Tokbox JavaScript Style Guide() {
A mostly reasonable approach to JavaScript
Table of Contents
- Types
- Objects
- Arrays
- Strings
- Functions
- Properties
- Variables
- Hoisting
- Conditional Expressions & Equality
- Blocks
- Comments
- Whitespace
- Commas
- Semicolons
- Type Casting & Coercion
- Naming Conventions
- Accessors
- Constructors
- Events
- Modules
- jQuery
- ECMAScript 5 Compatibility
- Testing
- Performance
- Resources
- Contributors
- License
Types
-
Primitives: When you access a primitive type you work directly on its value
string
number
boolean
null
undefined
var foo = 1;var bar = foo;bar = 9;console; // => 1, 9 -
Complex: When you access a complex type you work on a reference to its value
object
array
function
var foo = 1 2bar = foo;bar0 = 9;console; // => 9, 9
Objects
-
Use the literal syntax for object creation.
// badvar item = ;// goodvar item = {}; -
Don't use reserved words as keys. It won't work in IE8. More info
// badvar superman =default: clark: 'kent'private: true;// goodvar superman =defaults: clark: 'kent'hidden: true; -
Use readable synonyms in place of reserved words.
// badvar superman =class: 'alien';// badvar superman =klass: 'alien';// goodvar superman =type: 'alien';
Arrays
-
Use the literal syntax for array creation
// badvar items = ;// goodvar items = ; -
If you don't know array length use Array#push.
var someStack = ;// badsomeStacksomeStacklength = 'abracadabra';// goodsomeStack; -
When you need to copy an array use Array#slice. jsPerf
var len = itemslength;var itemsCopy = ;var i;// badfor i = 0; i < len; i++itemsCopyi = itemsi;// gooditemsCopy = items; -
To convert an array-like object to an array, use Array#slice.
{var args = Arrayprototypeslice;...}
Strings
-
Use single quotes
''
for strings// badvar name = "Bob Parr";// goodvar name = 'Bob Parr';// badvar fullName = "Bob " + thislastName;// goodvar fullName = 'Bob ' + thislastName; -
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
// badvar 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.';// badvar 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.';// goodvar 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 items;var messages;var length;var i;messages =state: 'success'message: 'This one worked.'state: 'success'message: 'This one worked as well.'state: 'error'message: 'This one did not work.';length = messageslength;// bad{items = '<ul>';for i = 0; i < length; i++items += '<li>' + messagesimessage + '</li>';return items + '</ul>';}// good{items = ;for i = 0; i < length; i++itemsi = messagesimessage;return '<ul><li>' + items + '</li></ul>';}
Functions
-
Function expressions:
// anonymous function expressionvar {return true;};// named function expressionvar {return true;};// immediately-invoked function expression (IIFE){console;}; -
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.// badif currentUser{console;}// goodvar test;if currentUser{console;}; -
Never name a parameter
arguments
, this will take precedence over thearguments
object that is given to every function scope.// bad{// ...stuff...}// good{// ...stuff...}
Properties
-
Use dot notation when accessing properties.
var luke =jedi: trueage: 28;// badvar isJedi = luke'jedi';// goodvar isJedi = lukejedi; -
Use subscript notation
[]
when accessing properties with a variable.var luke =jedi: trueage: 28;{return lukeprop;}var isJedi = ;
Variables
-
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.// badsuperPower = ;// goodvar superPower = ; -
Use one
var
declaration for each variable and declare each variable on a newline.// badvar items =goSportsTeam = truedragonball = 'z';// goodvar items = ;var goSportsTeam = true;var dragonball = 'z'; -
Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.
// badvar i len dragonball;var items = ;var goSportsTeam = true;// badvar i items = ;var dragonball;var goSportsTeam = true;var len;// goodvar items = ;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{;console;//..other stuff..var name = ;if name === 'test'return false;return name;}// good{var name = ;;console;//..other stuff..if name === 'test'return false;return name;}// bad{var name = ;if !argumentslengthreturn false;return true;}// good{if !argumentslengthreturn false;var name = ;return true;}
Hoisting
-
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){console; // => 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.{console; // => undefinedvar declaredButNotAssigned = true;}// The interpreter is hoisting the variable// declaration to the top of the scope.// Which means our example could be rewritten as:{var declaredButNotAssigned;console; // => undefineddeclaredButNotAssigned = true;} -
Anonymous function expressions hoist their variable name, but not the function assignment.
