JavaScript Programming Style Guide

The focus of this style guide is to keep consistency among developers in the team. Break a style rule if applying the rule would make the code less readable. If you have questions about style and do not see a discussion in this guide ask a developer, then add content to this guide. This guide IS opinionated. If you have a different opinion, make your case for change.

Table of Contents

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
Performance

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 = new Object();

// good
var item = {};

Don't use reserved words as keys.

// 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 = new Array();

// 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,
    itemsCopy = [],
    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.

function trigger() {
  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 100 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,
    messages,
    length,
    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
function inbox(messages) {
  items = '<ul>';

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

  return items + '</ul>';
}

// good
function inbox(messages) {
  items = [];

  for ( i = 0; i < length; i++ ) {
    // use direct assignment in this case because we're micro-optimizing.
    items[i] = '<li>' + messages[i].message + '</li>';
  }

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

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Functions

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...
}

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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
};

function getProp(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 unless necessary.
```
// bad
superPower = new SuperPower();

// good
var superPower = new SuperPower();
```

Use one var declaration per variable set.

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

// good
var items = getItems(),
    goSportsTeam = true,
    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(),
    goSportsTeam = true,
    dragonball,
    length,
    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 - unnecessary function call
function() {
  var name = getName();

  if ( !arguments.length ) {
    return false;
  }

  this.setFirstName(name);

  return true;
}

// good
function() {
  var name;

  if ( !arguments.length ) {
    return false;
  }

  name = getName();
  this.setFirstName(name);

  return true;
}

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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)
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.

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Comparison Operators & Equality

Use === and !== over == and !=.
Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract 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 )
  return false;

// bad (readability)
if ( test ) return false;

// good
if ( test ) {
  return false;
}

// bad
function() { return false; }

// good
function() {
  return false;
}

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

Comment at the beginning of each file to advise the reader to the general purpose of the file, if the code is not self-explanatory to a junior dev.
Use a single paragraph to outline the use of the file within the system. If more than one paragraph should be used the code should be refactored into more files.

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
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;
}

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.

function Calculator() {

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

  return this;
}

Use // TODO: to annotate solutions to problems.

function Calculator() {

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

  return this;
}

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Whitespace

Use soft tabs set to 4 spaces.

// bad
function() {
∙∙var name;
}

// bad
function() {
∙var name;
}

// good
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'
});

Place 1 space before the opening parenthesis in control statements (if, while etc.), one space after the opening parenthesis, and one space before the closing parenthesis.
```
// bad
if(isJedi) {
  fight ();
}

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

Place no space before the argument list in function calls and declarations.

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

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

Set off operators with spaces.

// bad
var x=y+5;

// good
var x = y + 5;

Place no whitespace immediately inside brackets or braces.

// bad
var foo = [ 1, 2, 3 ];

// good
var foo = [1, 2, 3];

Place no space immediately before a comma or semicolon.

// bad
var foo = [1 , 2 , 3] ;

// good
var foo = [1, 2, 3];

Immediately before the open parenthesis that starts the argument list of a function call.
```
// bad
var foo = bar (a, b);

// good
var foo = bar(a, b);
```

Immediately before the open parenthesis that starts an indexing.

// bad
var foo = someList [index];

// good
var foo = someList[index];

Use spaces immediately after colon when defining associative array.

// bad
var foo = {
    key:'value',
}

// good
var foo = {
    key: 'value',
}

Use spaces before and after colon when used with ternary operators.

// bad
a = typeof a !== 'undefined' ? a:42;

// good
a = typeof a !== 'undefined' ? a : 42;

Use spaces around arithmetic, comparison and binary operators.
```
// bad
var foo = a+b;

// good
var foo = a + b;
```

Function definitions should have their opening brace on the same line as the name with a single space separating them.

// bad
function foo(a, b){
  // Do something
}

// bad
function foo(a, b)
{
  // Do something
}

// good
function foo(a, b) {
  // Do something
}

If a comment is necessary for an else condition, the comment should be the first line inside the else brackets.

// bad
if ( foo === true ) {
  // Do something
// Comme```javascript
// bad
res += Broadcasts.find({$and: [{page: page._id}, {time: {$gt: gate, $lt: now}}]}).count();

// good
res += Broadcasts.find({
    $and: [
        {page: page._id},
        {time: {
          $gt: gate,
          $lt: now
        }}
    ]}).count();

End files with a single newline character.

// bad
(function(global) {
  // ...stuff...
})(this);

// bad
(function(global) {
  // ...stuff...
})(this);↵
↵

// good
(function(global) {
  // ...stuff...
})(this);↵

Limit the use of long long method chains. When long is necessary, break method chain into multiple lines.

Optionally, use indentation and leading dot to indicate method chain wrapping lines. Leading dot emphasizes that the line is a method call, not a new statement.

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

// good
var $items = $('#items');

$items.find('.selected').highlight()
$items.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.

```javascript
// 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'

) { console.log(that); }; }

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

Name your functions, at least while debugging. This is helpful for stack traces. Name can be removed for deployment.

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

// good
var log = function log(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
class CheckBox {
  // ...
}
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() ) {
  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];
};

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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!

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 || (o();

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

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

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Performance

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This styleguide is largely influenced by AirBNB and JQuery style guides