JavaScript Style Guide

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Airbnb JavaScript Style Guide

A mostly reasonable approach to JavaScript

Table of Contents

  1. Types
  2. Objects
  3. Arrays
  4. Strings
  5. Functions
  6. Properties
  7. Variables
  8. Hoisting
  9. Comparison Operators & Equality
  10. Blocks
  11. Comments
  12. Whitespace
  13. Commas
  14. Semicolons
  15. Type Casting & Coercion
  16. Naming Conventions
  17. Accessors
  18. Constructors
  19. Events
  20. Modules
  21. jQuery
  22. ECMAScript 5 Compatibility
  23. Testing
  24. Performance
  25. Resources
  26. In the Wild
  27. Translation
  28. The JavaScript Style Guide Guide
  29. Chat With Us About Javascript
  30. Contributors
  31. License

Types

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

    • string
    • number
    • boolean
    • null
    • undefined

    javascript 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

    javascript var foo = [1, 2]; var bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9

Objects

  • Use the literal syntax for object creation.

    javascript // bad var item = new Object(); // good var item = {};

  • Don't use reserved words as keys. It won't work in IE8. More info.

    javascript // bad var superman = { default: { clark: 'kent' }, private: true }; // good var superman = { defaults: { clark: 'kent' }, hidden: true };

  • Use readable synonyms in place of reserved words.

    javascript // bad var superman = { class: 'alien' }; // bad var superman = { klass: 'alien' }; // good var superman = { type: 'alien' };

Arrays

  • Use the literal syntax for array creation.

    javascript // bad var items = new Array(); // good var items = [];

  • Use Array#push instead of direct assignment to add items to an array.

    javascript var someStack = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra');

  • When you need to copy an array use Array#slice. jsPerf

    javascript 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.

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

Strings

  • Use single quotes '' for strings.

    javascript // 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.

  • Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.

    javascript // 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.

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

Functions

  • Function expressions:

    javascript // 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.

    javascript // 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.

    javascript // bad function nope(name, options, arguments) { // ...stuff... } // good function yup(name, options, args) { // ...stuff... }

Properties

  • Use dot notation when accessing properties.

    javascript 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.

    javascript 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.

    javascript // 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.

    javascript // 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.

    javascript // 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;

Hoisting

  • Variable declarations get hoisted to the top of their scope, but their assignment does not.

    javascript // 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.

    javascript 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.

    javascript 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.

    javascript function example() { superPower(); // => Flying function superPower() { console.log('Flying'); } }

  • For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

Comparison Operators & Equality

  • Use === and !== over == and !=.
  • Comparison operators 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 evaluate to false if +0, -0, or NaN, otherwise true
    • Strings evaluate to false if an empty string '', otherwise true

    javascript if ([0]) { // true // An array is an object, objects evaluate to true }

  • Use shortcuts.

    javascript // 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.

    javascript // 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.

    javascript // bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }

Comments

  • Use /** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

    ```javascript // 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.

    javascript // 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.

    javascript function Calculator() { // FIXME: shouldn't use a global here total = 0; return this; }

  • Use // TODO: to annotate solutions to problems.

    javascript function Calculator() { // TODO: total should be configurable by an options param this.total = 0; return this; }

Whitespace

  • Use soft tabs set to 2 spaces.

    javascript // bad function() { ∙∙∙∙var name; } // bad function() { ∙var name; } // good function() { ∙∙var name; }

  • Place 1 space before the leading brace.

    javascript // 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.

    javascript // 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.

    ```javascript // bad var x=y+5;

    // good var x = y + 5; ```

  • End files with a single newline character.

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

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

    javascript // 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.

    javascript // 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

    javascript // 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;

Commas

  • Leading commas: Nope.

    javascript // 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' ];

Semicolons

  • Yup.

    javascript // 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; })();

    Read more.

Type Casting & Coercion

  • Perform type coercion at the beginning of the statement.
  • Strings:

    javascript // => 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.

    javascript 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);

  • 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.

    ```javascript // 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; ```
  • Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:

    javascript 2147483647 >> 0 //=> 2147483647 2147483648 >> 0 //=> -2147483648 2147483649 >> 0 //=> -2147483647

  • Booleans:

    javascript 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.

    javascript // bad function q() { // ...stuff... } // good function query() { // ..stuff.. }

  • Use camelCase when naming objects, functions, and instances.

    javascript // bad var OBJEcttsssss = {}; 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.

    javascript // 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.

    javascript // bad this.__firstName__ = 'Panda'; this.firstName_ = 'Panda'; // good this._firstName = 'Panda';

  • When saving a reference to this use _this.

    javascript // 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.

    javascript // 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. `javascript // 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');

Accessors

  • Accessor functions for properties are not required.
  • If you do

Richard Qiu

Life is a gift ! I love waking up in the moring and not konowing what's going to hanppen,or who I'm going to meeting,where I'm going to wind up. so, keep optimistic and enjoy life!

Shanghai