{console; // => undefined; // => TypeError anonymous is not a functionvar {console;};} -
Named function expressions hoist the variable name, not the function name or the function body.
{console; // => undefined; // => TypeError named is not a function; // => ReferenceError superPower is not definedvar {console;};}// the same is true when the function name// is the same as the variable name.{console; // => undefined; // => TypeError named is not a functionvar {console;}} -
Function declarations hoist their name and the function body.
{; // => Flying{console;}} -
For more information refer to JavaScript Scoping & Hoisting by Ben Cherry
Conditional Expressions & Equality
-
Use
===
and!==
over==
and!=
. -
Conditional expressions are evaluated using coercion with the
ToBoolean
method and always follow these simple rules:- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string
''
, otherwise true
if 0// true// An array is an object, objects evaluate to true -
Use shortcuts.
// badif name !== ''// ...stuff...// goodif name// ...stuff...// badif collectionlength > 0// ...stuff...// goodif collectionlength// ...stuff... -
For more information see Truth Equality and JavaScript by Angus Croll
Blocks
-
Use braces with all blocks.
// badif testreturn false;// badif test return false;// goodif testreturn false;// bad{ return false; }// good{return false;}
Comments
-
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{// ...stuff...return element;}// good/*** make() returns a new element* based on the passed in tag name** @param <String> tag* @return <Element> element*/{// ...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.// badvar active = true; // is current tab// good// is current tabvar active = true;// bad{console;// set the default type to 'no type'var type = this_type || 'no type';return type;}// good{console;// set the default type to 'no type'var type = this_type || 'no type';return type;} -
Prefixing your comments with
FIXME
orTODO
helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions areFIXME -- need to figure this out
orTODO -- need to implement
. -
Use
// FIXME:
to annotate problems{// FIXME: shouldn't use a global heretotal = 0;return this;} -
Use
// TODO:
to annotate solutions to problems{// TODO: total should be configurable by an options paramthistotal = 0;return this;}
**[⬆ back to top](#table-of-contents)**
## Whitespace
- Use soft tabs set to 2 spaces
```javascript
// bad
function() {
∙∙∙∙var name;
}
// bad
function() {
∙var name;
}
// good
function() {
∙∙var name;
}
```
- Place 1 space before the leading brace.
```javascript
// 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.
```javascript
// bad
var x=y+5;
// good
var x = y + 5;
```
- Place an empty newline at the end of the file.
```javascript
// bad
(function(global) {
// ...stuff...
})(this);
```
```javascript
// good
(function(global) {
// ...stuff...
})(this);
```
- Use indentation when making long method chains.
```javascript
// 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);
```
**[⬆ back to top](#table-of-contents)**
## Commas
- Leading commas: **Nope.**
```javascript
// bad
var story = [
once
, upon
, aTime
];
// good
var story = [
once,
upon,
aTime
];
// 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](http://es5.github.io/#D)):
> 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'
];
```
**[⬆ back to top](#table-of-contents)**
## Semicolons
- **Yup.**
```javascript
// bad
(function() {
var name = 'Skywalker'
return name
})()
// good
(function() {
var name = 'Skywalker';
return name;
})();
// good
;(function() {
var name = 'Skywalker';
return name;
})();
```
**[⬆ back to top](#table-of-contents)**
## Type Casting & Coercion
- Perform type coercion at the beginning of the statement.
- Strings:
```javascript
// => 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.
```javascript
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](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.
- **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109)
```javascript
// 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;
```
- Booleans:
```javascript
var age = 0;
// bad
var hasAge = new Boolean(age);
// good
var hasAge = Boolean(age);
// good
var hasAge = !!age;
```
**[⬆ back to top](#table-of-contents)**
## Naming Conventions
- Avoid single letter names. Be descriptive with your naming.
```javascript
// bad
function q() {
// ...stuff...
}
// good
function query() {
// ..stuff..
}
```
- Use camelCase when naming objects, functions, and instances
```javascript
// 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
```javascript
// 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
```javascript
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
// good
this._firstName = 'Panda';
```
- When saving a reference to `this` use `self`.
```javascript
// bad
function() {
var _this = this;
return function() {
console.log(_this);
};
}
// bad
function() {
var that = this;
return function() {
console.log(that);
};
}
// good
function() {
var self = this;
return function() {
console.log(self);
};
}
```
- Name your functions. This is helpful for stack traces.
