-Airbnb JavaScript Style Guide() {
+Airbnb JavaScript Style Guide() {A mostly reasonable approach to JavaScript
+
Other Style Guides
-Table of Contents
+Table of Contents- Types @@ -48,7 +49,7 @@
- Strings
- Functions
- Arrow Functions -
- Constructors +
- Classes & Constructors
- Modules
- Iterators and Generators
- Properties @@ -79,11 +80,13 @@
-Types
+Types + +-
-
1.1 Primitives: When you access a primitive type you work directly on its value.
+1.1 Primitives: When you access a primitive type you work directly on its value.
-
-
1.2 Complex: When you access a complex type you work on a reference to its value.
+1.2 Complex: When you access a complex type you work on a reference to its value.
object
@@ -121,49 +126,51 @@ bar[0] = ⬆ back to top
-
-
2.1 Use
+constfor all of your references; avoid usingvar.2.1 Use
constfor 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.
eslint rules:
+prefer-const,no-const-assign.+// bad +var a = 1; +var b = 2; -
- // good - const a = 1; - const b = 2;// bad - var a = 1; - var b = 2; +// good +const a = 1; +const b = 2;
-
-
2.2 If you must reassign references, use
+letinstead ofvar.2.2 If you must reassign references, use
letinstead ofvar. eslint:no-varjscs:disallowVar
-Why?
letis block-scoped rather than function-scoped likevar.eslint rules:
+no-var.+// bad +var count = 1; +if (true) { + count += 1; +} -
- // good, use the let. - let count = 1; - if (true) { - count += 1; - }// bad - var count = 1; - if (true) { - count += 1; - } +// good, use the let. +let count = 1; +if (true) { + count += 1; +}
-
-
2.3 Note that both
+letandconstare block-scoped.2.3 Note that both
letandconstare block-scoped.- +// const and let only exist in the blocks they are defined in. { @@ -178,22 +185,24 @@ bar[0] = ⬆ back to top
-Objects
+Objects + +-
-
3.1 Use the literal syntax for object creation.
+3.1 Use the literal syntax for object creation. eslint:
-no-new-objecteslint rules:
+no-new-object.+// bad +const item = new Object(); -
- // good - const item = {};// bad - const item = new Object(); +// good +const 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.
+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:
disallowIdentifierNames+ +// bad const superman = { @@ -206,9 +215,11 @@ bar[0] = : { clark: 'kent' }, hidden: true, };
-
-
3.3 Use readable synonyms in place of reserved words.
+3.3 Use readable synonyms in place of reserved words. jscs:
disallowIdentifierNames- +// bad const superman = { @@ -225,133 +236,178 @@ bar[0] = : 'alien', };
-
-
3.4 Use computed property names when creating objects with dynamic property names.
+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.
+// good +const obj = { + id: 5, + name: 'San Francisco', + [getKey('enabled')]: true, +};- function getKey(k) { - return `a key named ${k}`; - } +function getKey(k) { + return `a key named ${k}`; +} - // bad - const obj = { - id: 5, - name: 'San Francisco', - }; - obj[getKey('enabled')] = true; +// bad +const obj = { + id: 5, + name: 'San Francisco', +}; +obj[getKey('enabled')] = true; - // good - const obj = { - id: 5, - name: 'San Francisco', - [getKey('enabled')]: true, - };
-
-
-
-
3.5 Use object method shorthand.
+3.5 Use object method shorthand. eslint:
-object-shorthandjscs:requireEnhancedObjectLiteralseslint rules:
+object-shorthand.- - +// bad +const atom = { + value: 1, -
- addValue(value) { - return atom.value + value; - }, - };// bad - const atom = { - value: 1, + addValue: function (value) { + return atom.value + value; + }, +}; - addValue: function (value) { - return atom.value + value; - }, - }; +// good +const atom = { + value: 1, - // good - const atom = { - value: 1, + addValue(value) { + return atom.value + value; + }, +};
-
-
3.6 Use property value shorthand.
+3.6 Use property value shorthand. eslint:
object-shorthandjscs:requireEnhancedObjectLiterals
-Why? It is shorter to write and descriptive.
eslint rules:
+object-shorthand.+const lukeSkywalker = 'Luke Skywalker'; -
- // good - const obj = { - lukeSkywalker, - };const lukeSkywalker = 'Luke Skywalker'; +// bad +const obj = { + lukeSkywalker: lukeSkywalker, +}; - // bad - const obj = { - lukeSkywalker: lukeSkywalker, - }; +// good +const obj = { + lukeSkywalker, +};
-
-
3.7 Group your shorthand properties at the beginning of your object declaration.
+3.7 Group your shorthand properties at the beginning of your object declaration.
-Why? It's easier to tell which properties are using the shorthand.
+ + +const anakinSkywalker = 'Anakin Skywalker'; - const lukeSkywalker = 'Luke Skywalker'; +
+// good +const obj = { + lukeSkywalker, + anakinSkywalker, + episodeOne: 1, + twoJediWalkIntoACantina: 2, + episodeThree: 3, + mayTheFourth: 4, +};const anakinSkywalker = 'Anakin Skywalker'; +const lukeSkywalker = 'Luke Skywalker'; - // bad - const obj = { - episodeOne: 1, - twoJediWalkIntoACantina: 2, - lukeSkywalker, - episodeThree: 3, - mayTheFourth: 4, - anakinSkywalker, - }; +// 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, - };
+ -
+
3.8 Only quote properties that are invalid identifiers. eslint:
+ +quote-propsjscs:disallowQuotedKeysInObjects+
+ +Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
++ + +// bad +const bad = { + 'foo': 3, + 'bar': 4, + 'data-blah': 5, +}; + +// good +const good = { + foo: 3, + bar: 4, + 'data-blah': 5, +};
+ -
+
3.9 Do not call
+ +Object.prototypemethods directly, such ashasOwnProperty,propertyIsEnumerable, andisPrototypeOf.+
+ +Why? These methods may be shadowed by properties on the object in question - consider
+{ hasOwnProperty: false }- or, the object may be a null object (Object.create(null)).// bad +console.log(object.hasOwnProperty(key)); + +// good +console.log(Object.prototype.hasOwnProperty.call(object, key)); + +// best +const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope. +/* or */ +const has = require('has'); +… +console.log(has.call(object, key));
-
-
4.1 Use the literal syntax for array creation.
+4.1 Use the literal syntax for array creation. eslint:
-no-array-constructoreslint rules:
+no-array-constructor.+// bad +const items = new Array(); -
- // good - const items = [];// bad - const items = new Array(); +// good +const items = [];
-
-
4.2 Use Array#push instead of direct assignment to add items to an array.
+4.2 Use Array#push instead of direct assignment to add items to an array.
- +const someStack = []; @@ -361,10 +417,10 @@ someStack[someStack.length// good someStack.push('abracadabra');
-
-
4.3 Use array spreads
+...to copy arrays.4.3 Use array spreads
...to copy arrays.+const itemsCopy = [...items]; + +// bad const len = items.length; @@ -376,50 +432,105 @@ someStack[someStack.length// good -const itemsCopy = [...items];
-
-
4.4 To convert an array-like object to an array, use Array#from.
+4.4 To convert an array-like object to an array, use Array.from.
