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JavaScript Style Guide

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Merge pull request #1 from voy/master

Initial bundle of GoodData customizations
latest commit 4e8d7b6645
Vojtech Jasny voy authored January 03, 2013
Octocat-spinner-32 linters Bump minor version for addition of single quotes setting November 13, 2012
Octocat-spinner-32 README.md Removed modules section, updated TOC December 14, 2012
README.md

GoodData JavaScript Style Guide() {

GoodData flavor of the mostly reasonable approach to JavaScript by AirBnb

Table of Contents

  1. Types
  2. Objects
  3. Arrays
  4. Strings
  5. Functions
  6. Properties
  7. Variables
  8. Conditional Expressions & Equality
  9. Blocks
  10. Comments
  11. Whitespace
  12. Leading Commas
  13. Semicolons
  14. Type Casting & Coercion
  15. Naming Conventions
  16. Accessors
  17. Constructors
  18. jQuery
  19. ES5 Compatibility
  20. Testing
  21. Performance
  22. Resources
  23. In the Wild
  24. The JavaScript Style Guide Guide
  25. Contributors
  26. License

Types

  • Primitives: When you access a primitive type you work directly on its value

    • String
    • Number
    • Boolean
    • null
    • undefined
    var foo = 1,
        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],
        bar = foo;
    
    bar[0] = 9;
    
    console.log(foo[0], bar[0]); // => 9, 9
    

    [⬆]

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 = {
        class: 'superhero',
        default: { clark: kent },
        private: true
    };
    
    // good
    var superman = {
        klass: 'superhero',
        defaults: { clark: kent },
        hidden: true
    };
    

    [⬆]

Arrays

  • Use the literal syntax for array creation

    // bad
    var items = new Array();
    
    // good
    var items = [];
    
  • When you are managing array length use direct assignment over Array#push. jsPerf

    var hundredOdds = [],
        i;
    
    // bad
    for (i = 0; i < 100; i++) {
        hundredOdds.push(i * 2 + 1);
    }
    
    // good
    for (i = 0; i < 100; i++) {
        hundredOdds[i] = i * 2 + 1;
    }
    
  • If you don't know array length use Array#push.

    var someStack = [],
    
    // bad
    someStack[someStack.length] = 'abracadabra';
    
    // good
    someStack.push('abracadabra');
    
  • When you need to copy an array use Array() constructor or Array#slice. jsPerf

    var len = items.length,
        itemsCopy = [],
        i;
    
    // bad
    for (i = 0; i < len; i++) {
        itemsCopy[i] = items[i];
    }
    
    // good
    itemsCopy = Array.apply(null, items);
    
    // good
    itemsCopy = items.slice(0)
    

    [⬆]

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.

    // 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 programatically 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++) {
            items[i] = messages[i].message;
        }
    
        return '<ul><li>' + items.join('</li><li>') + '</li></ul>';
    }
    

    [⬆]

  • You can also use the String#fmt function provided by Ember.js.

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
    if (currentUser) {
        var test = function test() {
            console.log('Yup.');
        };
    }
    
  • Never name a parameter arguments, this will take precendence 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...
    }
    

    [⬆]

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');
    

    [⬆]

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();
    
  • You can use one var declaration for multiple variables, but if the right hand side of the assignment is the result of a function call, prefer to use multiple var declarations. When using developers tools this eases stepping into the function for debugging.

    // bad
    var items = getItems(),
        stuff = getStuff();
    
    // good
    var items = getItems();
    var stuff = getStuff();
    
  • Use one var declaration for multiple variables and declare each variable on a newline.

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

    [⬆]

Conditional Expressions & Equality

  • Use === and !== over == and !=.
  • Conditional expressions 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 evalute 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

    [⬆]

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;
    }
    

    [⬆]

Comments

  • Use /** ... */ for multiline 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 emptyline 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;
    }
    

    [⬆]

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'
    });
    

    [⬆]

  • Use indentation when making long method chains.

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

Leading Commas

  • Nope.

    // bad
    var once
      , upon
      , aTime;
    
    // good
    var 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'
    };
    

    [⬆]

Semicolons

  • Yup.

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

  • 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.

    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);
    
    // 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;
    
  • Booleans:

    var age = 0;
    
    // bad
    var hasAge = new Boolean(age);
    
    // good
    var hasAge = Boolean(age);
    
    // good
    var hasAge = !!age;
    

    [⬆]

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 this-is-my-object = {};
    function c() {};
    var u = new user({
        name: 'Bob Parr'
    });
    
    // good
    var thisIsMyObject = {};
    function thisIsMyFunction() {};
    var user = new User({
        name: 'Bob Parr'
    });
    
  • 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'
    });
    
  • 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;
        return function() {
          console.log(self);
        };
    }
    
    // bad
    function() {
        var that = this;
        return function() {
          console.log(that);
        };
    }
    
    // good
    function() {
        var _this = this;
        return function() {
          console.log(_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);
    };
    

    [⬆]

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];
    };
    

    [⬆]

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 || (options = {});
        this.name = options.name || 'no name';
    }
    
    Jedi.prototype.getName = function getName() {
        return this.name;
    };
    
    Jedi.prototype.toString = function toString() {
        return 'Jedi - ' + this.getName();
    };
    

    [⬆]

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'). jsPerf

  • Use find with scoped jQuery object queries.

    // bad
    $('.sidebar', 'ul').hide();
    
    // bad
    $('.sidebar').find('ul').hide();
    
    // good
    $('.sidebar ul').hide();
    
    // good
    $('.sidebar > ul').hide();
    
    // good (slower)
    $sidebar.find('ul');
    
    // good (faster)
    $($sidebar[0]).find('ul');
    

    [⬆]

ECMAScript 5 Compatability

Testing

  • Yup.

    function() {
        return true;
    }
    

    [⬆]

Performance

Resources

Read This

Other Styleguides

Other Styles

Books

Blogs

In the Wild

This is a list of organizations that are using this style guide. Send us a pull request or open an issue and we'll add you to the list.

The JavaScript Style Guide Guide

Contributors

License

(The MIT License)

Copyright (c) 2012 Airbnb

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

[⬆]

};

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