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BigchainDB JavaScript Style Guide

For consistent JavaScript across BigchainDB-related repos.

Build StatusGreenkeeper badgejs ascribe

Introduction

At ascribe we write a lot of JavaScript and value quality code. Since all of us liked Airbnb's JavaScript Style Guide, we figured that we can just fork it and change it to our needs.

Usage

Use the provided ESlint packages under packages/ and refer to their documentation for detailed usage:

Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 Styles
  28. Testing
  29. Performance
  30. Resources
  31. In the Wild
  32. Contributors
  33. License

Types

  • 1.1Primitives: When you access a primitive type you work directly on its value.

    • string
    • number
    • boolean
    • null
    • undefined
    constfoo=1;letbar=foo;bar=9;console.log(foo,bar);// => 1, 9

  • 1.2Complex: When you access a complex type you work on a reference to its value.

    • object
    • array
    • function
    constfoo=[1,2];constbar=foo;bar[0]=9;console.log(foo[0],bar[0]);// => 9, 9

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References

  • 2.1 Use const for all of your references; avoid using var.

    Why? This ensures that you can't reassign your references (mutation), which can lead to bugs and difficult to comprehend code.

    // badvara=1;varb=2;// goodconsta=1;constb=2;

  • 2.2 If you must mutate references, use let instead of var.

    Why? let is block-scoped rather than function-scoped like var.

    // badvarcount=1;if(true){count+=1;}// good, use the let.letcount=1;if(true){count+=1;}

  • 2.3 Note that both let and const are block-scoped.

    // const and let only exist in the blocks they are defined in.{leta=1;constb=1;}console.log(a);// ReferenceErrorconsole.log(b);// ReferenceError

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Objects

  • 3.1 Use the literal syntax for object creation.

    // badconstitem=newObject();// goodconstitem={};

  • 3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code.

    // badconstsuperman={default: {clark: 'kent'},private: true,};// goodconstsuperman={defaults: {clark: 'kent'},hidden: true,};

  • 3.3 Use readable synonyms in place of reserved words.

    // badconstsuperman={class: 'alien',};// badconstsuperman={klass: 'alien',};// goodconstsuperman={type: 'alien',};

  • 3.4 Use computed property names when creating objects with dynamic property names.

    Why? They allow you to define all the properties of an object in one place.

    functiongetKey(k){return`a key named ${k}`;}// badconstobj={id: 5,name: 'Berlin',};obj[getKey('enabled')]=true;// goodconstobj={id: 5,name: 'Berlin',[getKey('enabled')]: true,};

  • 3.5 Use object method shorthand.

    // badconstatom={value: 1,addValue: function(value){returnatom.value+value;},};// goodconstatom={value: 1,addValue(value){returnatom.value+value;},};

  • 3.6 Use property value shorthand.

    Why? It is shorter to write and descriptive.

    constlukeSkywalker='Luke Skywalker';// badconstobj={lukeSkywalker: lukeSkywalker,};// goodconstobj={
lukeSkywalker,};

  • 3.7 Group your shorthand properties at the beginning of your object declaration.

    Why? It's easier to tell which properties are using the shorthand.

    constanakinSkywalker='Anakin Skywalker';constlukeSkywalker='Luke Skywalker';// badconstobj={episodeOne: 1,twoJediWalkIntoACantina: 2,
lukeSkywalker,episodeThree: 3,mayTheFourth: 4,
anakinSkywalker,};// goodconstobj={
lukeSkywalker,
anakinSkywalker,episodeOne: 1,twoJediWalkIntoACantina: 2,episodeThree: 3,mayTheFourth: 4,};

  • 3.8 Prefer quoting only properties that are invalid identifiers, but always ensure that all properties are consistently quoted.

    Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many javascript engines.

    // badconstbad={foo: 3,bar: 4,'data-blah': 5};// goodconstgood={'foo': 3,'bar': 4,'data-blah': 5};// betterconstbetter={foo: 3,bar: 4,dataBlah: 5};

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Arrays

  • 4.1 Use the literal syntax for array creation.

    // badconstitems=newArray();// goodconstitems=[];

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

    constsomeStack=[];// badsomeStack[someStack.length]='abracadabra';// goodsomeStack.push('abracadabra');

  • 4.3 Use array spreads ... to copy arrays.

