JavaScript Style Guide

BigSemantics JavaScript Style Guide

Based on Airbnb's JavaScript (ES5) Style Guide with modifications

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;
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.

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

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Strings

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, consider using Array#join instead of string concatenation.

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
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

Use immediately-invoked function expressions (IIFEs) to create a local lexical scope.

// Variables defined inside an IIFE won't be hoisted outside of the IIFE.
(function () {
  var name = 'Picaso';
  console.log('Welcome to the Internet, ' + name);
}());

Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers may interpret it differently.

// 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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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 = new SuperPower();

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

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') {
    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;

// good
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 indentation level as your if.

// 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. Use JSDoc.

// bad
// Returns a new element based on the passed in tag name.
//
// @param {String} tag - The tag name of the new element.
// @return {Element} The generated element.
function make(tag) {

  // ...stuff...

  return element;
}

// good
/**
 * Returns a new element based on the passed in tag name
 *
 * @param {String} tag
 *   The tag name of the new element.
 * @return {Element}
 *   The generated 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 2 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.). Place no space before the argument list in function calls and declarations.

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

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

// 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;

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);

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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'
};

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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';

When using parseInt, always specify the radix.

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);

Booleans:

var age = 0;

// bad
var hasAge = new Boolean(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
function q() {
  // ...stuff...
}

// good
function query() {
  // ..stuff..
}

Use camelCase when naming objects, functions, and instances.

// bad
var OBJEcttsssss = {};
var this_is_my_object = {};
var o = {};
function c() {}

// good
var thisIsMyObject = {};
function thisIsMyFunction() {}

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'
});

Don't save references to this. Use Function#bind.

// bad
function () {
  var self = this;
  return function () {
    console.log(self);
  };
}

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

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

// good
function () {
  return function () {
    console.log(this);
  }.bind(this);
}

Name your functions. This is helpful for stack traces.

// ok but not recommended
var log = function (msg) {
  console.log(msg);
};

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

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

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;
}

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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 jump() {
  this.jumping = true;
  return true;
};

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

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

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

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

var luke = new Jedi();

luke.jump()
  .setHeight(20);

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JavaScript Style Guide

BigSemantics JavaScript Style Guide

Table of Contents

Objects

Arrays

Strings

Functions

Variables

Hoisting

Comparison Operators & Equality

Blocks

Comments

Whitespace

Commas

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Constructors

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