-+const foo = document.querySelectorAll('.foo'); -const nodes = Array.from(foo);
+ + +const foo = document.querySelectorAll('.foo'); +const nodes = Array.from(foo);
+ -
+
4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint:
+ +array-callback-return// good +[1, 2, 3].map((x) => { + const y = x + 1; + return x * y; +}); + +// good +[1, 2, 3].map(x => x + 1); + +// bad +const flat = {}; +[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { + const flatten = memo.concat(item); + flat[index] = flatten; +}); + +// good +const flat = {}; +[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { + const flatten = memo.concat(item); + flat[index] = flatten; + return flatten; +}); + +// bad +inbox.filter((msg) => { + const { subject, author } = msg; + if (subject === 'Mockingbird') { + return author === 'Harper Lee'; + } else { + return false; + } +}); + +// good +inbox.filter((msg) => { + const { subject, author } = msg; + if (subject === 'Mockingbird') { + return author === 'Harper Lee'; + } + + return false; +});
-
-
5.1 Use object destructuring when accessing and using multiple properties of an object.
+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.
+ +// bad - function getFullName(user) { - const firstName = user.firstName; - const lastName = user.lastName; +
+// best +function getFullName({ firstName, lastName }) { + return `${firstName} ${lastName}`; +}// bad +function getFullName(user) { + const firstName = user.firstName; + const lastName = user.lastName; - return `${firstName} ${lastName}`; - } + return `${firstName} ${lastName}`; +} - // good - function getFullName(obj) { - const { firstName, lastName } = obj; - return `${firstName} ${lastName}`; - } +// good +function getFullName(user) { + const { firstName, lastName } = user; + return `${firstName} ${lastName}`; +} - // best - function getFullName({ firstName, lastName }) { - return `${firstName} ${lastName}`; - }
-
-
5.2 Use array destructuring.
+5.2 Use array destructuring. jscs:
requireArrayDestructuring+ +const arr = [1, 2, 3, 4]; @@ -429,54 +540,62 @@ someStack[someStack.length// good const [first, second] = arr;
-
-
5.3 Use object destructuring for multiple return values, not array destructuring.
+5.3 Use object destructuring for multiple return values, not array destructuring. jscs:
disallowArrayDestructuringReturn
-Why? You can add new properties over time or change the order of things without breaking call sites.
// bad - function processInput(input) { - // then a miracle occurs - return [left, right, top, bottom]; - } +
+// the caller selects only the data they need +const { left, top } = processInput(input);// 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); +// 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 }; - } +// good +function processInput(input) { + // then a miracle occurs + return { left, right, top, bottom }; +} - // the caller selects only the data they need - const { left, right } = processInput(input);
-
-
6.1 Use single quotes
+''for strings.6.1 Use single quotes
-''for strings. eslint:quotesjscs:validateQuoteMarkseslint rules:
+quotes.+// bad +const name = "Capt. Janeway"; -
- // good - const name = 'Capt. Janeway';// bad - const name = "Capt. Janeway"; +// good +const name = 'Capt. Janeway';
- 6.2 Strings longer than 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.
+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.
- +// 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.'; @@ -492,68 +611,104 @@ someStack[someStack.length'of Batman. When you stop to think about how Batman had anything to do ' + 'with this, you would get nowhere fast.';
-
-
6.4 When programmatically building up strings, use template strings instead of concatenation.
+6.4 When programmatically building up strings, use template strings instead of concatenation. eslint:
prefer-templatetemplate-curly-spacingjscs:requireTemplateStrings
-Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.
eslint rules:
+prefer-template.+ +// bad +function sayHi(name) { + return 'How are you, ' + name + '?'; +} -
+// good +function sayHi(name) { + return `How are you, ${name}?`; +}// 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, '?'].join(); - } +// bad +function sayHi(name) { + return `How are you, ${ name }?`; +} - // good - function sayHi(name) { - return `How are you, ${name}?`; - }
-
-6.5 Never use
eval()on a string, it opens too many vulnerabilities.
+
-References
+References + +-Arrays
+Arrays + +-Destructuring
+Destructuring + +-Strings
+Strings + +6.5 Never use
+ + +eval()on a string, it opens too many vulnerabilities.
+ -
+
6.6 Do not unnecessarily escape characters in strings. eslint:
+ +no-useless-escape+
+ +Why? Backslashes harm readability, thus they should only be present when necessary.
++// bad +const foo = '\'this\' \i\s \"quoted\"'; + +// good +const foo = '\'this\' is "quoted"'; +const foo = `'this' is "quoted"`;
-Functions
+Functions + +-
-
7.1 Use function declarations instead of function expressions.
+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.
+ +// bad - const foo = function () { - }; +
+// good +function foo() { +}// bad +const foo = function () { +}; - // good - function foo() { - }
-
-
7.2 Function expressions:
+7.2 Wrap immediately invoked function expressions in parentheses. eslint:
+ +wrap-iifejscs:requireParenthesesAroundIIFE+
Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
++}()); + +// immediately-invoked function expression (IIFE) -(() => { +(function () { console.log('Welcome to the Internet. Please follow me.'); -})();
- 7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.
-
-
7.4 Note: ECMA-262 defines a
+blockas a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint:
+ + +no-loop-func
+ -
+
7.4 Note: ECMA-262 defines a
blockas a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.+ +// bad if (currentUser) { @@ -569,12 +724,14 @@ someStack[someStack.lengthconsole.log('Yup.'); }; }
-
-
7.5 Never name a parameter
+arguments. This will take precedence over theargumentsobject that is given to every function scope.7.5 Never name a parameter
arguments. This will take precedence over theargumentsobject that is given to every function scope.- +// bad -function nope(name, options, arguments) { +function nope(name, options, arguments) { // ...stuff... } @@ -583,30 +740,30 @@ someStack[someStack.length// ...stuff... }
-
-
7.6 Never use
+arguments, opt to use rest syntax...instead.7.6 Never use
arguments, opt to use rest syntax...instead. eslint:prefer-rest-params-
-Why?
+...is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like likearguments.Why?
...is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like likearguments.- +// bad - function concatenateAll() { - const args = Array.prototype.slice.call(arguments); - return args.join(''); - } +
+// good +function concatenateAll(...args) { + return args.join(''); +}// bad +function concatenateAll() { + const args = Array.prototype.slice.call(arguments); + return args.join(''); +} - // good - function concatenateAll(...args) { - return args.join(''); - }
-
-
7.7 Use default parameter syntax rather than mutating function arguments.
+7.7 Use default parameter syntax rather than mutating function arguments.
+ +// really bad function handleThings(opts) { @@ -629,9 +786,11 @@ someStack[someStack.lengthfunction handleThings(opts = {}) { // ... }
-
-
7.8 Avoid side effects with default parameters.
+7.8 Avoid side effects with default parameters.
Why? They are confusing to reason about.
@@ -639,16 +798,18 @@ someStack[someStack.lengthvar b = 1; // bad -function count(a = b++) { +function count(a = b++) { console.log(a); } count(); // 1 count(); // 2 count(3); // 3 count(); // 3
+ + -
-
7.9 Always put default parameters last.
+7.9 Always put default parameters last.
-// bad function handleThings(opts = {}, name) { @@ -659,12 +820,11 @@ someStack[someStack.lengthfunction handleThings(name, opts = {}) { // ... }
-
-
+
+
-
-
7.10 Never use the Function constructor to create a new function.
+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.
@@ -675,9 +835,11 @@ someStack[someStack.length// still bad var subtract = Function('a', 'b', 'return a - b'); + + -
-
7.11 Spacing in a function signature.
+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.