    // badconstlen=items.length;constitemsCopy=[];leti;for(i=0;i<len;i++){itemsCopy[i]=items[i];}// goodconstitemsCopy=[...items];

  • 4.4 To convert an array-like object to an array, use Array#from.

    constfoo=document.querySelectorAll('.foo');constnodes=Array.from(foo);

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Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object.

    Why? Destructuring saves you from creating temporary references for those properties.

    // badfunctiongetFullName(user){constfirstName=user.firstName;constlastName=user.lastName;return`${firstName}${lastName}`;}// goodfunctiongetFullName(obj){const{ firstName, lastName }=obj;return`${firstName}${lastName}`;}// bestfunctiongetFullName({ firstName, lastName }){return`${firstName}${lastName}`;}

  • 5.2 When destructuring requires multiple lines, follow formatting rules for objects:

    // badconst{first: {
nested
},
second }=obj;// badconst{first: {
nested
},
second }=obj;// goodconst{first: {
nested
},
second
}=obj;

  • 5.3 Use array destructuring.

    constarr=[1,2,3,4];// badconstfirst=arr[0];constsecond=arr[1];// goodconst[first,second]=arr;

  • 5.4 Use object destructuring for multiple return values, not array destructuring.

    Why? You can add new properties over time or change the order of things without breaking call sites.

    // badfunctionprocessInput(input){// then a miracle occursreturn[left,right,top,bottom];}// the caller needs to think about the order of return dataconst[left,__,top]=processInput(input);// goodfunctionprocessInput(input){// then a miracle occursreturn{ left, right, top, bottom };}// the caller selects only the data they needconst{ left, right }=processInput(input);

  • 5.5 You can use destructuring and an object spread operator to filter out specific properties while keeping the other properties in a new object.

    // badconstval=obj.value;deleteobj.value;// goodconst{value: val, ...otherObj}=obj;// otherObj will hold all other properties of obj except for value

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Strings

  • 6.1 Use single quotes '' for strings.

    // badconstname="Capt. Janeway";// goodconstname='Capt. Janeway';

  • 6.2 When using (single- or double) quotes in a string, use the other literal ('' or "").

    // badconstname="What a \"nice\" day!";// badconstname='Let\'s go to Rosi\'s!';// goodconstname='What a "nice" day!';// goodconstname="Let's go to Rosi's!";

  • 6.3 Strings longer than 100 characters should be written across multiple lines using string concatenation.

    // badconsterrorMessage='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.';// badconsterrorMessage='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.';// goodconsterrorMessage='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.';

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

  • 6.5 When programmatically building up strings, use template strings instead of concatenation.

    Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

    // badfunctionsayHi(name){return'How are you, '+name+'?';}// badfunctionsayHi(name){return['How are you, ',name,'?'].join();}// goodfunctionsayHi(name){return`How are you, ${name}?`;}

  • 6.6NEVER use eval() on a string, it opens too many vulnerabilities.

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Functions

  • 7.1 Use function declarations instead of function expressions.

    Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.

    // badconstfoo=function(){};// goodfunctionfoo(){}

  • 7.2 Immediately-invoked function expressions should use arrow functions as opposed to traditional functions:

    // immediately-invoked function expression (IIFE)(()=>{console.log('Welcome to the Internet. Please follow me.');})();

  • 7.3NEVER 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.

  • 7.4Note: 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.

    // badif(currentUser){functiontest(){console.log('Nope.');}}// goodlettest;if(currentUser){test=()=>{console.log('Yup.');};}

  • 7.5NEVER name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

    // badfunctionnope(name,options,arguments){// ...stuff...}// goodfunctionyup(name,options,args){// ...stuff...}

  • 7.6NEVER use arguments, opt to use rest syntax ... instead.

    Why? ... is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like like arguments.

    // badfunctionconcatenateAll(){constargs=Array.prototype.slice.call(arguments);returnargs.join('');}// goodfunctionconcatenateAll(...args){returnargs.join('');}

  • 7.7 Use default parameter syntax rather than mutating function arguments.

