JavaScript Style Guide

Aug 1, 2015

A mostly reasonable approach to JavaScript. See full: https://github.com/duyet/javascript

Types
Objects
Arrays
Strings
Functions
Properties
Variables
Hoisting
Comparison Operators & Equality
Blocks
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Constructors
Events
Modules
jQuery
ECMAScript 5 Compatibility
Testing
Performance
Resources
In the Wild
Translation
The JavaScript Style Guide Guide
Chat With Us About Javascript
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.log(foo, bar); // => 1, 9

Complex: When you access a complex type you work on a reference to its value.
object
array
function

var foo = [1, 2];
var bar = foo;

bar[0] =9;

console.log(foo[0], bar[0]); // => 9, 9

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Objects

Use the literal syntax for object creation.

// bad
var item =newObject();

// 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
};

Use readable synonyms in place of reserved words.

// bad
var superman = {
  class:'alien'
};

// bad
var superman = {
  klass:'alien'
};

// good
var superman = {
  type:'alien'
};

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Arrays

Use the literal syntax for array creation.

// bad
var items =newArray();

// good
var items = [];

Use Array#push instead of direct assignment to add items to an array.

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 = [];
var i;

// bad
for (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.

functiontrigger() {
var args =Array.prototype.slice.call(arguments);
  ...
}

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Strings

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 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 = messages.length;

// bad
functioninbox(messages) {
  items ='<ul>';

for (i =0; i < length; i++) {
    items +='<li>'+ messages[i].message +'</li>';
  }

return items +'</ul>';
}

// good
functioninbox(messages) {
  items = [];

for (i =0; i < length; i++) {
    items[i] ='<li>'+ messages[i].message +'</li>';
  }

return'<ul>'+ items.join('') +'</ul>';
}

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Functions

Function expressions:

// anonymous function expression
varanonymous=function() {
returntrue;
};

// named function expression
varnamed=functionnamed() {
returntrue;
};

// 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) {
functiontest() {
console.log('Nope.');
  }
}

// good
var test;
if (currentUser) {
test=functiontest() {
console.log('Yup.');
  };
}

Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

// bad
functionnope(name, options, arguments) {
// ...stuff...
}

// good
functionyup(name, options, args) {
// ...stuff...
}

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Properties

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.

var luke = {
  jedi:true,
  age:28
};

functiongetProp(prop) {
return luke[prop];
}

var isJedi = getProp('jedi');

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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.

// bad
superPower =newSuperPower();

// good
var superPower =newSuperPower();

Use one var declaration per variable. 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.

// bad
var items = getItems(),
    goSportsTeam =true,
    dragonball ='z';

// bad
// (compare to above, and try to spot the mistake)
var items = getItems(),
    goSportsTeam =true;
    dragonball ='z';

// good
var items = getItems();
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.

// bad
var i, len, dragonball,
    items = getItems(),
    goSportsTeam =true;

// bad
var i;
var items = getItems();
var dragonball;
var goSportsTeam =true;
var 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') {
returnfalse;
  }

return name;
}

// good
function() {
var name = getName();

test();
console.log('doing stuff..');

//..other stuff..

if (name ==='test') {
returnfalse;
  }

return name;
}

// bad
function() {
var name = getName();

if (!arguments.length) {
returnfalse;
  }

returntrue;
}

// good
function() {
if (!arguments.length) {
returnfalse;
  }

var name = getName();

returntrue;
}

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Hoisting

Variable declarations get hoisted to the top of their scope, but their assignment does not.

// we know this wouldn't work (assuming there
// is no notDefined global variable)
functionexample() {
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.
functionexample() {
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:
functionexample() {
var declaredButNotAssigned;
console.log(declaredButNotAssigned); // => undefined
  declaredButNotAssigned =true;
}

Anonymous function expressions hoist their variable name, but not the function assignment.

functionexample() {
console.log(anonymous); // => undefined

  anonymous(); // => TypeError anonymous is not a function

varanonymous=function() {
console.log('anonymous function expression');
  };
}

Named function expressions hoist the variable name, not the function name or the function body.

functionexample() {
console.log(named); // => undefined

  named(); // => TypeError named is not a function

  superPower(); // => ReferenceError superPower is not defined

varnamed=functionsuperPower() {
console.log('Flying');
  };
}

// the same is true when the function name
// is the same as the variable name.
functionexample() {
console.log(named); // => undefined

  named(); // => TypeError named is not a function

varnamed=functionnamed() {
console.log('named');
  }
}

Function declarations hoist their name and the function body.

functionexample() {
  superPower(); // => Flying

functionsuperPower() {
console.log('Flying');
  }
}

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Comparison Operators & Equality

Use === and !== over == and !=.
Comparison operators 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.

