JS Style Guide
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README.md

README.md

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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