    // really badfunctionhandleThings(opts){// No! We shouldn't mutate function arguments.// Double bad: if opts is falsy it'll be set to an object which may// be what you want but it can introduce subtle bugs.opts=opts||{};// ...}// still badfunctionhandleThings(opts){if(opts===void0){opts={};}// ...}// goodfunctionhandleThings(opts={}){// ...}

  • 7.8 Avoid side effects with default parameters.

    Why? They are confusing to reason about.

    varb=1;// badfunctioncount(a=b++){console.log(a);}count();// 1count();// 2count(3);// 3count();// 3

  • 7.9 Always put default parameters last.

    // badfunctionhandleThings(opts={},name){// ...}// goodfunctionhandleThings(name,opts={}){// ...}

  • 7.10NEVER use the Function constructor to create a new function.

    Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

    // badvaradd=newFunction('a','b','return a + b');// still badvarsubtract=Function('a','b','return a - b');

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Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation.

    Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

    Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.

    // bad[1,2,3].map(function(x){consty=x+1;returnx*y;});// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});

  • 8.2 If the function body consists of a single expression, feel free to omit the braces and use the implicit return. Otherwise use a return statement.

    Why? Syntactic sugar. It reads well when multiple functions are chained together.

    Why not? If you plan on returning an object.

    // good[1,2,3].map(number=>`A string containing the ${number}.`);// bad[1,2,3].map(number=>{constnextNumber=number+1;`A string containing the ${nextNumber}.`;});// good[1,2,3].map(number=>{constnextNumber=number+1;return`A string containing the ${nextNumber}.`;});

  • 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

    Why? It shows clearly where the function starts and ends.

    // bad[1,2,3].map(number=>'As time went by, the string containing the '+`${number} became much longer. So we needed to break it over multiple `+'lines.');// good[1,2,3].map(number=>(`As time went by, the string containing the ${number} became much `+'longer. So we needed to break it over multiple lines.'));

  • 8.4 If your function only takes a single argument, feel free to omit the parentheses.

    Why? Less visual clutter.

    // good[1,2,3].map(x=>x*x);// good[1,2,3].reduce((y,x)=>x+y);

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Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

    Why? class syntax is more concise and easier to reason about.

    // badfunctionQueue(contents=[]){this._queue=[...contents];}Queue.prototype.pop=function(){constvalue=this._queue[0];this._queue.splice(0,1);returnvalue;}// goodclassQueue{constructor(contents=[]){this._queue=[...contents];}pop(){constvalue=this._queue[0];this._queue.splice(0,1);returnvalue;}}

  • 9.2 Use extends for inheritance.

    Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

    // badconstinherits=require('inherits');functionPeekableQueue(contents){Queue.apply(this,contents);}inherits(PeekableQueue,Queue);PeekableQueue.prototype.peek=function(){returnthis._queue[0];}// goodclassPeekableQueueextendsQueue{peek(){returnthis._queue[0];}}

  • 9.3 Methods can return this to help with method chaining.

    // badJedi.prototype.jump=function(){this.jumping=true;returntrue;};Jedi.prototype.setHeight=function(height){this.height=height;};constluke=newJedi();luke.jump();// => trueluke.setHeight(20);// => undefined// goodclassJedi{jump(){this.jumping=true;returnthis;}setHeight(height){this.height=height;returnthis;}}constluke=newJedi();luke.jump().setHeight(20);

  • 9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

    classJedi{constructor({ name ='no name'}={}){this.name=name;}getName(){returnthis.name;}toString(){return`Jedi - ${this.getName()}`;}}

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Modules

  • 10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

    Why? Modules are the future, let's start using the future now.

    // badconstAirbnbStyleGuide=require('./AirbnbStyleGuide');module.exports=AirbnbStyleGuide.es6;// okimportAirbnbStyleGuidefrom'./AirbnbStyleGuide';exportdefaultAirbnbStyleGuide.es6;// bestimport{es6}from'./AirbnbStyleGuide';exportdefaultes6;

  • 10.2 Do not use wildcard imports.

    Why? This makes sure you have a single default export.

    // badimport*asAirbnbStyleGuidefrom'./AirbnbStyleGuide';// goodimportAirbnbStyleGuidefrom'./AirbnbStyleGuide';

  • 10.3And do not export directly from an import.

    Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

    // bad// filename es6.jsexport{es6asdefault}from'./airbnbStyleGuide';// good// filename es6.jsimport{es6}from'./AirbnbStyleGuide';exportdefaultes6;

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Iterators and Generators

  • 11.1 Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of unless there is a substantial performance disadvantage by doing so.

    Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side-effects.

    constnumbers=[1,2,3,4,5];// badletsum=0;for(letnumofnumbers){sum+=num;}sum===15;// goodletsum=0;numbers.forEach((num)=>sum+=num);sum===15;// best (use the functional force, Luke)constsum=numbers.reduce((total,num)=>total+num,0);sum===15;

  • 11.2 Only use for-in if you know exactly what you're doing. If unsure, prefer the options given in 11.1.

  • 11.3 Don't use generators for now.

    Why? They don't transpile well to ES5.

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Properties

  • 12.1 Use dot notation when accessing properties.

    constluke={jedi: true,age: 28,};// badconstisJedi=luke['jedi'];// goodconstisJedi=luke.jedi;

  • 12.2 Use subscript notation [] when accessing properties with a variable.

    constluke={jedi: true,age: 28,};functiongetProp(prop){returnluke[prop];}constisJedi=getProp('jedi');

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Variables

  • 13.1 Always use const or let 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.

    // badsuperPower=newSuperPower();// goodconstsuperPower=newSuperPower();

  • 13.2 Use one const or let declaration per variable.

    Why? 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.

    // badconstitems=getItems(),goSportsTeam=true,dragonball='z';// bad// (compare to above, and try to spot the mistake)constitems=getItems(),goSportsTeam=true;dragonball='z';// goodconstitems=getItems();constgoSportsTeam=true;constdragonball='z';

  • 13.3 Group all your consts and then group all your lets.

    Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

    // badleti,len,dragonball,items=getItems(),goSportsTeam=true;// badleti;constitems=getItems();letdragonball;constgoSportsTeam=true;letlen;// goodconstgoSportsTeam=true;constitems=getItems();letdragonball;leti;letlength;

  • 13.4 Assign variables where you need them, but place them in a reasonable place.

    Why? let and const are block scoped and not function scoped.

    // goodfunction(){test();console.log('doing stuff..');//..other stuff..constname=getName();if(name==='test'){returnfalse;}returnname;}// bad - unnecessary function callfunction(hasName){constname=getName();if(!hasName){returnfalse;}this.setFirstName(name);returntrue;}// goodfunction(hasName){if(!hasName){returnfalse;}constname=getName();this.setFirstName(name);returntrue;}

    Note that referencing a variable declared by let or const before they are set results in a reference error, including typeof (see Why typeof is no longer "safe")

    if(condition){console.log(typeofvalue);// ReferenceError!letvalue="blue";}

  • 13.5 Avoid declaring unused variables, however the cases where it can be convenient (such as filtering some properties out of an object or destructuring an array, for example), prefix the variable name with ignored:

    // badconst{
first,// ignored
second,// ignored
third
}=winners;// goodconst{first: ignoredFirst,// ignoredsecond: ignoredSecond,// ignored
third
}=winners;

    Note that our ESLint configuration is set up to error on any unused variable unless it is prefixed by ignored. An exception to this is argument names; any arguments listed before the first one used is OK:

    // bad -- `second` is unusedfunction(first,second){returnfirst;}// good -- `first` is listed before the used `second` argumentfunction(first,second){returnsecond;}

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Hoisting

  • 14.1var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

    // 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(){letdeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}// using const and letfunctionexample(){console.log(declaredButNotAssigned);// => throws a ReferenceErrorconsole.log(typeofdeclaredButNotAssigned);// => throws a ReferenceErrorconstdeclaredButNotAssigned=true;}

  • 14.2 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');};}

  • 14.3 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');}}

  • 14.4 Function declarations hoist their name and the function body.

    functionexample(){superPower();// => FlyingfunctionsuperPower(){console.log('Flying');}}

  • 14.5 ES6 imports are hoisted to the beginning of their module while modules imported through requires (ie. CommonJS modules) are not.

