JavaScript Styleguide

A mostly reasonable approach to JavaScript

Table of Contents

1. Objects
2. Arrays
3. Strings
4. Functions
5. Properties
6. Variables
7. Blocks
8. Comments
9. Whitespace
10. Commas
11. Semicolons
12. Naming Conventions
13. Accessors
14. Constructors
15. Events
16. jQuery

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();

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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.';

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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.

  // 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. 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;

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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 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
  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

• 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;

• 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'
  };

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Semicolons

• Yup.

  // bad
  (function () {
    var name = 'Skywalker'
    return name
  })()
  
  // good
  (function () {
    var name = 'Skywalker';
    return name;
  })();

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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() {}

• 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.

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

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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;
  }

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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);

• 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();
  };

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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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jQuery

• Prefix jQuery object variables with a $.

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

• Cache jQuery lookups.

  // bad
  function setSidebar() {
    $('.sidebar').hide();
  
    // ...stuff...
  
    $('.sidebar').css({
      'background-color': 'pink'
    });
  }
  
  // good
  function setSidebar() {
    var $sidebar = $('.sidebar');
    $sidebar.hide();
  
    // ...stuff...
  
    $sidebar.css({
      'background-color': 'pink'
    });
  }

• For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul').

• 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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