// 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.

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Blocks

Use braces with all multi-line blocks.

// bad
if (test)
returnfalse;

// good
if (test) returnfalse;

// good
if (test) {
returnfalse;
}

// bad
function() { returnfalse; }

// good
function() {
returnfalse;
}

If you’re using multi-line blocks with if and else, put else on the same line as your if block’s closing brace.

// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

// good
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}

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Comments

Use /** ... */ for multi-line 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
functionmake(tag) {

// ...stuff...

return element;
}

// good
/**
 * make() returns a new element
 * based on the passed in tag name
 *
 * @param {String} tag
 * @return {Element} element
 */
functionmake(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
functiongetType() {
console.log('fetching type...');
// set the default type to 'no type'
var type =this._type ||'no type';

return type;
}

// good
functiongetType() {
console.log('fetching type...');

// set the default type to 'no type'
var type =this._type ||'no type';

return type;
}

Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME -- need to figure this out or TODO -- need to implement.
Use // FIXME: to annotate problems.

functionCalculator() {

// FIXME: shouldn't use a global here
  total =0;

returnthis;
}

Use // TODO: to annotate solutions to problems.

functionCalculator() {

// TODO: total should be configurable by an options param
this.total =0;

returnthis;
}

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Whitespace

Use soft tabs set to 2 spaces.

// bad
function() {
∙∙∙∙var name;
}

// bad
function() {
∙var name;
}

// good
function() {
∙∙var name;
}

Place 1 space before the leading brace.

// bad
functiontest(){
console.log('test');
}

// good
functiontest() {
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'
});

Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space before the argument list in function calls and declarations.

// bad
if(isJedi) {
  fight ();
}

// good
if (isJedi) {
  fight();
}

// bad
functionfight () {
console.log ('Swooosh!');
}

// good
functionfight() {
console.log('Swooosh!');
}

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. Use a leading dot, which emphasizes that the line is a method call, not a new statement.

// bad
$('#items').find('.selected').highlight().end().find('.open').updateCount();

// 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').classed('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')
    .classed('led', true)
    .attr('width',  (radius + margin) *2)
  .append('svg:g')
    .attr('transform', 'translate('+ (radius + margin) +','+ (radius + margin) +')')
    .call(tron.led);

Leave a blank line after blocks and before the next statement

// bad
if (foo) {
return bar;
}
return baz;

// good
if (foo) {
return bar;
}

return baz;

// bad
var obj = {
foo:function() {
  },
bar:function() {
  }
};
return obj;

// good
var obj = {
foo:function() {
  },

bar:function() {
  }
};

return obj;

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Commas

Leading commas: Nope.

// 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):

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.

// bad
var hero = {
    firstName:'Kevin',
    lastName:'Flynn',
  };

var heroes = [
'Batman',
'Superman',
  ];

// good
var hero = {
    firstName:'Kevin',
    lastName:'Flynn'
  };

var heroes = [
'Batman',
'Superman'
  ];

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Semicolons

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;
})();

Type Casting & Coercion

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 =newNumber(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//=> -2147483647

Booleans:

var age =0;

// bad
var hasAge =newBoolean(age);

// good
var hasAge =Boolean(age);

// good
var hasAge =!!age;

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Naming Conventions

Avoid single letter names. Be descriptive with your naming.

// bad
functionq() {
// ...stuff...
}

// good
functionquery() {
// ..stuff..
}

Use camelCase when naming objects, functions, and instances.