    // This worksfoo();import{foo}from'my_module';// This will import 'imported_module' before 'required_module'require('required_module');import'imported_module';

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

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Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=. Avoid == and != because they are 'loose' equality comparisons, only evaluating equality after coercing both values following confusing and difficult to remember rules (see MDN).

  • 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract 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([]){// true// An array is an object, objects evaluate to true}

  • 15.3 Use shortcuts.

    // badif(name!==''){// ...stuff...}// goodif(name){// ...stuff...}// badif(collection.length>0){// ...stuff...}// goodif(collection.length){// ...stuff...}

  • 15.4 For more information see Truth Equality and JavaScript by Angus Croll.

  • 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

    Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

    // badswitch(foo){case1:
letx=1;break;case2:
consty=2;break;case3:
functionf(){}break;default:
classC{}}// goodswitch(foo){case1: {letx=1;break;}case2: {consty=2;break;}case3: {functionf(){}break;}case4:
bar();break;default: {classC{}}}

  • 15.6 Indent one full level for case statements.

    // badswitch(foo){case1:
break;default:
break;}// badswitch(foo){case1:
break;default:
break;}// goodswitch(foo){case1:
break;default:
break;}

  • 15.7 Ternaries should not be nested and generally be single line expressions.

    // badconstfoo=maybe1>maybe2
? "bar"
: value1>value2 ? "baz" : null;// betterconstmaybeNull=value1>value2 ? 'baz'
: null;constfoo=maybe1>maybe2
? 'bar'
: maybeNull;// bestconstmaybeNull=value1>value2 ? 'baz' : null;constfoo=maybe1>maybe2 ? 'bar' : maybeNull;

  • 15.8 Avoid unneeded ternary statements.

    // badconstfoo=a ? a : b;constbar=c ? true : false;constbaz=c ? false : true;// goodconstfoo=a||b;constbar=!!c;constbaz=!c;

  • 15.9 Use any of the following styles for multi-line ternary statements:

    // goodconstfoo=thisisasuperlongexpression ? value
: otherValue;// goodconstfoo=thisisasuperlongexpression
? value : otherValue;// goodconstfoo=thisisasuperlongexpression
? value
: otherValue;

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Blocks

  • 16.1 Use braces with all multi-line blocks.

    // badif(test)returnfalse;// goodif(test)returnfalse;// goodif(test){returnfalse;}// badfunction(){returnfalse;}// goodfunction(){returnfalse;}

  • 16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace.

    // badif(test){thing1();thing2();}else{thing3();}// goodif(test){thing1();thing2();}else{thing3();}

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Comments

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

    // 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;}

  • 17.2 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.

    // badconstactive=true;// is current tab// good// is current tabconstactive=true;// badfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'consttype=this._type||'no type';returntype;}// goodfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'consttype=this._type||'no type';returntype;}

  • 17.3 Always put a single space between where your comment starts (ie. /*, /**, or //) and the comment.

  • 17.4 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.

  • 17.5 Use // FIXME: to annotate problems.

    classCalculatorextendsAbacus{constructor(){super();// FIXME: shouldn't use a global heretotal=0;}}

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

    classCalculatorextendsAbacus{constructor(){super();// TODO: total should be configurable by an options paramthis.total=0;}}

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Whitespace

  • 18.1 Use soft tabs set to 4 spaces.

    // goodfunction(){∙∙∙∙constname;}// badfunction(){∙constname;}// badfunction(){∙∙constname;}

  • 18.2 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',});

  • 18.3 Place 1 space before the opening parenthesis in control statements (if, while etc.) and anonymous function declarations. Place no space before the argument list in function calls and named declarations.

    // badif(isJedi){fight();}// goodif(isJedi){fight();}// badfunction(){console.log('Anonymous');}// good -- easier to tell this is a function decarlation rather than function callfunction(){console.log('Anonymous');}// badfunctionfight(){console.log('Swooosh!');}// goodfunctionfight(){console.log('Swooosh!');}

  • 18.4 Set off operators with spaces.

    // badconstx=y+5;// goodconstx=y+5;

  • 18.5 End files with a single newline character.

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

  • 18.6 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();// badconstrequest=fetch('/users').then(...).catch(...).finally(...);// goodconstrequest=fetch('/users').then(...).catch(...).finally(...);

  • 18.7 Leave a blank line after blocks and before the next statement.

