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A mostly reasonable approach to JavaScript
Primitives: When you access a primitive type you work directly on its value
var foo = 1;
var bar = foo;
bar = 9;
console.log(foo, bar); // => 1, 9Complex: When you access a complex type you work on a reference to its value
var foo = [1, 2];
var bar = foo;
bar[0] = 9;
console.log(foo[0], bar[0]); // => 9, 9Use the literal syntax for object creation.
// bad
var item = new Object();
// good
var item = {};Don't use reserved words as keys. It won't work in IE8. More info
// bad
var superman = {
default: { clark: 'kent' },
private: true
};
// good
var superman = {
defaults: { clark: 'kent' },
hidden: true
};
// bad
service.getRestaunrant()
.catch(function() {});
// good
service.getRestaunrant()
['catch'](function() {});Use readable synonyms in place of reserved words.
// bad
var superman = {
class: 'alien'
};
// bad
var superman = {
klass: 'alien'
};
// good
var superman = {
type: 'alien'
};Use the literal syntax for array creation
// bad
var items = new Array();
// good
var items = [];If you don't know array length use Array#push.
var someStack = [];
// bad
someStack[someStack.length] = 'abracadabra';
// good
someStack.push('abracadabra');When you need to copy an array use Array#slice. jsPerf
var len = items.length;
var itemsCopy = [];
// bad
for (var i = 0; i < len; i++) {
itemsCopy[i] = items[i];
}
// good
itemsCopy = items.slice();To convert an array-like object to an array, use Array#slice.
function trigger() {
var args = Array.prototype.slice.call(arguments);
...
}Use single quotes '' for strings
// bad
var name = "Bob Parr";
// good
var name = 'Bob Parr';
// bad
var fullName = "Bob " + this.lastName;
// good
var fullName = 'Bob ' + this.lastName;Strings longer than 80 characters should be written across multiple lines using string concatenation.
Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion
// bad
var errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
// bad
var errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';
// good
var 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.';When programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE: jsPerf.
var messages = [{
state: 'success',
message: 'This one worked.'
}, {
state: 'success',
message: 'This one worked as well.'
}, {
state: 'error',
message: 'This one did not work.'
}];
var length = messages.length;
// bad
function inbox(messages) {
var items = '<ul>';
for (var i = 0; i < length; i++) {
items += '<li>' + messages[i].message + '</li>';
}
return items + '</ul>';
}
// good
function inbox(messages) {
var items = [];
for (var i = 0; i < length; i++) {
items[i] = messages[i].message;
}
return '<ul><li>' + items.join('</li><li>') + '</li></ul>';
}Function expressions:
// anonymous function expression
var anonymous = function() {
return true;
};
// named function expression
var named = function named() {
return true;
};
// immediately-invoked function expression (IIFE)
(function() {
console.log('Welcome to the Internet. Please follow me.');
})();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.
Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.
// bad
if (currentUser) {
function test() {
console.log('Nope.');
}
}
// good
var test;
if (currentUser) {
test = function test() {
console.log('Yup.');
};
}Never name a parameter arguments, this will take precedence over the arguments object that is given to every function scope.
// bad
function nope(name, options, arguments) {
// ...stuff...
}
// good
function yup(name, options, args) {
// ...stuff...
}Use dot notation when accessing properties.
var luke = {
jedi: true,
age: 28
};
// bad
var isJedi = luke['jedi'];
// good
var isJedi = luke.jedi;Use subscript notation [] when accessing properties with a variable or computed properties in ES6.
var luke = {
jedi: true,
age: 28
};
function getProp(prop) {
return luke[prop];
}
var isJedi = getProp('jedi');
// ES6
var age = luke['ag' + 'e'];Avoid using reserved words as property names. They will work in modern browsers (unlike IE8) but cause confusion. Find a good synonym instead (but don't get to creative).
// bad
var luke = {
typeof: 'jedi',
age: 28
};
// bad
var luke = {
brand: 'jedi',
age: 28
};
// good
var luke = {
group: 'jedi',
age: 28
};Always use var 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.
// bad
superPower = new SuperPower();
// good
var superPower = new SuperPower();Use one var declaration and one line for each variable you are declaring. This makes your declarations more readable.
// bad
var i, length, valid = true, model = {};
// bad
var items = getItems(), goSportsTeam = true;
var dragonball = 'z';
// good
var items = getItems();
var goSportsTeam = true;
var dragonball = 'z';When assigning variables, give each one its own line. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.
// bad
var i, len, dragonball;
var items = getItems();
var goSportsTeam = true;
// bad
var i, items = getItems(),
dragonball,
goSportsTeam = true,
len;
// good
var items = getItems();
var goSportsTeam = true;
var dragonball;
var length;
var i;Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.