@@ -691,17 +853,64 @@ someStack[someStack.length// good const x = function () {}; const y = function a() {}; + + +
+ -
+
7.12 Never mutate parameters. eslint:
+ +no-param-reassign+
+ +Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
++ + +// bad +function f1(obj) { + obj.key = 1; +}; + +// good +function f2(obj) { + const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1; +};
+ -
+
7.13 Never reassign parameters. eslint:
+ +no-param-reassign+
+ +Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the
+argumentsobject. It can also cause optimization issues, especially in V8.// bad +function f1(a) { + a = 1; +} + +function f2(a) { + if (!a) { a = 1; } +} + +// good +function f3(a) { + const b = a || 1; +} + +function f4(a = 1) { +}
-Arrow Functions
+Arrow Functions + +-
-
8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation.
+8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint:
prefer-arrow-callback,arrow-spacingjscs:requireArrowFunctions
-Why? It creates a version of the function that executes in the context of
@@ -709,152 +918,193 @@ someStack[someStack.lengthWhy not? If you have a fairly complicated function, you might move that logic out into its own function declaration.this, which is usually what you want, and is a more concise syntax.eslint rules:
+prefer-arrow-callback,arrow-spacing.+// 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; - });// bad - [1, 2, 3].map(function (x) { - const y = x + 1; - return x * y; - }); +// good +[1, 2, 3].map((x) => { + const y = x + 1; + return x * y; +});
-
-
8.2 If the function body consists of a single expression, feel free to omit the braces and use the implicit return. Otherwise use a
+returnstatement.8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a
returnstatement. eslint:arrow-parens,arrow-body-stylejscs:disallowParenthesesAroundArrowParam,requireShorthandArrowFunctions
-Why? Syntactic sugar. It reads well when multiple functions are chained together.
- -Why not? If you plan on returning an object.
eslint rules:
+arrow-parens,arrow-body-style.+ +// bad +[1, 2, 3].map(number => { + const nextNumber = number + 1; + `A string containing the ${nextNumber}.`; +}); -
+// good +[1, 2, 3].map((number, index) => ({ + index: number +}));// good - [1, 2, 3].map(number => `A string containing the ${number}.`); +// good +[1, 2, 3].map(number => `A string containing the ${number}.`); - // bad - [1, 2, 3].map(number => { - const nextNumber = number + 1; - `A string containing the ${nextNumber}.`; - }); +// good +[1, 2, 3].map((number) => { + const nextNumber = number + 1; + return `A string containing the ${nextNumber}.`; +}); - // good - [1, 2, 3].map(number => { - const nextNumber = number + 1; - return `A string containing the ${nextNumber}.`; - });
-
-
8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
+8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
-Why? It shows clearly where the function starts and ends.
+ +// bad - [1, 2, 3].map(number => 'As time went by, the string containing the ' + - `${number} became much longer. So we needed to break it over multiple ` + - 'lines.' - ); +
+// good +[1, 2, 3].map(number => ( + `As time went by, the string containing the ${number} became much ` + + 'longer. So we needed to break it over multiple lines.' +));// bad +[1, 2, 3].map(number => 'As time went by, the string containing the ' + + `${number} became much longer. So we needed to break it over multiple ` + + 'lines.' +); - // good - [1, 2, 3].map(number => ( - `As time went by, the string containing the ${number} became much ` + - 'longer. So we needed to break it over multiple lines.' - ));
-
-
8.4 If your function only takes a single argument, feel free to omit the parentheses.
+8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint:
arrow-parensjscs:disallowParenthesesAroundArrowParam
-Why? Less visual clutter.
eslint rules:
+arrow-parens.+ + +// bad +[1, 2, 3].map((x) => x * x); -
+// good +[1, 2, 3].map(number => ( + `A long string with the ${number}. It’s so long that we’ve broken it ` + + 'over multiple lines!' +)); + +// bad +[1, 2, 3].map(x => { + const y = x + 1; + return x * y; +}); + +// good +[1, 2, 3].map((x) => { + const y = x + 1; + return x * y; +});// good - [1, 2, 3].map(x => x * x); +// good +[1, 2, 3].map(x => x * x); - // good - [1, 2, 3].reduce((y, x) => x + y);
+ -
+
8.5 Avoid confusing arrow function syntax (
+ +=>) with comparison operators (<=,>=). eslint:no-confusing-arrow// bad +const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize; + +// bad +const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize; + +// good +const itemHeight = (item) => { return item.height > 256 ? item.largeSize : item.smallSize; };
-Constructors
+Classes & Constructors + +-
-
9.1 Always use
+class. Avoid manipulatingprototypedirectly.9.1 Always use
class. Avoid manipulatingprototypedirectly.
-Why?
classsyntax is more concise and easier to reason about.+// bad - function Queue(contents = []) { - this._queue = [...contents]; +
- - // good - class Queue { - constructor(contents = []) { - this._queue = [...contents]; - } - pop() { - const value = this._queue[0]; - this._queue.splice(0, 1); - return value; - } - }// bad +function Queue(contents = []) { + this.queue = [...contents]; +} +Queue.prototype.pop = function () { + const value = this.queue[0]; + this.queue.splice(0, 1); + return value; +}; + + +// good +class Queue { + constructor(contents = []) { + this.queue = [...contents]; } - Queue.prototype.pop = function () { - const value = this._queue[0]; - this._queue.splice(0, 1); + pop() { + const value = this.queue[0]; + this.queue.splice(0, 1); return value; } +}
-
-
9.2 Use
+extendsfor inheritance.9.2 Use
extendsfor inheritance.
-Why? It is a built-in way to inherit prototype functionality without breaking
instanceof.+// bad - const inherits = require('inherits'); - function PeekableQueue(contents) { - Queue.apply(this, contents); - } - inherits(PeekableQueue, Queue); - PeekableQueue.prototype.peek = function () { +
- // good - class PeekableQueue extends Queue { - peek() { - return this._queue[0]; - } - }// bad +const inherits = require('inherits'); +function PeekableQueue(contents) { + Queue.apply(this, contents); +} +inherits(PeekableQueue, Queue); +PeekableQueue.prototype.peek = function () { + return this._queue[0]; +} + +// good +class PeekableQueue extends Queue { + peek() { return this._queue[0]; } +}
-
-
9.3 Methods can return
+thisto help with method chaining.9.3 Methods can return
thisto help with method chaining.+ +// bad -Jedi.prototype.jump = function () { +Jedi.prototype.jump = function () { this.jumping = true; return true; }; -Jedi.prototype.setHeight = function (height) { +Jedi.prototype.setHeight = function (height) { this.height = height; }; @@ -879,9 +1129,11 @@ someStack[someStack.lengthluke.jump() .setHeight(20);
-
-
9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
+9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
+ + +class Jedi { constructor(options = {}) { @@ -896,132 +1148,318 @@ someStack[someStack.lengthreturn `Jedi - ${this.getName()}`; } }
+ -
+
9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint:
+ +no-useless-constructor+ + +// bad +class Jedi { + constructor() {} + + getName() { + return this.name; + } +} + +// bad +class Rey extends Jedi { + constructor(...args) { + super(...args); + } +} + +// good +class Rey extends Jedi { + constructor(...args) { + super(...args); + this.name = 'Rey'; + } +}
+ -
+
9.6 Avoid duplicate class members. eslint:
+ +no-dupe-class-members+
+ +Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.
