
ing1();
ing2();
se {
ing3();
17.1 Use /** ... */
for multi-line comments. Include a description, specify types and values for all parameters and return values.
ad
ake() returns a new element
ased on the passed in tag name
param {String} tag
return {Element} element
tion make(tag) {
...stuff...
turn element;
ood
ake() returns a new element
ased on the passed in tag name
param {String} tag
return {Element} element
tion make(tag) {
...stuff...
turn element;
17.2 Use //
for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it's on the first line of a block.
ad
t active = true; // is current tab
ood
s current tab
t active = true;
ad
tion getType() {
nsole.log('fetching type...');
set the default type to 'no type'
nst type = this._type || 'no type';
turn type;
ood
tion getType() {
nsole.log('fetching type...');
set the default type to 'no type'
nst type = this._type || 'no type';
turn type;
lso good
tion getType() {
set the default type to 'no type'
nst type = this._type || 'no type';
turn type;
17.3 Prefixing your comments with FIXME
or TODO
helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME -- need to figure this out
or TODO -- need to implement
.
17.4 Use // FIXME:
to annotate problems.
s Calculator extends Abacus {
nstructor() {
super();
// FIXME: shouldn't use a global here
total = 0;
17.5 Use // TODO:
to annotate solutions to problems.
s Calculator extends Abacus {
nstructor() {
super();
// TODO: total should be configurable by an options param
this.total = 0;
18.1 Use soft tabs set to 2 spaces. eslint: indent
jscs: validateIndentation
ad
tion foo() {
const name;
ad
tion bar() {
st name;
ood
tion baz() {
nst name;
18.2 Place 1 space before the leading brace. eslint: space-before-blocks
jscs: requireSpaceBeforeBlockStatements
ad
tion test(){
nsole.log('test');
ood
tion test() {
nsole.log('test');
ad
set('attr',{
e: '1 year',
eed: 'Bernese Mountain Dog',
ood
set('attr', {
e: '1 year',
eed: 'Bernese Mountain Dog',
18.3 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: space-after-keywords
, space-before-keywords
jscs: requireSpaceAfterKeywords
ad
sJedi) {
ght ();
ood
isJedi) {
ght();
ad
tion fight () {
nsole.log ('Swooosh!');
ood
tion fight() {
nsole.log('Swooosh!');
18.4 Set off operators with spaces. eslint: space-infix-ops
jscs: requireSpaceBeforeBinaryOperators
, requireSpaceAfterBinaryOperators
ad
t x=y+5;
ood
t x = y + 5;
18.5 End files with a single newline character.
ad
ction (global) {
...stuff...
his);
avascript
ad
ction (global) {
...stuff...
his);?
avascript
ood
ction (global) {
...stuff...
his);?
18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which
emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call
no-whitespace-before-property
ad
items').find('.selected').highlight().end().find('.open').updateCount();
ad
items').
nd('.selected').
highlight().
end().
nd('.open').
updateCount();
ood
items')
ind('.selected')
.highlight()
.end()
ind('.open')
.updateCount();
ad
t 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);
ood
t leds = stage.selectAll('.led')
.data(data)
nter().append('svg:svg')
.classed('led', true)
.attr('width', (radius + margin) * 2)
ppend('svg:g')
.attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
.call(tron.led);
ood
t leds = stage.selectAll('.led').data(data);
18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks
ad
foo) {
turn bar;
rn baz;
ood
foo) {
turn bar;
rn baz;
ad
t obj = {
o() {
r() {
rn obj;
ood
t obj = {
o() {
r() {
rn obj;
ad
t arr = [
nction foo() {
nction bar() {
rn arr;
ood
t arr = [
nction foo() {
nction bar() {
rn arr;
18.8 Do not pad your blocks with blank lines. eslint: padded-blocks
jscs: disallowPaddingNewlinesInBlocks
ad
tion bar() {
nsole.log(foo);
lso bad
baz) {
nsole.log(qux);
se {
nsole.log(foo);
ood
tion bar() {
nsole.log(foo);
ood
baz) {
nsole.log(qux);
se {
nsole.log(foo);
18.9 Do not add spaces inside parentheses. eslint: space-in-parens
jscs: disallowSpacesInsideParentheses
ad
tion bar( foo ) {
turn foo;
ood
tion bar(foo) {
turn foo;
ad
foo ) {
nsole.log(foo);
ood
foo) {
nsole.log(foo);
18.10 Do not add spaces inside brackets. eslint: array-bracket-spacing
jscs: disallowSpacesInsideArrayBrackets
ad
t foo = [ 1, 2, 3 ];
ole.log(foo[ 0 ]);
ood
t foo = [1, 2, 3];
ole.log(foo[0]);
18.11 Add spaces inside curly braces. eslint: object-curly-spacing
jscs: disallowSpacesInsideObjectBrackets
ad
t foo = {clark: 'kent'};
ood
t foo = { clark: 'kent' };
18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len
jscs: maximumLineLength
Why? This ensures readability and maintainability.