// bad
function() {
test();
console.log('doing stuff..');
//..other stuff..
var name = getName();
if (name === 'test') {
return false;
}
return name;
}
// good
function() {
var name = getName();
test();
console.log('doing stuff..');
//..other stuff..
if (name === 'test') {
return false;
}
return name;
}
// bad
function() {
var name = getName();
if (!arguments.length) {
return false;
}
return true;
}
// good
function() {
if (!arguments.length) {
return false;
}
var name = getName();
return true;
}Variable declarations get hoisted to the top of their scope, their assignment does not.
// we know this wouldn't work (assuming there
// is no notDefined global variable)
function example() {
console.log(notDefined); // => throws a ReferenceError
}
// creating a variable declaration after you
// reference the variable will work due to
// variable hoisting. Note: the assignment
// value of `true` is not hoisted.
function example() {
console.log(declaredButNotAssigned); // => undefined
var declaredButNotAssigned = true;
}
// The interpreter is hoisting the variable
// declaration to the top of the scope.
// Which means our example could be rewritten as:
function example() {
var declaredButNotAssigned;
console.log(declaredButNotAssigned); // => undefined
declaredButNotAssigned = true;
}Anonymous function expressions hoist their variable name, but not the function assignment.
function example() {
console.log(anonymous); // => undefined
anonymous(); // => TypeError anonymous is not a function
var anonymous = function() {
console.log('anonymous function expression');
};
}Named function expressions hoist the variable name, not the function name or the function body.
function example() {
console.log(named); // => undefined
named(); // => TypeError named is not a function
superPower(); // => ReferenceError superPower is not defined
var named = function superPower() {
console.log('Flying');
};
}
// the same is true when the function name
// is the same as the variable name.
function example() {
console.log(named); // => undefined
named(); // => TypeError named is not a function
var named = function named() {
console.log('named');
}
}Function declarations hoist their name and the function body.
function example() {
superPower(); // => Flying
function superPower() {
console.log('Flying');
}
}For more information refer to JavaScript Scoping & Hoisting by Ben Cherry
Use === and !== over == and !=.
Conditional expressions are evaluated using coercion with the ToBoolean method and always follow these simple rules:
if ([0]) {
// true
// An array is an object, objects evaluate to true
}Use shortcuts.
// bad
if (name !== '') {
// ...stuff...
}
// good
if (name) {
// ...stuff...
}
// bad
if (collection.length > 0) {
// ...stuff...
}
// good
if (collection.length) {
// ...stuff...
}For more information see Truth Equality and JavaScript by Angus Croll
Use braces with all multi-line blocks.
// bad
if (test)
return false;
// good
if (test) return false;
// good
if (test) {
return false;
}
// bad
function() { return false; }
// good
function() {
return false;
}Use /** ... */ for multiline comments. Include a description, specify types and values for all parameters and return values.
// bad
// make() returns a new element
// based on the passed in tag name
//
// @param <String> tag
// @return <Element> element
function make(tag) {
// ...stuff...
return element;
}
// good
/**
* make() returns a new element
* based on the passed in tag name
*
* @param <String> tag
* @return <Element> element
*/
function make(tag) {
// ...stuff...
return element;
}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.
// bad
var active = true; // is current tab
// good
// is current tab
var active = true;
// bad
function getType() {
console.log('fetching type...');
// set the default type to 'no type'
var type = this._type || 'no type';
return type;
}
// good
function getType() {
console.log('fetching type...');
// set the default type to 'no type'
var type = this._type || 'no type';
return type;
}Use soft tabs set to 4 spaces. Generally, Visual Studio will take care of this, but if you are adding code try to keep to this standard.
// good
function() {
∙∙∙∙var name;
}
// bad
function() {
∙var name;
}
// bad
function() {
∙∙var name;
}Place 1 space before the leading brace.
// 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'
});Set off operators with spaces.
// bad
var x=y+5;
// good
var x = y + 5;End files with a single newline character.
// bad
(function(global) {
// ...stuff...
})(this);// bad
(function(global) {
// ...stuff...
})(this);↵
↵// good
(function(global) {
// ...stuff...
})(this);↵Use indentation when making long method chains.
// bad
$('#items').find('.selected').highlight().end().find('.open').updateCount();
// good
$('#items')
.find('.selected')
.highlight()
.end()
.find('.open')
.updateCount();
// bad
var leds = stage.selectAll('.led').data(data).enter().append('svg:svg').class('led', true)
.attr('width', (radius + margin) * 2).append('svg:g')
.attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
.call(tron.led);
// good
var leds = stage.selectAll('.led')
.data(data)
.enter().append('svg:svg')
.class('led', true)
.attr('width', (radius + margin) * 2)
.append('svg:g')
.attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
.call(tron.led);Leading commas: Nope.