+// bad +class Foo { + bar() { return 1; } + bar() { return 2; } +} + +// good +class Foo { + bar() { return 1; } +} + +// good +class Foo { + bar() { return 2; } +}
-Modules
+Modules + +-
-
10.1 Always use modules (
+import/export) over a non-standard module system. You can always transpile to your preferred module system.10.1 Always use modules (
import/export) over a non-standard module system. You can always transpile to your preferred module system.
-Why? Modules are the future, let's start using the future now.
+ +// bad - const AirbnbStyleGuide = require('./AirbnbStyleGuide'); - module.exports = AirbnbStyleGuide.es6; +
+// best +import { es6 } from './AirbnbStyleGuide'; +export default es6;// bad +const AirbnbStyleGuide = require('./AirbnbStyleGuide'); +module.exports = AirbnbStyleGuide.es6; - // ok - import AirbnbStyleGuide from './AirbnbStyleGuide'; - export default AirbnbStyleGuide.es6; +// ok +import AirbnbStyleGuide from './AirbnbStyleGuide'; +export default AirbnbStyleGuide.es6; - // best - import { es6 } from './AirbnbStyleGuide'; - export default es6;
-
-
10.2 Do not use wildcard imports.
+10.2 Do not use wildcard imports.
-Why? This makes sure you have a single default export.
+ +// bad - import * as AirbnbStyleGuide from './AirbnbStyleGuide'; +
+// good +import AirbnbStyleGuide from './AirbnbStyleGuide';// bad +import * as AirbnbStyleGuide from './AirbnbStyleGuide'; - // good - import AirbnbStyleGuide from './AirbnbStyleGuide';
-
-
10.3 And do not export directly from an import.
+10.3 And do not export directly from an import.
+Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.
+ + +// bad +// filename es6.js +export { es6 as default } from './airbnbStyleGuide'; + +// good +// filename es6.js +import { es6 } from './AirbnbStyleGuide'; +export default es6;
+ -
+
10.4 Only import from a path in one place. +eslint:
+ +no-duplicate-imports+
+Why? Having multiple lines that import from the same path can make code harder to maintain.
++ import foo, { named1, named2 } from 'foo'; + + // good + import foo, { + named1, + named2, + } from 'foo'; + + +// bad - // filename es6.js - export { es6 as default } from './airbnbStyleGuide'; + import foo from 'foo'; + // … some other imports … // + import { named1, named2 } from 'foo'; // good - // filename es6.js - import { es6 } from './AirbnbStyleGuide'; - export default es6;
+ -
+
10.5 Do not export mutable bindings. +eslint:
+ +import/no-mutable-exports+
+ +Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
++ + +// bad + let foo = 3; + export { foo } + + // good + const foo = 3; + export { foo }
+ -
+
10.6 In modules with a single export, prefer default export over named export. +eslint:
+ +import/prefer-default-export+ + +// bad + export function foo() {} + + // good + export default function foo() {}
+ -
+
10.7 Put all
+ +imports above non-import statements. +eslint:import/imports-first+
+ +Why? Since
+imports are hoisted, keeping them all at the top prevents surprising behavior.// bad + import foo from 'foo'; + foo.init(); + + import bar from 'bar'; + + // good + import foo from 'foo'; + import bar from 'bar'; + + foo.init();
-Iterators and Generators
+Iterators and Generators + +-
-
11.1 Don't use iterators. Prefer JavaScript's higher-order functions like
+map()andreduce()instead of loops likefor-of.11.1 Don't use iterators. Prefer JavaScript's higher-order functions like
map()andreduce()instead of loops likefor-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.
+Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.
eslint rules:
+no-iterator.+const numbers = [1, 2, 3, 4, 5]; -
- // best (use the functional force) - const sum = numbers.reduce((total, num) => total + num, 0); - sum === 15;const numbers = [1, 2, 3, 4, 5]; +// bad +let sum = 0; +for (let num of numbers) { + sum += num; +} - // bad - let sum = 0; - for (let num of numbers) { - sum += num; - } +sum === 15; - sum === 15; +// good +let sum = 0; +numbers.forEach(num => 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;
-
-
11.2 Don't use generators for now.
+11.2 Don't use generators for now.
+ + +Why? They don't transpile well to ES5.
+ -
+
11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint:
+ +generator-star-spacing+
+ +Why?
+functionand*are part of the same conceptual keyword -*is not a modifier forfunction,function*is a unique construct, different fromfunction.// bad +function * foo() { +} + +const bar = function * () { +} + +const baz = function *() { +} + +const quux = function*() { +} + +function*foo() { +} + +function *foo() { +} + +// very bad +function +* +foo() { +} + +const wat = function +* +() { +} + +// good +function* foo() { +} + +const foo = function* () { +}
-Properties
+Properties + +-
-
12.1 Use dot notation when accessing properties.
+12.1 Use dot notation when accessing properties. eslint:
-dot-notationjscs:requireDotNotationeslint rules:
+dot-notation.+const luke = { + jedi: true, + age: 28, +}; -
- // good - const isJedi = luke.jedi;const luke = { - jedi: true, - age: 28, - }; +// bad +const isJedi = luke['jedi']; - // bad - const isJedi = luke['jedi']; +// good +const isJedi = luke.jedi;
-
-
12.2 Use subscript notation
+[]when accessing properties with a variable.12.2 Use bracket notation
[]when accessing properties with a variable.const luke = { jedi: true, @@ -1039,127 +1477,126 @@ someStack[someStack.length⬆ back to top
-Variables
+Variables + +-
-
13.1 Always use
+constto declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.13.1 Always use
constto declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.+ +// bad superPower = new SuperPower(); // good const superPower = new SuperPower();
-
-
13.2 Use one
+constdeclaration per variable.13.2 Use one
constdeclaration per variable. eslint:one-varjscs: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.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.eslint rules:
+one-var.+// bad +const items = getItems(), + goSportsTeam = true, + dragonball = 'z'; -
- // good - const items = getItems(); - const goSportsTeam = true; - const dragonball = 'z';// 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'; - // bad - // (compare to above, and try to spot the mistake) - const items = getItems(), - goSportsTeam = true; - dragonball = 'z'; +// good +const items = getItems(); +const goSportsTeam = true; +const dragonball = 'z';
-
-
13.3 Group all your
+consts and then group all yourlets.13.3 Group all your
consts and then group all yourlets.
-Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.
+ +// bad - let i, len, dragonball, - items = getItems(), - goSportsTeam = true; +
+// good +const goSportsTeam = true; +const items = getItems(); +let dragonball; +let i; +let length;// bad +let i, len, dragonball, + items = getItems(), + goSportsTeam = true; - // bad - let i; - const items = getItems(); - let dragonball; - const goSportsTeam = true; - let len; +// bad +let i; +const items = getItems(); +let dragonball; +const goSportsTeam = true; +let len; - // good - const goSportsTeam = true; - const items = getItems(); - let dragonball; - let i; - let length;
-
-
13.4 Assign variables where you need them, but place them in a reasonable place.
+13.4 Assign variables where you need them, but place them in a reasonable place.