ad
t foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';
ad
ax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
ood
t foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
hatever wizard constrains a helpful ally. The counterpart ascends!';
ood
ax({
thod: 'POST',
l: 'https://airbnb.com/',
ta: { name: 'John' },
one(() => console.log('Congratulations!'))
ail(() => console.log('You have failed this city.'));
19.1 Leading commas: Nope. eslint: comma-style
jscs: requireCommaBeforeLineBreak
ad
t story = [
once
upon
aTime
ood
t story = [
ce,
on,
ime,
ad
t hero = {
firstName: 'Ada'
lastName: 'Lovelace'
birthYear: 1815
superPower: 'computers'
ood
t hero = {
rstName: 'Ada',
stName: 'Lovelace',
rthYear: 1815,
perPower: 'computers',
19.2 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.
ad - git diff without trailing comma
t hero = {
firstName: 'Florence',
lastName: 'Nightingale'
lastName: 'Nightingale',
inventorOf: ['coxcomb graph', 'modern nursing']
ood - git diff with trailing comma
t hero = {
firstName: 'Florence',
lastName: 'Nightingale',
inventorOf: ['coxcomb chart', 'modern nursing'],
ad
t hero = {
rstName: 'Dana',
stName: 'Scully'
t heroes = [
atman',
uperman'
ood
t hero = {
rstName: 'Dana',
stName: 'Scully',
t heroes = [
atman',
uperman',
20.1 Yup. eslint: semi
jscs: requireSemicolons
ad
ction () {
nst name = 'Skywalker'
turn name
ood
=> {
nst name = 'Skywalker';
turn name;
;
ood (guards against the function becoming an argument when two files with IIFEs are concatenated)
=> {
nst name = 'Skywalker';
turn name;
;
21.1 Perform type coercion at the beginning of the statement.
21.2 Strings:
> this.reviewScore = 9;
ad
t totalScore = this.reviewScore + '';
ood
t totalScore = String(this.reviewScore);
21.3 Numbers: Use Number
for type casting and parseInt
always with a radix for parsing strings. eslint: radix
t inputValue = '4';
ad
t val = new Number(inputValue);
ad
t val = +inputValue;
ad
t val = inputValue >> 0;
ad
t val = parseInt(inputValue);
ood
t val = Number(inputValue);
ood
t val = parseInt(inputValue, 10);
21.4 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.
ood
arseInt was the reason my code was slow.
itshifting the String to coerce it to a
umber made it a lot faster.
t val = inputValue >> 0;
21.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
483647 >> 0 //=> 2147483647
483648 >> 0 //=> -2147483648
483649 >> 0 //=> -2147483647
21.6 Booleans:
t age = 0;
ad
t hasAge = new Boolean(age);
ood
t hasAge = Boolean(age);
ood
t hasAge = !!age;
22.1 Avoid single letter names. Be descriptive with your naming.
ad
tion q() {
...stuff...
ood
tion query() {
..stuff..