// bad
var hero = {
firstName: 'Bob'
, lastName: 'Parr'
, heroName: 'Mr. Incredible'
, superPower: 'strength'
};
// good
var hero = {
firstName: 'Bob',
lastName: 'Parr',
heroName: 'Mr. Incredible',
superPower: 'strength'
};Additional trailing comma: Nope. This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 (source):
Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.
```javascript
// bad
var hero = {
firstName: 'Kevin',
lastName: 'Flynn',
};
var heroes = [
'Batman',
'Superman',
];
// good
var hero = {
firstName: 'Kevin',
lastName: 'Flynn'
};
var heroes = [
'Batman',
'Superman'
];
```
Yup.
// bad
(function() {
var name = 'Skywalker'
return name
})()
// good
(function() {
var name = 'Skywalker';
return name;
})();
// good (guards against the function becoming an argument when two files with IIFEs are concatenated)
;(function() {
var name = 'Skywalker';
return name;
})();Perform type coercion at the beginning of the statement.
Strings:
// => this.reviewScore = 9;
// bad
var totalScore = this.reviewScore + '';
// good
var totalScore = '' + this.reviewScore;
// bad
var totalScore = '' + this.reviewScore + ' total score';
// good
var totalScore = this.reviewScore + ' total score';Use parseInt for Numbers and always with a radix for type casting.
var inputValue = '4';
// bad
var val = new Number(inputValue);
// bad
var val = +inputValue;
// bad
var val = inputValue >> 0;
// bad
var val = parseInt(inputValue);
// good
var val = Number(inputValue);
// good
var val = parseInt(inputValue, 10);If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.
// good
/**
* parseInt was the reason my code was slow.
* Bitshifting the String to coerce it to a
* Number made it a lot faster.
*/
var val = inputValue >> 0;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 //=> -2147483647Booleans:
var age = 0;
// bad
var hasAge = new Boolean(age);
// good
var hasAge = Boolean(age);
// good
var hasAge = !!age;Avoid single letter names. Be descriptive with your naming.
// bad
function q() {
// ...stuff...
}
// good
function query() {
// ..stuff..
}Use camelCase when naming objects, functions, and instances
// bad
var OBJEcttsssss = {};
var this_is_my_object = {};
function c() {}
var u = new user({
name: 'Bob Parr'
});
// good
var thisIsMyObject = {};
function thisIsMyFunction() {}
var user = new User({
name: 'Bob Parr'
});Use PascalCase when naming constructors or classes
// bad
function user(options) {
this.name = options.name;
}
var bad = new user({
name: 'nope'
});
// good
function User(options) {
this.name = options.name;
}
var good = new User({
name: 'yup'
});Use a leading underscore _ when naming private properties
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
// good
this._firstName = 'Panda';When saving a reference to this use _this.
// bad
function() {
var self = this;
return function() {
console.log(self);
};
}
// bad
function() {
var that = this;
return function() {
console.log(that);
};
}
// good
function() {
var _this = this;
return function() {
console.log(_this);
};
}Note: IE8 and below exhibit some quirks with named function expressions. See http://kangax.github.io/nfe/ for more info.
Accessor functions for properties are not required
If you do make accessor functions use getVal() and setVal('hello')
// bad
dragon.age();
// good
dragon.getAge();
// bad
dragon.age(25);
// good
dragon.setAge(25);If the property is a boolean, use isVal() or hasVal()
// bad
if (!dragon.age()) {
return false;
}
// good
if (!dragon.hasAge()) {
return false;
}It's okay to create get() and set() functions, but be consistent.
function Jedi(options) {
options || (options = {});
var lightsaber = options.lightsaber || 'blue';
this.set('lightsaber', lightsaber);
}
Jedi.prototype.set = function(key, val) {
this[key] = val;
};
Jedi.prototype.get = function(key) {
return this[key];
};Assign methods to the prototype object, instead of overwriting the prototype with a new object. Overwriting the prototype makes inheritance impossible: by resetting the prototype you'll overwrite the base!
function Jedi() {
console.log('new jedi');
}
// bad
Jedi.prototype = {
fight: function fight() {
console.log('fighting');
},
block: function block() {
console.log('blocking');
}
};
// good
Jedi.prototype.fight = function fight() {
console.log('fighting');
};
Jedi.prototype.block = function block() {
console.log('blocking');
};Methods can return this to help with method chaining.