-Why?
letandconstare block scoped and not function scoped.// good - function () { - test(); - console.log('doing stuff..'); +
+ const name = getName(); + + if (name === 'test') { + this.setName(''); + return false; + } + + return name; +}// bad - unnecessary function call +function checkName(hasName) { + const name = getName(); - //..other stuff.. - - const name = getName(); - - if (name === 'test') { - return false; - } - - return name; + if (hasName === 'test') { + return false; } - // bad - unnecessary function call - function (hasName) { - const name = getName(); - - if (!hasName) { - return false; - } - - this.setFirstName(name); - - return true; + if (name === 'test') { + this.setName(''); + return false; } - // good - function (hasName) { - if (!hasName) { - return false; - } + return name; +} - const name = getName(); - this.setFirstName(name); +// good +function checkName(hasName) { + if (hasName === 'test') { + return false; + } - return true; - }
-Hoisting
+Hoisting + +-
-
14.1
+vardeclarations get hoisted to the top of their scope, their assignment does not.constandletdeclarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.14.1
vardeclarations get hoisted to the top of their scope, their assignment does not.constandletdeclarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.+ +// we know this wouldn't work (assuming there // is no notDefined global variable) @@ -1176,7 +1613,7 @@ superPower = new var declaredButNotAssigned = true; } -// The interpreter is hoisting the variable +// the interpreter is hoisting the variable // declaration to the top of the scope, // which means our example could be rewritten as: function example() { @@ -1191,9 +1628,11 @@ superPower = new console.log(typeof declaredButNotAssigned); // => throws a ReferenceError const declaredButNotAssigned = true; }
-
-
14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
+14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
+ +function example() { console.log(anonymous); // => undefined @@ -1204,9 +1643,11 @@ superPower = new console.log('anonymous function expression'); }; }
-
-
14.3 Named function expressions hoist the variable name, not the function name or the function body.
+14.3 Named function expressions hoist the variable name, not the function name or the function body.
+ +function example() { console.log(named); // => undefined @@ -1231,9 +1672,11 @@ superPower = new console.log('named'); } }
-
-
14.4 Function declarations hoist their name and the function body.
+14.4 Function declarations hoist their name and the function body.
function example() { superPower(); // => Flying @@ -1249,17 +1692,23 @@ superPower = new ⬆ back to top
-Comparison Operators & Equality
+Comparison Operators & Equality + +-
-15.1 Use
===and!==over==and!=.
- -
-
15.2 Conditional statements such as the
+ifstatement evaluate their expression using coercion with theToBooleanabstract method and always follow these simple rules:15.1 Use
-===and!==over==and!=. eslint:eqeqeqeslint rules:
+ +eqeqeq.
+ -
+
15.2 Conditional statements such as the
+ +ifstatement evaluate their expression using coercion with theToBooleanabstract method and always follow these simple rules:-
+
- +Objects evaluate to true -
Objects evaluate to true
- Undefined evaluates to false @@ -1273,15 +1722,19 @@ superPower = new Numbers evaluate to false if +0, -0, or NaN, otherwise true
-
-
Strings evaluate to false if an empty string
+Strings evaluate to false if an empty string'', otherwise true'', otherwise true +
+
if ([0]) { +
+ +if ([0] && []) { // true - // An array is an object, objects evaluate to true + // an array (even an empty one) is an object, objects will evaluate to true }
- -
+15.3 Use shortcuts.
+15.3 Use shortcuts.
+ + +// bad if (name !== '') { @@ -1302,18 +1755,113 @@ superPower = new if (collection.length) { // ...stuff... }
- +
+15.4 For more information see Truth Equality and JavaScript by Angus Croll.
+ + +- +
+15.5 Use braces to create blocks in
+ +caseanddefaultclauses that contain lexical declarations (e.g.let,const,function, andclass).+
+ +Why? Lexical declarations are visible in the entire
+switchblock but only get initialized when assigned, which only happens when itscaseis reached. This causes problems when multiplecaseclauses attempt to define the same thing.eslint rules:
+ +no-case-declarations.+ + +// bad + switch (foo) { + case 1: + let x = 1; + break; + case 2: + const y = 2; + break; + case 3: + function f() {} + break; + default: + class C {} + } + + // good + switch (foo) { + case 1: { + let x = 1; + break; + } + case 2: { + const y = 2; + break; + } + case 3: { + function f() {} + break; + } + case 4: + bar(); + break; + default: { + class C {} + } + }
- +
+15.6 Ternaries should not be nested and generally be single line expressions.
+ +eslint 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;
- +
-15.7 Avoid unneeded ternary statements.
+ +eslint rules:
+ +no-unneeded-ternary.// bad +const foo = a ? a : b; +const bar = c ? true : false; +const baz = c ? false : true; + +// good +const foo = a || b; +const bar = !!c; +const baz = !c;
15.4 For more information see Truth Equality and JavaScript by Angus Croll.
-Blocks
+Blocks + +-
-
16.1 Use braces with all multi-line blocks.
+16.1 Use braces with all multi-line blocks.
+ +// bad if (test) @@ -1328,18 +1876,17 @@ superPower = new // bad -function () { return false; } +function foo() { return false; } // good -function () { +function bar() { return false; }
-
-
16.2 If you're using multi-line blocks with
- -ifandelse, putelseon the same line as your -ifblock's closing brace.eslint rules:
+brace-style.16.2 If you're using multi-line blocks with
ifandelse, putelseon the same line as yourifblock's closing brace. eslint:brace-stylejscs:disallowNewlineBeforeBlockStatements// bad if (test) { @@ -1363,11 +1910,13 @@ superPower = new ⬆ back to top
-Comments
+Comments + +-
-
17.1 Use
+/** ... */for multi-line comments. Include a description, specify types and values for all parameters and return values.17.1 Use
/** ... */for multi-line comments. Include a description, specify types and values for all parameters and return values.+ +// bad // make() returns a new element @@ -1387,8 +1936,8 @@ superPower = new * make() returns a new element * based on the passed in tag name * - * @param {String} tag - * @return {Element} element + * @param {String} tag + * @return {Element} element */ function make(tag) { @@ -1396,9 +1945,11 @@ superPower = new return 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.17.2 Use
//for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it's on the first line of a block.-// bad const active = true; // is current tab @@ -1433,12 +1984,18 @@ superPower = new return type; }
- 17.3 Prefixing your comments with
FIXMEorTODOhelps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions areFIXME -- need to figure this outorTODO -- need to implement.
--
-
17.4 Use
-// FIXME:to annotate problems.class Calculator extends Abacus { + + +
- +
+17.3 Prefixing your comments with
+ + +FIXMEorTODOhelps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions areFIXME: -- need to figure this outorTODO: -- need to implement.- +
17.4 Use
+ +// FIXME:to annotate problems.+ +class Calculator extends Abacus { constructor() { super(); @@ -1446,11 +2003,13 @@ superPower = new = 0; } }
- -
17.5 Use
+// TODO:to annotate solutions to problems.17.5 Use
-// TODO:to annotate solutions to problems.class Calculator extends Abacus { +
+ +class Calculator extends Abacus { constructor() { super(); @@ -1464,124 +2023,128 @@ superPower = new ⬆ back to top
-Whitespace
+Whitespace + +-
-
18.1 Use soft tabs set to 2 spaces.
- -eslint rules:
- -indent.-// bad - function () { - ∙∙∙∙const name; - } - - // bad - function () { - ∙const name; - } - - // good - function () { - ∙∙const name; - }
