A mostly reasonable approach to JavaScript
- Types
- Objects
- Arrays
- Strings
- Functions
- Properties
- Variables
- Hoisting
- Comparison Operators & Equality
- Blocks
- Comments
- Whitespace
- Commas
- Semicolons
- Type Casting & Coercion
- Naming Conventions
- Accessors
- Constructors
- Modules
- jQuery
- ECMAScript 5 Compatibility
- Testing
- Performance
- Editorconfig Setup / Use
- Resources
- In the Wild
- The JavaScript Style Guide Guide
- Contributors
- License
Primitives: When you access a primitive type you work directly on its value.
stringnumberbooleannullundefined
varfoo=1;varbar=foo;bar=9;console.log(foo,bar);// => 1, 9
Complex: When you access a complex type you work on a reference to its value.
objectarrayfunction
varfoo=[1,2];varbar=foo;bar[0]=9;console.log(foo[0],bar[0]);// => 9, 9
Use the literal syntax for object creation.
// badvaritem=newObject();// goodvaritem={};
Don't use reserved words as keys. It won't work in IE8. More info.
// badvarsuperman={class: 'superhero',default: {clark: 'kent'},private: true};// goodvarsuperman={cssClass: 'superhero',defaults: {clark: 'kent'},hidden: true};
Use readable synonyms in place of reserved words.
// badvarsuperman={class: 'alien'};// badvarsuperman={klass: 'alien'};// goodvarsuperman={type: 'alien'};
Use the literal syntax for array creation.
// badvaritems=newArray();// goodvaritems=[];
Use Array#push instead of direct assignment to add items to an array.
varsomeStack=[];// badsomeStack[someStack.length]='abracadabra';// goodsomeStack.push('abracadabra');
When you need to copy an array use Array#slice. jsPerf
varlen=items.length;varitemsCopy=[];vari;// badfor(i=0;i<len;i++){itemsCopy[i]=items[i];}// gooditemsCopy=items.slice();
To convert an array-like object to an array, use Array#slice.
functiontrigger(){varargs=Array.prototype.slice.call(arguments); ... }
Use single quotes
''for strings.// badvarname="Bob Parr";// goodvarname='Bob Parr';// badvarfullName="Bob "+this.lastName;// goodvarfullName='Bob '+this.lastName;
Function expressions:
// anonymous function as event handler$el.on('click',function(){returntrue;});// optional name to aid debugging with stack traces$el.on('click',functionelClick(){returntrue;});// anonymous function as methodMyObject.prototype.isTrue=function(){returntrue;};// immediately-invoked function expression (IIFE)(function(){console.log('Welcome to the Internet. Please follow me.');})();
Function declarations:
// use a named declaration when appropriate mainly// for internal helper logic and organizationfunctionnamedFunction(){returntrue;}
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
blockas a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.// badif(currentUser){functiontest(){console.log('Nope.');}}// goodvartest;if(currentUser){test=functiontest(){console.log('Yup.');};}
Never name a parameter
arguments. This will take precedence over theargumentsobject that is given to every function scope.// badfunctionnope(name,options,arguments){// ...stuff...}// goodfunctionyup(name,options,args){// ...stuff...}
Use dot notation when accessing properties.
varluke={jedi: true,age: 28};// badvarisJedi=luke['jedi'];// goodvarisJedi=luke.jedi;
Use subscript notation
[]when accessing properties with a variable.varluke={jedi: true,age: 28};functiongetProp(prop){returnluke[prop];}varisJedi=getProp('jedi');
Always use
varto declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.// badsuperPower=newSuperPower();// goodvarsuperPower=newSuperPower();
Use one
vardeclaration 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.// badvaritems=getItems(),goSportsTeam=true,dragonball='z';// bad// (compare to above, and try to spot the mistake)varitems=getItems(),goSportsTeam=true;dragonball='z';// goodvaritems=getItems();vargoSportsTeam=true;vardragonball='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. Multiple unassigned variables can be declared in a single-line var declaration.
// goodvaritems=getItems();vargoSportsTeam=true;vari,j,length;
Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.