```javascript
// bad
var log = function(msg) {
console.log(msg);
};
// good
var log = function log(msg) {
console.log(msg);
};
```
**[⬆ back to top](#table-of-contents)**
## Accessors
- Accessor functions for properties are not required
- If you do make accessor functions use val() and val('hello')
```javascript
// bad
dragon.getAge();
// good
dragon.age();
// bad
dragon.setAge(25);
// good
dragon.age(25);
```
- If the property is a boolean, use isVal() or hasVal()
```javascript
// bad
if (!dragon.age()) {
return false;
}
// good
if (!dragon.hasAge()) {
return false;
}
```
**[⬆ back to top](#table-of-contents)**
## Constructors
- Prefer factories over prototypes
```javascript
// bad
function Widget(name) {
this.privateField = 'Hello ';
this.name = name;
}
Widget.prototype.publicMethod = function() {
return this.privateField + this.name + '!';
}
var widget = new Widget('Testing');
// good
var createWidget = function(name) {
var api = new (function Widget(){})();
var privateField = 'Hello ';
api.publicMethod = function() {
return privateField + name + '!';
};
return api;
}
var widget = createWidget('Testing!');
-
Use prototypes only when an extremely large number of objects will be created (think in the millions)
-
You're probably better off not using prototype then either.
-
If you are using it though, 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!
{console;}// badJediprototype ={console;}{console;};// goodJediprototype {console;};Jediprototype {console;}; -
Methods can return
this
to help with method chaining.// badvar {var api = {};var jumping = false;var height;api {jumping = true;return true;};api {height = newHeight;};return api;};var luke = ;luke; // => trueluke // => undefined// goodvar {var api = {};var jumping = false;var height;api {jumping = true;return api;};api {height = newHeight;return api;};return api;};var luke = ;luke; -
It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
var {var api = new { };options || options = {};var name = optionsname || 'no name';api {return name;};api {return 'Jedi - ' + name;};return api;};
Events
-
When attaching data payloads to events (whether DOM events, TokBox trigger or node.js EventEmitter), 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:
// bad;...;prefer:
// good;...;
Modules
-
The module should start with a
!
. This ensures that if a malformed module forgets to include a final semicolon there aren't errors in production when the scripts get concatenated. Explanation -
If your module is adding to the global scope, add a method called noConflict() that sets the exported module to the previous version and returns this one.
-
Always declare
'use strict';
at the top of the module.// fancyInput/fancyInput.js! {'use strict';var previousFancyInput = globalcreateFancyInput;{var options = options || {};}createFancyInput {globalcreateFancyInput = previousFancyInput;return createFancyInput;};globalcreateFancyInput = createFancyInput;}this;
jQuery
See You Might Not Need jQuery.
ECMAScript 5 Compatibility
- Refer to Kangax's ES5 compatibility table
Testing
-
Yup.
{return true;}
Performance
- On Layout & Web Performance
- String vs Array Concat
- Try/Catch Cost In a Loop
- Bang Function
- innerHTML vs textContent for script text
- Long String Concatenation
- Loading...
Resources
Read This
Other Styleguides
Other Styles
- Naming this in nested functions - Christian Johansen
- Conditional Callbacks
- Popular JavaScript Coding Conventions on Github
Further Reading
- Understanding JavaScript Closures - Angus Croll
- Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
- You Might Not Need jQuery - Zack Bloom & Adam Schwartz
- ES6 Features - Luke Hoban
Books
- JavaScript: The Good Parts - Douglas Crockford
- JavaScript Patterns - Stoyan Stefanov
- Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
- High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
- Maintainable JavaScript - Nicholas C. Zakas
- JavaScript Web Applications - Alex MacCaw
- Pro JavaScript Techniques - John Resig
- Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
- Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
- Human JavaScript - Henrik Joreteg
- Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
- JSBooks
- Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Blogs
- DailyJS
- JavaScript Weekly
- JavaScript, JavaScript...
- Bocoup Weblog
- Adequately Good
- NCZOnline
- Perfection Kills
- Ben Alman
- Dmitry Baranovskiy
- Dustin Diaz
- nettuts
In the Wild
The list of organizations using similar style guides (including translations) can be found at the original from Airbnb.
Contributors
License
(The MIT License)
Copyright (c) 2014 Tokbox Inc. 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.