- -
-
18.2 Place 1 space before the leading brace.
- -eslint rules:
- -space-before-blocks.-// 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', - });
- -
-
18.3 Place 1 space before the opening parenthesis in control statements (
- -if,whileetc.). Place no space before the argument list in function calls and declarations.eslint rules:
- -space-after-keywords,space-before-keywords.-// bad - if(isJedi) { - fight (); - } - - // good - if (isJedi) { - fight(); - } - - // bad - function fight () { - console.log ('Swooosh!'); - } - - // good - function fight() { - console.log('Swooosh!'); - }
- -
-
18.4 Set off operators with spaces.
- -eslint rules:
- -space-infix-ops.-// bad - const x=y+5; - - // good - const x = y + 5;
- -
-
18.5 End files with a single newline character.
+18.1 Use soft tabs set to 2 spaces. eslint:
indentjscs:validateIndentation+ + +// bad -(function (global) { +function foo() { +∙∙∙∙const name; +} + +// bad +function bar() { +∙const name; +} + +// good +function baz() { +∙∙const name; +}
+ -
+
18.2 Place 1 space before the leading brace. eslint:
+ +space-before-blocksjscs:requireSpaceBeforeBlockStatements+ + +// bad +function test(){ + console.log('test'); +} + +// good +function test() { + console.log('test'); +} + +// bad +dog.set('attr',{ + age: '1 year', + breed: 'Bernese Mountain Dog', +}); + +// good +dog.set('attr', { + age: '1 year', + breed: 'Bernese Mountain Dog', +});
+ -
+
18.3 Place 1 space before the opening parenthesis in control statements (
+ +if,whileetc.). Place no space between the argument list and the function name in function calls and declarations. eslint:keyword-spacingjscs:requireSpaceAfterKeywords+ + +// bad +if(isJedi) { + fight (); +} + +// good +if (isJedi) { + fight(); +} + +// bad +function fight () { + console.log ('Swooosh!'); +} + +// good +function fight() { + console.log('Swooosh!'); +}
+ -
+
18.4 Set off operators with spaces. eslint:
+ +space-infix-opsjscs:requireSpaceBeforeBinaryOperators,requireSpaceAfterBinaryOperators+ + +// bad +const x=y+5; + +// good +const x = y + 5;
+ -
+
18.5 End files with a single newline character.
+ +// bad +(function (global) { // ...stuff... })(this);
// bad -(function (global) { +(function (global) { // ...stuff... })(this);↵ ↵
+ +// good -(function (global) { +(function (global) { // ...stuff... })(this);↵
-
-
18.6 Use indentation when making long method chains. Use a leading dot, which -emphasizes that the line is a method call, not a new statement.
+18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which +emphasizes that the line is a method call, not a new statement. eslint:
newline-per-chained-callno-whitespace-before-property+ .call(tron.led); + +// good +const leds = stage.selectAll('.led').data(data);// bad $('#items').find('.selected').highlight().end().find('.open').updateCount(); // bad $('#items'). - find('.selected'). + find('.selected'). highlight(). end(). - find('.open'). + find('.open'). updateCount(); // good @@ -1593,7 +2156,7 @@ emphasizes that the line is a method call, not a new statement. .updateCount(); // bad -const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').class('led', true) +const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true) .attr('width', (radius + margin) * 2).append('svg:g') .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')') .call(tron.led); @@ -1606,10 +2169,15 @@ emphasizes that the line is a method call, not a new statement. .attr('width', (radius + margin) * 2) .append('svg:g') .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')') - .call(tron.led);
- -
18.7 Leave a blank line after blocks and before the next statement.
+18.7 Leave a blank line after blocks and before the next statement. jscs:
requirePaddingNewLinesAfterBlocks+ +// bad if (foo) { @@ -1663,208 +2231,236 @@ emphasizes that the line is a method call, not a new statement. ]; return arr;
- -
18.8 Do not pad your blocks with blank lines.
+18.8 Do not pad your blocks with blank lines. eslint:
-padded-blocksjscs:disallowPaddingNewlinesInBlockseslint rules:
+padded-blocks.+// bad +function bar() { -
- // good - if (baz) { - console.log(qux); - } else { - console.log(foo); - }// bad - function bar() { + console.log(foo); - console.log(foo); +} - } +// also bad +if (baz) { - // also bad - if (baz) { + console.log(qux); +} else { + console.log(foo); - console.log(qux); - } else { - console.log(foo); +} - } +// good +function bar() { + console.log(foo); +} - // good - function bar() { - console.log(foo); - } +// good +if (baz) { + console.log(qux); +} else { + console.log(foo); +}
- -
18.9 Do not add spaces inside parentheses.
+18.9 Do not add spaces inside parentheses. eslint:
-space-in-parensjscs:disallowSpacesInsideParentheseseslint rules:
+space-in-parens.+// bad +function bar( foo ) { + return foo; +} -
- // good - if (foo) { - console.log(foo); - }// bad - function bar( foo ) { - return foo; - } +// good +function bar(foo) { + return foo; +} - // good - function bar(foo) { - return foo; - } +// bad +if ( foo ) { + console.log(foo); +} - // bad - if ( foo ) { - console.log(foo); - } +// good +if (foo) { + console.log(foo); +}
- -
18.10 Do not add spaces inside brackets.
+18.10 Do not add spaces inside brackets. eslint:
-array-bracket-spacingjscs:disallowSpacesInsideArrayBracketseslint rules:
+array-bracket-spacing.+// bad +const foo = [ 1, 2, 3 ]; +console.log(foo[ 0 ]); -
- // good - const foo = [1, 2, 3]; - console.log(foo[0]);// bad - const foo = [ 1, 2, 3 ]; - console.log(foo[ 0 ]); +// good +const foo = [1, 2, 3]; +console.log(foo[0]);
- -
+18.11 Add spaces inside curly braces.
+18.11 Add spaces inside curly braces. eslint:
-object-curly-spacingjscs:requireSpacesInsideObjectBracketseslint rules:
+object-curly-spacing.+ +// bad +const foo = {clark: 'kent'}; -
- // good - const foo = { clark: 'kent' };// bad - const foo = {clark: 'kent'}; +// good +const foo = { clark: 'kent' };
- +
18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint:
+ +max-lenjscs:maximumLineLength+
+ +Why? This ensures readability and maintainability.
+// bad +const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!'; + +// bad +$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.')); + +// good +const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' + + 'Whatever wizard constrains a helpful ally. The counterpart ascends!'; + +// good +$.ajax({ + method: 'POST', + url: 'https://airbnb.com/', + data: { name: 'John' }, +}) + .done(() => console.log('Congratulations!')) + .fail(() => console.log('You have failed this city.'));
-Commas
+Commas + +-
-
19.1 Leading commas: Nope.
+19.1 Leading commas: Nope. eslint:
-comma-stylejscs:requireCommaBeforeLineBreakeslint rules:
+comma-style.+// bad +const story = [ + once + , upon + , aTime +]; -
- // good - const hero = { - firstName: 'Ada', - lastName: 'Lovelace', - birthYear: 1815, - superPower: 'computers', - };// bad - const story = [ - once - , upon - , aTime - ]; +// good +const story = [ + once, + upon, + aTime, +]; - // good - const story = [ - once, - upon, - aTime, - ]; +// bad +const hero = { + firstName: 'Ada' + , lastName: 'Lovelace' + , birthYear: 1815 + , superPower: 'computers' +}; - // bad - const hero = { - firstName: 'Ada' - , lastName: 'Lovelace' - , birthYear: 1815 - , superPower: 'computers' - }; +// good +const hero = { + firstName: 'Ada', + lastName: 'Lovelace', + birthYear: 1815, + superPower: 'computers', +};