// badfunction(){test();console.log('doing stuff..');//..other stuff..varname=getName();if(name==='test'){returnfalse;}returnname;}// goodfunction(){varname=getName();test();console.log('doing stuff..');//..other stuff..if(name==='test'){returnfalse;}returnname;}// bad - unnecessary function callfunction(){varname=getName();if(!arguments.length){returnfalse;}this.setFirstName(name);returntrue;}// goodfunction(){varname;if(!arguments.length){returnfalse;}name=getName();this.setFirstName(name);returntrue;}
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)functionexample(){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.functionexample(){console.log(declaredButNotAssigned);// => undefinedvardeclaredButNotAssigned=true;}// The interpreter is hoisting the variable// declaration to the top of the scope,// which means our example could be rewritten as:functionexample(){vardeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}
Anonymous function expressions hoist their variable name, but not the function assignment.
functionexample(){console.log(anonymous);// => undefinedanonymous();// => TypeError anonymous is not a functionvaranonymous=function(){console.log('anonymous function expression');};}
Named function expressions hoist the variable name, not the function name or the function body.
functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionsuperPower();// => ReferenceError superPower is not definedvarnamed=functionsuperPower(){console.log('Flying');};}// the same is true when the function name// is the same as the variable name.functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionvarnamed=functionnamed(){console.log('named');};}
Function declarations hoist their name and the function body.
functionexample(){superPower();// => FlyingfunctionsuperPower(){console.log('Flying');}}
For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.
Use
===and!==over==and!=.Conditional statements such as the
ifstatement evaluate their expression using coercion with theToBooleanabstract 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.
// badif(name!==''){// ...stuff...}// goodif(name){// ...stuff...}// badif(collection.length>0){// ...stuff...}// goodif(collection.length){// ...stuff...}
For more information see Truth Equality and JavaScript by Angus Croll.
Use braces with all multi-line blocks.
// badif(test)returnfalse;// acceptable (only for very simple statements and early function return)if(test)returnfalse;// goodif(test){returnfalse;}// badfunction(){returnfalse;}// goodfunction(){returnfalse;}
If you're using multi-line blocks with
ifandelse, putelseon the same line as yourifblock's closing brace.// badif(test){thing1();thing2();}else{thing3();}// goodif(test){thing1();thing2();}else{thing3();}
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} elementfunctionmake(tag){// ...stuff...returnelement;}// good/** * make() returns a new element * based on the passed in tag name * * @param {String} tag * @return {Element} element */functionmake(tag){// ...stuff...returnelement;}
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.// badvaractive=true;// is current tab// good// is current tabvaractive=true;// badfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}// goodfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}
Prefixing your comments with
FIXMEorTODOhelps 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 areFIXME -- need to figure this outorTODO -- need to implement.Use
// FIXME:to annotate problems.functionCalculator(){// FIXME: shouldn't use a global heretotal=0;returnthis;}
Use
// TODO:to annotate solutions to problems.functionCalculator(){// TODO: total should be configurable by an options paramthis.total=0;returnthis;}
Use tabs. One per indent level. This allows developers to choose their preferred display width.
// badfunction(){∙∙varname;}// badfunction(){∙∙∙∙varname;}// goodfunction(){varname;}
Place 1 space before the leading brace.
// badfunctiontest(){console.log('test');}// goodfunctiontest(){console.log('test');}// baddog.set('attr',{age: '1 year',breed: 'Bernese Mountain Dog'});// gooddog.set('attr',{age: '1 year',breed: 'Bernese Mountain Dog'});
Place 1 space before the opening parenthesis in control statements (
if,whileetc.). Place no space before the argument list in function calls and declarations.// badif(isJedi){fight();}// goodif(isJedi){fight();}// badfunctionfight(){console.log('Swooosh!');}// goodfunctionfight(){console.log('Swooosh!');}
Place 1 space before anonymous function parenthesis.
// badfunction(){varname;}// goodfunction(){varname;}
Set off operators with spaces.
// badvarx=y+5;// goodvarx=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. Long method chains should be avoided except in cases of a performance benefit or significant readability benefit.
// bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// good$('#items').find('.selected').highlight().end().find('.open').updateCount();// badvarleds=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);// goodvarleds=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.