    // badif(foo){returnbar;}returnbaz;// goodif(foo){returnbar;}returnbaz;// badconstobj={foo(){},bar(){},};returnobj;// goodconstobj={foo(){},bar(){},};returnobj;// badconstarr=[functionfoo(){},functionbar(){},];returnarr;// goodconstarr=[functionfoo(){},functionbar(){},];returnarr;

  • 18.8 Break long logical operations into multiple lines, leaving operators at the end of the line and intenting the later lines to the first line's first operand.

    // badif(aReallyReallyLongExpr&&anotherSuperLongExpr&&wowSoManyExpr&&longExprToCheckTheWorldIsOk){
...
}// goodif(aReallyReallyLongExpr&&anotherSuperLongExpr&&wowSoManyExpr&&longExprToCheckTheWorldIsOk){
...
}// goodwhile(aReallyReallyLongExpr&&anotherSuperLongExpr&&wowSoManyExpr&&longExprToCheckTheWorldIsOk){
...
}

  • 18.9 Do not pad your blocks with blank lines.

    // badfunctionbar(){console.log(foo);}// also badif(baz){console.log(qux);}else{console.log(foo);}// goodfunctionbar(){console.log(foo);}// goodif(baz){console.log(qux);}else{console.log(foo);}

  • 18.10 Do not add spaces inside parentheses.

    // badfunctionbar(foo){returnfoo;}// goodfunctionbar(foo){returnfoo;}// badif(foo){console.log(foo);}// goodif(foo){console.log(foo);}

  • 18.11 Do not add spaces inside brackets.

    // badconstfoo=[1,2,3];console.log(foo[0]);// goodconstfoo=[1,2,3];console.log(foo[0]);

  • 18.12 Add spaces inside curly braces.

    // badconstfoo={clark: 'kent'};// goodconstfoo={clark: 'kent'};

  • 18.13 Avoid having lines of code that are longer than 100 characters (including whitespace).

    Why? This ensures readability and maintainability.

    // badconstfoo='Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';// bad$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'}}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));// goodconstfoo='Whatever national crop flips the window. The cartoon reverts within the screw. '+'Whatever wizard constrains a helpful ally. The counterpart ascends!';// good$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'},}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));

    In some cases, you can go slightly over the limit (urls, code that's just slightly over), but our ESLint configuration is set up to warn on code lines that are over 105 characters.

  • 18.14 When a function call needs to be broken up into multiple lines, put arguments on a separate line, indented four spaces:

    // badconstfoo=funcCall(this,is,a,really,reallllyyyyyyy,long,function,call);// goodconstfoo=funcCall(this,is,a,really,reallllyyyyyyy,long,function,call);

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Commas

  • 19.1 Leading commas: Nope.

    // badconststory=[once,upon,aTime];// goodconststory=[once,upon,aTime,];// badconsthero={firstName: 'Ada',lastName: 'Lovelace',birthYear: 1815,superPower: 'computers'};// goodconsthero={firstName: 'Ada',lastName: 'Lovelace',birthYear: 1815,superPower: 'computers',};

  • 19.2 Additional trailing comma: Yup.

    Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the trailing comma problem in legacy browsers.

    // bad - git diff without trailing commaconsthero={firstName: 'Florence',-lastName: 'Nightingale'+lastName: 'Nightingale',+inventorOf: ['coxcomb graph','modern nursing']};// good - git diff with trailing commaconsthero={firstName: 'Florence',lastName: 'Nightingale',+inventorOf: ['coxcomb chart','modern nursing'],};// badconsthero={firstName: 'Dana',lastName: 'Scully'};constheroes=['Batman','Superman'];// goodconsthero={firstName: 'Dana',lastName: 'Scully',};constheroes=['Batman','Superman',];

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Semicolons

  • 20.1Nope.

    // bad(function(){constname='Skywalker';returnname;})();// good(()=>{constname='Skywalker'returnname})()

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Type Casting & Coercion

  • 21.1 Perform type coercion at the beginning of the statement.