-
-
19.2 Additional trailing comma: Yup.
- -eslint rules:
+no-comma-dangle.19.2 Additional trailing comma: Yup. eslint:
comma-danglejscs: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 graph', 'modern nursing'] - }; +
+const heroes = [ + 'Batman', + 'Superman', +];// bad - git diff without trailing comma +const hero = { + firstName: 'Florence', +- lastName: 'Nightingale' ++ lastName: 'Nightingale', ++ inventorOf: ['coxcomb graph', 'modern nursing'] +}; - // good - git diff with trailing comma - const hero = { - firstName: 'Florence', - lastName: 'Nightingale', - + inventorOf: ['coxcomb chart', 'modern nursing'], - }; +// good - git diff with trailing comma +const hero = { + firstName: 'Florence', + lastName: 'Nightingale', ++ inventorOf: ['coxcomb chart', 'modern nursing'], +}; - // bad - const hero = { - firstName: 'Dana', - lastName: 'Scully' - }; +// bad +const hero = { + firstName: 'Dana', + lastName: 'Scully' +}; - const heroes = [ - 'Batman', - 'Superman' - ]; +const heroes = [ + 'Batman', + 'Superman' +]; - // good - const hero = { - firstName: 'Dana', - lastName: 'Scully', - }; +// good +const hero = { + firstName: 'Dana', + lastName: 'Scully', +}; - const heroes = [ - 'Batman', - 'Superman', - ];
-Semicolons
+Semicolons + +-
-
20.1 Yup.
+20.1 Yup. eslint:
-semijscs:requireSemicolonseslint rules:
+semi.// bad +(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; +}());// bad - (function () { - const name = 'Skywalker' - return name - })() +// good +(function () { + const name = 'Skywalker'; + return name; +}()); - // good - (() => { - const name = 'Skywalker'; - return name; - })(); - - // good (guards against the function becoming an argument when two files with IIFEs are concatenated) - ;(() => { - const name = 'Skywalker'; - return name; - })();
@@ -1873,24 +2469,34 @@ emphasizes that the line is a method call, not a new statement.
- -21.1 Perform type coercion at the beginning of the statement. -
-
-
21.2 Strings:
+21.1 Perform type coercion at the beginning of the statement.
-// => this.reviewScore = 9; + + +
- +
21.2 Strings:
+ ++ +// => this.reviewScore = 9; // bad -const totalScore = this.reviewScore + ''; +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);
- -
21.3 Numbers: Use
+Numberfor type casting andparseIntalways with a radix for parsing strings.21.3 Numbers: Use
Numberfor type casting andparseIntalways with a radix for parsing strings. eslint:radix+ +const inputValue = '4'; @@ -1911,9 +2517,11 @@ emphasizes that the line is a method call, not a new statement. // good const val = parseInt(inputValue, 10);
- -
21.4 If for whatever reason you are doing something wild and
+parseIntis your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.21.4 If for whatever reason you are doing something wild and
parseIntis your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.+ +// good /** @@ -1922,16 +2530,20 @@ emphasizes that the line is a method call, not a new statement. * Number made it a lot faster. */ const val = inputValue >> 0;
- -
21.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
+21.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
+ +2147483647 >> 0 //=> 2147483647 2147483648 >> 0 //=> -2147483648 2147483649 >> 0 //=> -2147483647
- -
@@ -1949,11 +2561,13 @@ emphasizes that the line is a method call, not a new statement.21.6 Booleans:
+21.6 Booleans:
const age = 0; @@ -1941,7 +2553,7 @@ emphasizes that the line is a method call, not a new statement. // good const hasAge = Boolean(age); -// good +// best const hasAge = !!age;
-Naming Conventions
+Naming Conventions + +-
-
22.1 Avoid single letter names. Be descriptive with your naming.
+22.1 Avoid single letter names. Be descriptive with your naming.
+ +// bad function q() { @@ -1964,23 +2578,25 @@ emphasizes that the line is a method call, not a new statement. function query() { // ..stuff.. }
-
-
22.2 Use camelCase when naming objects, functions, and instances.
+22.2 Use camelCase when naming objects, functions, and instances. eslint:
-camelcasejscs:requireCamelCaseOrUpperCaseIdentifierseslint rules:
+camelcase.+// bad +const OBJEcttsssss = {}; +const this_is_my_object = {}; +function c() {} -
- // good - const thisIsMyObject = {}; - function thisIsMyFunction() {}// bad - const OBJEcttsssss = {}; - const this_is_my_object = {}; - function c() {} +// good +const thisIsMyObject = {}; +function thisIsMyFunction() {}
-
-
22.3 Use PascalCase when naming constructors or classes.
+22.3 Use PascalCase only when naming constructors or classes. eslint:
new-capjscs:requireCapitalizedConstructors+ +// bad function user(options) { @@ -2001,21 +2617,28 @@ emphasizes that the line is a method call, not a new statement. const good = new User({ name: 'yup', });
-
-
22.4 Use a leading underscore
+_when naming private properties.22.4 Do not use trailing or leading underscores. eslint:
-no-underscore-danglejscs:disallowDanglingUnderscoreseslint rules:
+no-underscore-dangle.+
-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'; +
+// good +this.firstName = 'Panda';// bad +this.__firstName__ = 'Panda'; +this.firstName_ = 'Panda'; +this._firstName = 'Panda'; - // good - this._firstName = 'Panda';
-
-
22.5 Don't save references to
+this. Use arrow functions or Function#bind.22.5 Don't save references to
this. Use arrow functions or Function#bind. jscs:disallowNodeTypes+ +// bad function foo() { @@ -2039,36 +2662,57 @@ emphasizes that the line is a method call, not a new statement. console.log(this); }; }
-
-
22.6 If your file exports a single class, your filename should be exactly the name of the class.
+22.6 A base filename should exactly match the name of its default export.
-+ +// file contents +
+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// 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'; +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'; +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';
-
-
22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
+22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
+ +function makeStyleGuide() { } export default makeStyleGuide;
-
-
22.8 Use PascalCase when you export a singleton / function library / bare object.
+22.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
const AirbnbStyleGuide = { es6: { @@ -2082,28 +2726,45 @@ emphasizes that the line is a method call, not a new statement.
-Accessors
+Accessors + +- -23.1 Accessor functions for properties are not required. -
-
-
23.2 If you do make accessor functions use getVal() and setVal('hello').
+23.1 Accessor functions for properties are not required.
-+// bad -dragon.age(); - -// good -dragon.getAge(); - -// bad -dragon.age(25); - -// good -dragon.setAge(25);
-
-
23.3 If the property is a
+boolean, useisVal()orhasVal().23.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').
+ ++ + +// bad +class Dragon { + get age() { + // ... + } + + set age(value) { + // ... + } +} + +// good +class Dragon { + getAge() { + // ... + } + + setAge(value) { + // ... + } +}
+ -
+
23.3 If the property/method is a
boolean, useisVal()orhasVal().+ +// bad if (!dragon.age()) { @@ -2114,9 +2775,11 @@ emphasizes that the line is a method call, not a new statement. if (!dragon.hasAge()) { return false; }
-
-
23.4 It's okay to create get() and set() functions, but be consistent.
+23.4 It's okay to create get() and set() functions, but be consistent.
class Jedi { constructor(options = {}) { @@ -2138,18 +2801,20 @@ emphasizes that the line is a method call, not a new statement.