22.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase
jscs: requireCamelCaseOrUpperCaseIdentifiers
ad
t OBJEcttsssss = {};
t this_is_my_object = {};
tion c() {}
ood
t thisIsMyObject = {};
tion thisIsMyFunction() {}
22.3 Use PascalCase when naming constructors or classes. eslint: new-cap
jscs: requireCapitalizedConstructors
ad
tion user(options) {
is.name = options.name;
t bad = new user({
me: 'nope',
ood
s User {
nstructor(options) {
this.name = options.name;
t good = new User({
me: 'yup',
22.4 Use a leading underscore _
when naming private properties. eslint: no-underscore-dangle
jscs: disallowDanglingUnderscores
ad
.__firstName__ = 'Panda';
.firstName_ = 'Panda';
ood
._firstName = 'Panda';
22.5 Don't save references to this
. Use arrow functions or Function#bind. jscs: disallowNodeTypes
ad
tion foo() {
nst self = this;
turn function () {
console.log(self);
ad
tion foo() {
nst that = this;
turn function () {
console.log(that);
ood
tion foo() {
turn () => {
console.log(this);
22.6 If your file exports a single class, your filename should be exactly the name of the class.
ile contents
s CheckBox {
...
rt default CheckBox;
n some other file
ad
rt CheckBox from './checkBox';
ad
rt CheckBox from './check_box';
ood
rt CheckBox from './CheckBox';
22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
tion makeStyleGuide() {
rt default makeStyleGuide;
22.8 Use PascalCase when you export a singleton / function library / bare object.
t AirbnbStyleGuide = {
6: {
rt default AirbnbStyleGuide;
23.1 Accessor functions for properties are not required.
23.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').
ad
on.age();
ood
on.getAge();
ad
on.age(25);
ood
on.setAge(25);
23.3 If the property is a boolean
, use isVal()
or hasVal()
.
ad
!dragon.age()) {
turn false;
ood
!dragon.hasAge()) {
turn false;
23.4 It's okay to create get() and set() functions, but be consistent.
s Jedi {
nstructor(options = {}) {
const lightsaber = options.lightsaber || 'blue';
this.set('lightsaber', lightsaber);
t(key, val) {
this[key] = val;
t(key) {
return this[key];
24.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass a hash instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
ad
is).trigger('listingUpdated', listing.id);
is).on('listingUpdated', (e, listingId) => {
do something with listingId
prefer:
ood
is).trigger('listingUpdated', { listingId: listing.id });
is).on('listingUpdated', (e, data) => {
do something with data.listingId
25.1 Prefix jQuery object variables with a $
. jscs: requireDollarBeforejQueryAssignment
ad
t sidebar = $('.sidebar');
ood
t $sidebar = $('.sidebar');
ood
t $sidebarBtn = $('.sidebar-btn');
25.2 Cache jQuery lookups.
ad
tion setSidebar() {
'.sidebar').hide();
...stuff...
'.sidebar').css({
'background-color': 'pink'
;
ood
tion setSidebar() {
nst $sidebar = $('.sidebar');
idebar.hide();
...stuff...
idebar.css({
'background-color': 'pink'
;
25.3 For DOM queries use Cascading $('.sidebar ul')
or parent > child $('.sidebar > ul')
. jsPerf
25.4 Use find
with scoped jQuery object queries.
ad
l', '.sidebar').hide();
ad
sidebar').find('ul').hide();
ood
sidebar ul').hide();
ood
sidebar > ul').hide();
ood
ebar.find('ul').hide();
27.1 This is a collection of links to the various ES6 features.
28.1 Yup.
tion foo() {
turn true;
28.2 No, but seriously:
Whichever testing framework you use, you should be writing tests!
Strive to write many small pure functions, and minimize where mutations occur.
Be cautious about stubs and mocks - they can make your tests more brittle.
We primarily use 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.
Learning ES6
Read This
Tools
Other Style Guides
Other Styles
Further Reading
Books
Blogs
Podcasts
This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.
This style guide is also available in other languages:
(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.