// bad
var OBJEcttsssss = {};
var this_is_my_object = {};
functionc() {}
var u =newuser({
  name:'Bob Parr'
});

// good
var thisIsMyObject = {};
functionthisIsMyFunction() {}
var user =newUser({
  name:'Bob Parr'
});

Use PascalCase when naming constructors or classes.

// bad
functionuser(options) {
this.name= options.name;
}

var bad =newuser({
  name:'nope'
});

// good
functionUser(options) {
this.name= options.name;
}

var good =newUser({
  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;
returnfunction() {
console.log(self);
  };
}

// bad
function() {
var that =this;
returnfunction() {
console.log(that);
  };
}

// good
function() {
var _this =this;
returnfunction() {
console.log(_this);
  };
}

Name your functions. This is helpful for stack traces.

// bad
varlog=function(msg) {
console.log(msg);
};

// good
varlog=functionlog(msg) {
console.log(msg);
};

Note: IE8 and below exhibit some quirks with named function expressions. See http://kangax.github.io/nfe/ for more info.
If your file exports a single class, your filename should be exactly the name of the class.

// file contents
classCheckBox {
// ...
}
module.exports = CheckBox;

// in some other file
// bad
var CheckBox =require('./checkBox');

// bad
var CheckBox =require('./check_box');

// good
var CheckBox =require('./CheckBox');

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Accessors

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()) {
returnfalse;
}

// good
if (!dragon.hasAge()) {
returnfalse;
}

It’s okay to create get() and set() functions, but be consistent.

functionJedi(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) {
returnthis[key];
};

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Constructors

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!

functionJedi() {
console.log('new jedi');
}

// bad
Jedi.prototype= {
fight:functionfight() {
console.log('fighting');
  },

block:functionblock() {
console.log('blocking');
  }
};

// good
Jedi.prototype.fight=functionfight() {
console.log('fighting');
};

Jedi.prototype.block=functionblock() {
console.log('blocking');
};

Methods can return this to help with method chaining.

// bad
Jedi.prototype.jump=function() {
this.jumping =true;
returntrue;
};

Jedi.prototype.setHeight=function(height) {
this.height= height;
};

var luke =newJedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
Jedi.prototype.jump=function() {
this.jumping =true;
returnthis;
};

Jedi.prototype.setHeight=function(height) {
this.height= height;
returnthis;
};

var luke =newJedi();

luke.jump()
  .setHeight(20);

It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

functionJedi(options) {
  options || (options = {});
this.name= options.name||'no name';
}

Jedi.prototype.getName=functiongetName() {
returnthis.name;
};

Jedi.prototype.toString=functiontoString() {
return'Jedi - '+this.getName();
};

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Events

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:

// bad
$(this).trigger('listingUpdated', listing.id);

...

$(this).on('listingUpdated', function(e, listingId) {
// do something with listingId
});

prefer:

// good
$(this).trigger('listingUpdated', { listingId : listing.id });

...

$(this).on('listingUpdated', function(e, data) {
// do something with data.listingId
});

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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
The file should be named with camelCase, live in a folder with the same name, and match the name of the single export.
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

!function(global) {
'use strict';

var previousFancyInput =global.FancyInput;

functionFancyInput(options) {
this.options= options || {};
  }

FancyInput.noConflict=functionnoConflict() {
global.FancyInput = previousFancyInput;
return FancyInput;
  };

global.FancyInput = FancyInput;
}(this);

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jQuery

Prefix jQuery object variables with a $.

// bad
var sidebar = $('.sidebar');

// good
var $sidebar = $('.sidebar');

Cache jQuery lookups.

// bad
functionsetSidebar() {
  $('.sidebar').hide();

// ...stuff...

  $('.sidebar').css({
'background-color':'pink'
  });
}

// good
functionsetSidebar() {
var $sidebar = $('.sidebar');
  $sidebar.hide();

// ...stuff...

  $sidebar.css({
'background-color':'pink'
  });
}

For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf
Use find with scoped jQuery object queries.

// bad
$('ul', '.sidebar').hide();

// bad
$('.sidebar').find('ul').hide();

// good
$('.sidebar ul').hide();

// good
$('.sidebar > ul').hide();

// good
$sidebar.find('ul').hide();

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