// bad
Jedi.prototype.jump = function() {
this.jumping = true;
return true;
};
Jedi.prototype.setHeight = function(height) {
this.height = height;
};
var luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined
// good
Jedi.prototype.jump = function() {
this.jumping = true;
return this;
};
Jedi.prototype.setHeight = function(height) {
this.height = height;
return this;
};
var luke = new Jedi();
luke.jump()
.setHeight(20);It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
function Jedi(options) {
options || (options = {});
this.name = options.name || 'no name';
}
Jedi.prototype.getName = function getName() {
return this.name;
};
Jedi.prototype.toString = function toString() {
return 'Jedi - ' + this.getName();
};One of the reasons people generally stay away from the 'this' keyword in javascript is the context of a function can change depending on how the function was called.
var Order = {
add: function add(product) {
this.products.push(product);
},
products: []
};
// bad
add({ name: 'swiss cheese'});
// good
Order.add({ name: 'grilled cheese' });Events will usually not call a callback function with its own context, it will use Window, or the event as context.
var orderSetup = {
updateForm: function() {
this.times = this.times.slice();
}
};
// bad
$scope.$on('user:logout', orderSetup.updateForm);
// good
$scope.$on('user:logout', function() {
orderSetup.updateForm();
});Angular is a framework built in Google and used all over the web.
Some good resources:
Keep in mind angular examples will not necessarily use our style guide (specifically, how dependencies are set). Make sure you read through this document and understand how we do angular.
angular.module('app').controller('orderSetupCtrl',['$scope', 'dependency', function($scope, dependency) {
this.method = function() {};
$scope.func = function() {};
}]);Naming:
Controller names should match the state they are in or a model they hold business logic for. They should all be affixed with 'Ctrl'.
We generally only use services or providers. Both are singletons, as in there is only one copy for each application. The big difference is that providers allow for setup in the config stage of the application bootstraping. This lets us set values on the provider, and customize them for the application. We use this in a series of different situations.
// Service
angular.module('app').service('exampleService', ['dependency',function(dependency) {
return {};
});
angular.module('app').provider('configService', function() {
var config = {};
this.config = function(_config) {
config = _config;
};
this.$get = ['dependency', function(dependency) {
return {};
}];
});Usage:
angular.module('controllers')
.config(['configServiceProvider', function(configServiceProvider) {
configServiceProvider.config({});
}])
.controller('exampleCtrl', ['exampleService', 'configService', function(exampleService, configService){
}]);Naming:
Service and provider names are always camel case, and do not include service or provider in the name.
// Service
angular.module('app').service('exampleService', ['dependency',function(dependency) {
return {};
});
angular.module('app').provider('configService', function() {
var config = {};
this.config = function(_config) {
config = _config;
};
this.$get = ['dependency', function(dependency) {
return {};
}];
});Usage:
angular.module('controllers')
.config(['configServiceProvider', function(configServiceProvider) {
configServiceProvider.config({});
}])
.controller('exampleCtrl', ['exampleService', 'configService', function(exampleService, configService){
}]);Naming:
Service and provider names are always camel case, and do not include service or provider in the name.
Angular's power shines through when creating complex forms. We have made some additions to handle validation and error handling.
<div ng-form="orderSetupForm" ng-submit="placeOrder()">
<h3>Order Setup</h3>
<div ono-form-errors></div>
<label>
Menu Type
<select ng-options="menu in values.menuTypes" ng-model="menuType"></select>
</label>
<button ng-click="orderSetupForm.$submit()">submit</button>
<button ng-click="goBack()">cancel</button>
</div>[ng-form] is an angular directive that we have extended. It creates a form object in scope which allows you to check the validity of the form and add to it, at any point within that scope. 'orderSetupForm.$submit()' triggers the form validation, and if that is successful, calls placeOrder on the current scope. If [ng-submit] is not used, the form directive will look for 'orderSetupFormSubmit' on scope to submit the form.
The function called by the form directive needs to return a promise object. This is how the form directive handles any errors.
Yup.
function() {
return true;
}Read This
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This is a list of organizations that are using this style guide. Send us a pull request or open an issue and we'll add you to the list.
This style guide is also available in other languages:
German: timofurrer/javascript-style-guide
Japanese: mitsuruog/javacript-style-guide
Portuguese: armoucar/javascript-style-guide
Chinese: adamlu/javascript-style-guide
Spanish: paolocarrasco/javascript-style-guide
Korean: tipjs/javascript-style-guide
French: nmussy/javascript-style-guide
Russian: uprock/javascript
Bulgarian: borislavvv/javascript
Catalan: fpmweb/javascript-style-guide
Polish: mjurczyk/javascript(The MIT License)
Copyright (c) 2014 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.
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