-Events
+Events + +-
-
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:
+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:
@@ -2158,9 +2823,9 @@ emphasizes that the line is a method call, not a new statement.// bad $(this).trigger('listingUpdated', listing.id); -... +... -$(this).on('listingUpdated', function (e, listingId) { +$(this).on('listingUpdated', (e, listingId) => { // do something with listingId });
@@ -2169,11 +2834,13 @@ emphasizes that the line is a method call, not a new statement.// good $(this).trigger('listingUpdated', { listingId: listing.id }); -... +... -$(this).on('listingUpdated', function (e, data) { +$(this).on('listingUpdated', (e, data) => { // do something with data.listingId });
-jQuery
+jQuery + +-
-
25.1 Prefix jQuery object variables with a
+$.25.1 Prefix jQuery object variables with a
$. jscs:requireDollarBeforejQueryAssignment+ +// bad const sidebar = $('.sidebar'); @@ -2183,9 +2850,11 @@ emphasizes that the line is a method call, not a new statement. // good const $sidebarBtn = $('.sidebar-btn');
-
-
25.2 Cache jQuery lookups.
+25.2 Cache jQuery lookups.
+ +// bad function setSidebar() { @@ -2209,10 +2878,16 @@ emphasizes that the line is a method call, not a new statement. 'background-color': 'pink' }); }
- 25.3 For DOM queries use Cascading
$('.sidebar ul')or parent > child$('.sidebar > ul'). jsPerf-
-
25.4 Use
+findwith scoped jQuery object queries.25.3 For DOM queries use Cascading
+ + +$('.sidebar ul')or parent > child$('.sidebar > ul'). jsPerf
+ -
+
25.4 Use
findwith scoped jQuery object queries.// bad $('ul', '.sidebar').hide(); @@ -2234,21 +2909,25 @@ emphasizes that the line is a method call, not a new statement.
-ECMAScript 5 Compatibility
+ECMAScript 5 Compatibility + +- -26.1 Refer to Kangax's ES5 compatibility table. +26.1 Refer to Kangax's ES5 compatibility table.
-ECMAScript 6 Styles
+ECMAScript 6 Styles + +- -27.1 This is a collection of links to the various es6 features. +27.1 This is a collection of links to the various ES6 features.
-
@@ -2270,18 +2949,22 @@ emphasizes that the line is a method call, not a new statement.
-
-
28.1 Yup.
+28.1 Yup.
-function () { +
+ +function foo() { return true; }
- -
28.2 No, but seriously:
+28.2 No, but seriously:- Whichever testing framework you use, you should be writing tests! @@ -2297,7 +2980,7 @@ emphasizes that the line is a method call, not a new statement.
- On Layout & Web Performance @@ -2307,13 +2990,14 @@ emphasizes that the line is a method call, not a new statement.
- jQuery Find vs Context, Selector
- innerHTML vs textContent for script text
- Long String Concatenation +
- Are Javascript functions like
map(),reduce(), andfilter()optimized for traversing arrays? - Loading...
-Performance
+Performance-Resources
+ResourcesLearning ES6
@@ -2340,7 +3024,7 @@ emphasizes that the line is a method call, not a new statement. ESlint - Airbnb Style .eslintrc- -JSHint - Airbnb Style .jshintrc +JSHint - Airbnb Style .jshintrc
- JSCS - Airbnb Style Preset @@ -2352,7 +3036,7 @@ emphasizes that the line is a method call, not a new statement.
Other Style Guides
-
-
- Google JavaScript Style Guide +
- Google JavaScript Style Guide
- jQuery Core Style Guidelines
- Principles of Writing Consistent, Idiomatic JavaScript
-In the Wild
+In the WildThis is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.
- +4Catalyzer: 4Catalyzer/javascript + +
- Aan Zee: AanZee/javascript
- @@ -2465,9 +3152,15 @@ emphasizes that the line is a method call, not a new statement. Apartmint: apartmint/javascript
- +Ascribe: ascribe/javascript + +
- Avalara: avalara/javascript
- +Avant: avantcredit/javascript + +
- Billabong: billabong/javascript
- @@ -2477,6 +3170,12 @@ emphasizes that the line is a method call, not a new statement. Blendle: blendle/javascript
- +Brainshark: brainshark/javascript + +
- +Chartboost: ChartBoost/javascript-style-guide + +
- ComparaOnline: comparaonline/javascript
- @@ -2486,6 +3185,9 @@ emphasizes that the line is a method call, not a new statement. DailyMotion: dailymotion/javascript
- +DoSomething: DoSomething/eslint-config + +
- Digitpaint digitpaint/javascript
- @@ -2495,6 +3197,12 @@ emphasizes that the line is a method call, not a new statement. Evernote: evernote/javascript-style-guide
- +Evolution Gaming: evolution-gaming/javascript + +
- +EvozonJs: evozonjs/javascript + +
- ExactTarget: ExactTarget/javascript
- @@ -2543,6 +3251,9 @@ emphasizes that the line is a method call, not a new statement. JSSolutions: JSSolutions/javascript
- +KickorStick: kickorstick/javascript + +
- Kinetica Solutions: kinetica/javascript
- @@ -2576,6 +3287,9 @@ emphasizes that the line is a method call, not a new statement. Orion Health: orionhealth/javascript
- +OutBoxSoft: OutBoxSoft/javascript + +
- Peerby: Peerby/javascript
- @@ -2588,7 +3302,7 @@ emphasizes that the line is a method call, not a new statement. React: /facebook/react/blob/master/CONTRIBUTING.md#style-guide
- -REI: reidev/js-style-guide +REI: reidev/js-style-guide
- Ripple: ripple/javascript-style-guide @@ -2606,18 +3320,27 @@ emphasizes that the line is a method call, not a new statement. StudentSphere: studentsphere/javascript
- +SysGarage: sysgarage/javascript-style-guide + +
- Target: target/javascript
- TheLadders: TheLadders/javascript
- +The Nerdery: thenerdery/javascript-standards + +
- T4R Technology: T4R-Technology/javascript
- VoxFeed: VoxFeed/javascript-style-guide
- +WeBox Studio: weboxstudio/javascript + +
- Weggo: Weggo/javascript
-
@@ -2631,7 +3354,7 @@ emphasizes that the line is a method call, not a new statement.
-Translation
+TranslationThis style guide is also available in other languages:
@@ -2678,35 +3401,38 @@ emphasizes that the line is a method call, not a new statement. -
Thai: lvarayut/javascript-style-guide
+ -
+
Vietnam: giangpii/javascript-style-guide
+
-The JavaScript Style Guide Guide
+The JavaScript Style Guide Guide-Chat With Us About JavaScript
+Chat With Us About JavaScript- Find us on gitter.
-Contributors
+Contributors-License
+License(The MIT License)
-Copyright (c) 2014 Airbnb
+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 @@ -2730,12 +3456,12 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-Amendments
+AmendmentsWe 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.
-};
+}; - -
-Testing
+Testing + +
-
-
- +
-Type Casting & Coercion
+Type Casting & Coercion + + - +
-
-
-
-15.1 Use
-
-