// badvarstory=[once,upon,aTime];// goodvarstory=[once,upon,aTime];// badvarhero={firstName: 'Bob',lastName: 'Parr',heroName: 'Mr. Incredible',superPower: 'strength'};// goodvarhero={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'
];
```
Yup.
// bad(function(){varname='Skywalker'returnname})()// good(function(){varname='Skywalker';returnname;})();// good (guards against the function becoming an argument when two files with IIFEs are concatenated);(function(){varname='Skywalker';returnname;})();
Perform type coercion at the beginning of the statement.
Strings:
// => this.reviewScore = 9;// badvartotalScore=this.reviewScore+'';// goodvartotalScore=''+this.reviewScore;// badvartotalScore=''+this.reviewScore+' total score';// goodvartotalScore=this.reviewScore+' total score';
Use
parseIntfor Numbers and always with a radix for type casting.varinputValue='4';// badvarval=newNumber(inputValue);// badvarval=+inputValue;// badvarval=inputValue>>0;// badvarval=parseInt(inputValue);// goodvarval=Number(inputValue);// goodvarval=parseInt(inputValue,10);
If for whatever reason you are doing something wild and
parseIntis your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.// good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */varval=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:
2147483647>>0//=> 21474836472147483648>>0//=> -21474836482147483649>>0//=> -2147483647
Booleans:
varage=0;// badvarhasAge=newBoolean(age);// goodvarhasAge=Boolean(age);// goodvarhasAge=!!age;
Avoid single letter names. Be descriptive with your naming.
// badfunctionq(){// ...stuff...}// goodfunctionquery(){// ..stuff..}
Use camelCase when naming objects, functions, and instances.
// badvarOBJEcttsssss={};varthis_is_my_object={};varo={};functionc(){}varu=newuser({name: 'Bob Parr'});// goodvarthisIsMyObject={};functionthisIsMyFunction(){};varuser=newUser({name: 'Bob Parr'});
Use PascalCase when naming constructors or classes.
// badfunctionuser(options){this.name=options.name;}varbad=newuser({name: 'nope'});// goodfunctionUser(options){this.name=options.name;}vargood=newUser({name: 'yup'});
Use a leading underscore
_when naming private properties.// badthis.__firstName__='Panda';this.firstName_='Panda';// goodthis._firstName='Panda';
When saving a reference to
thisuseself.// badfunction(){varthat=this;returnfunction(){console.log(that);};}// badfunction(){var_this=this;returnfunction(){console.log(_this);};}// goodfunction(){varself=this;returnfunction(){console.log(self);};}
Name your functions. This is helpful for stack traces.
// badvarlog=function(msg){console.log(msg);};// goodvarlog=functionlog(msg){console.log(msg);};
Accessor functions for properties are not required.
If you do make accessor functions use getVal() and setVal('hello').
// baddragon.age();// gooddragon.getAge();// baddragon.age(25);// gooddragon.setAge(25);
If the property is a boolean, use isVal() or hasVal().
// badif(!dragon.age()){returnfalse;}// goodif(!dragon.hasAge()){returnfalse;}
It's okay to create get() and set() functions, but be consistent.
functionJedi(options){options||(options={});varlightsaber=options.lightsaber||'blue';this.set('lightsaber',lightsaber);}Jedi.prototype.set=function(key,val){this[key]=val;};Jedi.prototype.get=function(key){returnthis[key];};
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!
functionJedi(){console.log('new jedi');}// badJedi.prototype={fight: functionfight(){console.log('fighting');},block: functionblock(){console.log('blocking');}};// goodJedi.prototype.fight=functionfight(){console.log('fighting');};Jedi.prototype.block=functionblock(){console.log('blocking');};
Methods can return
thisto help with method chaining.// badJedi.prototype.jump=function(){this.jumping=true;returntrue;};Jedi.prototype.setHeight=function(height){this.height=height;};varluke=newJedi();luke.jump();// => trueluke.setHeight(20);// => undefined// goodJedi.prototype.jump=function(){this.jumping=true;returnthis;};Jedi.prototype.setHeight=function(height){this.height=height;returnthis;};varluke=newJedi();luke.jump().setHeight(20);
It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
functionJedi(options){options||(options={});this.name=options.name||'no name';}Jedi.prototype.getName=functiongetName(){returnthis.name;};Jedi.prototype.toString=functiontoString(){return'Jedi - '+this.getName();};
- Use AMD. Justification:
- http://requirejs.org/docs/whyamd.html
- https://gist.github.com/4686136
Prefix jQuery object variables with a
$.// badvarsidebar=$('.sidebar');// goodvar$sidebar=$('.sidebar');
Cache jQuery lookups.