  • 21.2 Strings:

    // => this.reviewScore = 9;// badconsttotalScore=this.reviewScore+'';// goodconsttotalScore=String(this.reviewScore);

  • 21.3 Numbers: Use Number for type casting and parseInt always with a radix.

    constinputValue='4';// badconstval=newNumber(inputValue);// badconstval=+inputValue;// badconstval=inputValue>>0;// badconstval=parseInt(inputValue);// goodconstval=Number(inputValue);// goodconstval=parseInt(inputValue,10);

  • 21.4 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.

    // good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */constval=inputValue>>0;

  • 21.5Note: 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

  • 21.6 Booleans:

    constage=0;// badconsthasAge=newBoolean(age);// goodconsthasAge=Boolean(age);// goodconsthasAge=!!age;

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Naming Conventions

  • 22.1 Avoid single letter names. Be descriptive with your naming.

    // badfunctionq(){// ...stuff...}// goodfunctionquery(){// ..stuff..}

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

    // badconstOBJEcttsssss={};constthis_is_my_object={};functionc(){}// goodconstthisIsMyObject={};functionthisIsMyFunction(){}

  • 22.3 Use PascalCase when naming constructors or classes.

    // badfunctionuser(options){this.name=options.name;}constbad=newuser({name: 'nope',});// goodclassUserPascalCase{constructor(options){this.name=options.name;}}constgood=newUserPascalCase({name: 'yup',});

  • 22.4 Use a leading underscore _ when naming private properties.

    // badthis.__firstName__='Panda';this.firstName_='Panda';// goodthis._firstName='Panda';

  • 22.5 Don't save references to this. Use arrow functions or Function#bind.

    // badfunctionfoo(){constself=this;returnfunction(){console.log(self);};}// badfunctionfoo(){constthat=this;returnfunction(){console.log(that);};}// goodfunctionfoo(){return()=>{console.log(this);};}// goodfunctionfoo(){return(function(){console.log(this);}).bind(this);}

  • 22.6 If your file exports a single class, your filename should be exactly the name of the class, converted from PascalCase to snake_case.

    // file contentsclassCheckBox{// ...}exportdefaultCheckBox;// in some other file// badimportCheckBoxfrom'./checkBox';// badimportCheckBoxfrom'./CheckBox';// goodimportCheckBoxfrom'./check_box';

  • 22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.

    functionmakeStyleGuide(){}exportdefaultmakeStyleGuide;

  • 22.8 Use PascalCase when you export a singleton / function library / bare object.

    constAirbnbStyleGuide={es6: {}};exportdefaultAirbnbStyleGuide;

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Accessors

  • 23.1 Accessor functions for properties are not required.

  • 23.2 If you do make accessor functions use getVal() and setVal('hello').

    // baddragon.age();// gooddragon.getAge();// baddragon.age(25);// gooddragon.setAge(25);

  • 23.3 If the property is a boolean, use isVal() or hasVal().

    // badif(!dragon.age()){returnfalse;}// goodif(!dragon.hasAge()){returnfalse;}

  • 23.4 It's okay to create get() and set() functions, but be consistent.

    classJedi{constructor(options={}){constlightsaber=options.lightsaber||'blue';this.set('lightsaber',lightsaber);}set(key,val){this[key]=val;}get(key){returnthis[key];}}

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Events

  • 24.1 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

  • 25.1 Prefix jQuery object variables with a $.

    // badconstsidebar=$('.sidebar');// goodconst$sidebar=$('.sidebar');// goodconst$sidebarBtn=$('.sidebar-btn');

  • 25.2 Cache jQuery lookups.

    // badfunctionsetSidebar(){$('.sidebar').hide();// ...stuff...$('.sidebar').css({'background-color': 'pink'});}// goodfunctionsetSidebar(){const$sidebar=$('.sidebar');$sidebar.hide();// ...stuff...$sidebar.css({'background-color': 'pink'});}

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

  • 25.4 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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ECMAScript 5 Compatibility

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ECMAScript 6 Styles

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Testing

  • 28.1Yup.

    function(){returntrue;}

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Performance

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Resources

Learning ES6

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

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

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Contributors

License

(The MIT License)

Copyright (c) 2014 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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Amendments

We encourage you to fork this guide and change the rules to fit your team's style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

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