// badfunctionsetSidebar(){$('.sidebar').hide();// ...stuff...$('.sidebar').css({'background-color': 'pink'});}// goodfunctionsetSidebar(){var$sidebar=$('.sidebar');$sidebar.hide();// ...stuff...$sidebar.css({'background-color': 'pink'});}
For DOM queries use Cascading
$('.sidebar ul')or parent > child$('.sidebar > ul'). jsPerfUse
findwith scoped jQuery object queries.// bad$('ul','.sidebar').hide();// bad$('.sidebar').find('ul').hide();// good$('.sidebar ul').hide();// good$('.sidebar > ul').hide();// good (slower)$sidebar.find('ul');// good (faster)$($sidebar[0]).find('ul');
- Refer to Kangax's ES5 compatibility table.
Yup.
function(){returntrue;}
- On Layout & Web Performance
- String vs Array Concat
- Try/Catch Cost In a Loop
- Bang Function
- jQuery Find vs Context, Selector
- innerHTML vs textContent for script text
- Long String Concatenation
- Loading...
- Usually for a project with more than one developer involved, it is essentially important for the project to define and maintain a consistent coding style. Most code editors and IDEs, such as Vim, Emacs, Code::Blocks, provide settings related to coding styles, such as the width of tab, the size of indentation, end of line, etc. However, it is hard to provide the same settings for different Editors and IDEs: we have to maintain many config files for different editors and IDEs, such as .vimrc for Vim, .emacs for Emacs. In order to solve this, EditorConfig was born. By defining coding style in files named .editorconfig, the EditorConfig plugins for different editors and IDEs will automatically adjust your coding style.
- To use EditorConfig, you have to download the corresponding EditorConfig plugins for your Editor or IDE. Follow the installation instructions to install them.
- The "base" .editorconfig file goes in the root directory. You can have other config files in specific folders if wanted.
The only difference would be at the top of the file to remove
root = truefrom any file that isn't the root.
Read This
Other Style Guides
- Google JavaScript Style Guide
- jQuery Core Style Guidelines
- Principles of Writing Consistent, Idiomatic JavaScript
- JavaScript Standard Style
Other Styles
- Naming this in nested functions - Christian Johansen
- Conditional Callbacks - Ross Allen
- Popular JavaScript Coding Conventions on Github - JeongHoon Byun
- Multiple var statements in JavaScript, not superfluous - Ben Alman
Further Reading
- Understanding JavaScript Closures - Angus Croll
- Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
- You Might Not Need jQuery - Zack Bloom & Adam Schwartz
- ES6 Features - Luke Hoban
- Frontend Guidelines - Benjamin De Cock
Books
- JavaScript: The Good Parts - Douglas Crockford
- JavaScript Patterns - Stoyan Stefanov
- Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
- High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
- Maintainable JavaScript - Nicholas C. Zakas
- JavaScript Web Applications - Alex MacCaw
- Pro JavaScript Techniques - John Resig
- Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
- Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
- Human JavaScript - Henrik Joreteg
- Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
- JSBooks - Julien Bouquillon
- Third Party JavaScript - Ben Vinegar and Anton Kovalyov
- Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
- Eloquent JavaScript - Marijn Haverbeke
- You Don't Know JS - Kyle Simpson
Blogs
- DailyJS
- JavaScript Weekly
- JavaScript, JavaScript...
- Bocoup Weblog
- Adequately Good
- NCZOnline
- Perfection Kills
- Ben Alman
- Dmitry Baranovskiy
- Dustin Diaz
- nettuts
Podcasts
(The MIT License)
Copyright (c) 2012 Airbnb
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