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packages/ and refer to their documentation for detailed usage:
eslint-config-ascribe
eslint-config-ascribe-react
string
●number
●boolean
●null
●undefined
const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 91.2 Complex: When you access a complex type you work on a reference to its value. ●
object
●array
●function
const foo = [1, 2]; const bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9
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.
// bad var a = 1; var b = 2; // good const a = 1; const b = 2;2.2 If you must mutate references, use
let instead of var.
Why? let is block-scoped rather than function-scoped like var.
// bad var count = 1; if (true) { count += 1; } // good, use the let. let count = 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. { let a = 1; const b = 1; } console.log(a); // ReferenceError console.log(b); // ReferenceError
// bad const item = new Object(); // good const item = {};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.
// bad const superman = { default: { clark: 'kent' }, private: true, }; // good const superman = { defaults: { clark: 'kent' }, hidden: true, };3.3 Use readable synonyms in place of reserved words.
// bad const superman = { class: 'alien', }; // bad const superman = { klass: 'alien', }; // good const superman = { 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.
function getKey(k) { return `a key named ${k}`; } // bad const obj = { id: 5, name: 'Berlin', }; obj[getKey('enabled')] = true; // good const obj = { id: 5, name: 'Berlin', [getKey('enabled')]: true, };3.5 Use object method shorthand.
// bad const atom = { value: 1, addValue: function (value) { return atom.value + value; }, }; // good const atom = { value: 1, addValue(value) { return atom.value + value; }, };3.6 Use property value shorthand. Why? It is shorter to write and descriptive.
const lukeSkywalker = 'Luke Skywalker'; // bad const obj = { lukeSkywalker: lukeSkywalker, }; // good const obj = { 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.
const anakinSkywalker = 'Anakin Skywalker'; const lukeSkywalker = 'Luke Skywalker'; // bad const obj = { episodeOne: 1, twoJediWalkIntoACantina: 2, lukeSkywalker, episodeThree: 3, mayTheFourth: 4, anakinSkywalker, }; // good const obj = { 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.
// bad const bad = { foo: 3, bar: 4, 'data-blah': 5 }; // good const good = { 'foo': 3, 'bar': 4, 'data-blah': 5 }; // better const better = { foo: 3, bar: 4, dataBlah: 5 };
// bad const items = new Array(); // good const items = [];4.2 Use Array#push instead of direct assignment to add items to an array.
const someStack = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra');4.3 Use array spreads
... to copy arrays.
// bad const len = items.length; const itemsCopy = []; let i; for (i = 0; i < len; i++) { itemsCopy[i] = items[i]; } // good const itemsCopy = [...items];4.4 To convert an array-like object to an array, use Array#from.
const foo = document.querySelectorAll('.foo'); const nodes = Array.from(foo);
// bad function getFullName(user) { const firstName = user.firstName; const lastName = user.lastName; return `${firstName} ${lastName}`; } // good function getFullName(obj) { const { firstName, lastName } = obj; return `${firstName} ${lastName}`; } // best function getFullName({ firstName, lastName }) { return `${firstName} ${lastName}`; }5.2 When destructuring requires multiple lines, follow formatting rules for objects:
// bad const { first: { nested }, second } = obj; // bad const { first: { nested }, second } = obj; // good const { first: { nested }, second } = obj;5.3 Use array destructuring.
const arr = [1, 2, 3, 4]; // bad const first = arr[0]; const second = arr[1]; // good const [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.
// bad function processInput(input) { // then a miracle occurs return [left, right, top, bottom]; } // the caller needs to think about the order of return data const [left, __, top] = processInput(input); // good function processInput(input) { // then a miracle occurs return { left, right, top, bottom }; } // the caller selects only the data they need const { 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.
// bad const val = obj.value; delete obj.value; // good const { value: val, ...otherObj } = obj; // otherObj will hold all other properties of obj except for value
'' for strings.
// bad const name = "Capt. Janeway"; // good const name = 'Capt. Janeway';6.2 When using (single- or double) quotes in a string, use the other literal (
''or"").
// bad const name = "What a \"nice\" day!"; // bad const name = 'Let\'s go to Rosi\'s!'; // good const name = 'What a "nice" day!'; // good const name = "Let's go to Rosi's!";6.3 Strings longer than 100 characters should be written across multiple lines using string concatenation.
// bad const 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 const 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 const 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.';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.
// bad function sayHi(name) { return 'How are you, ' + name + '?'; } // bad function sayHi(name) { return ['How are you, ', name, '?'].join(); } // good function sayHi(name) { return `How are you, ${name}?`; }6.6 NEVER use eval() on a string, it opens too many vulnerabilities.
// bad const foo = function () { }; // good function foo() { }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.'); })();7.3 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. 7.4 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.
// bad if (currentUser) { function test() { console.log('Nope.'); } } // good let test; if (currentUser) { test = () => { console.log('Yup.'); }; }7.5 NEVER name a parameter
arguments. This will take precedence over the arguments object that is given to every function scope.
// bad function nope(name, options, arguments) { // ...stuff... } // good function yup(name, options, args) { // ...stuff... }7.6 NEVER 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.
// bad function concatenateAll() { const args = Array.prototype.slice.call(arguments); return args.join(''); } // good function concatenateAll(...args) { return args.join(''); }7.7 Use default parameter syntax rather than mutating function arguments.
// really bad function handleThings(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 bad function handleThings(opts) { if (opts === void 0) { opts = {}; } // ... } // good function handleThings(opts = {}) { // ... }7.8 Avoid side effects with default parameters. Why? They are confusing to reason about.
var b = 1; // bad function count(a= b++) { console.log(a); } count(); // 1 count(); // 2 count(3); // 3 count(); // 37.9 Always put default parameters last.
// bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... }7.10 NEVER 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.
// bad var add = new Function('a', 'b', 'return a + b'); // still bad var subtract = Function('a', 'b', 'return a - b');
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) { const y = x + 1; return x * y; }); // good [1, 2, 3].map((x) => { const y = x + 1; return x * 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 => { const nextNumber = number + 1; `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map(number => { const nextNumber = 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);
class. Avoid manipulating prototype directly.
Why? class syntax is more concise and easier to reason about.
// bad function Queue(contents = []) { this._queue = [...contents]; } Queue.prototype.pop = function() { const value = this._queue[0]; this._queue.splice(0, 1); return value; } // good class Queue { constructor(contents = []) { this._queue = [...contents]; } pop() { const value = this._queue[0]; this._queue.splice(0, 1); return value; } }9.2 Use
extends for inheritance.
Why? It is a built-in way to inherit prototype functionality without breaking instanceof.
// bad const inherits = require('inherits'); function PeekableQueue(contents) { Queue.apply(this, contents); } inherits(PeekableQueue, Queue); PeekableQueue.prototype.peek = function() { return this._queue[0]; } // good class PeekableQueue extends Queue { peek() { return this._queue[0]; } }9.3 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; }; const luke = new Jedi(); luke.jump(); // => true luke.setHeight(20); // => undefined // good class Jedi { jump() { this.jumping = true; return this; } setHeight(height) { this.height = height; return this; } } const luke = new Jedi(); 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.
class Jedi { constructor({ name = 'no name' } = {}) { this.name = name; } getName() { return this.name; } toString() { return `Jedi - ${this.getName()}`; } }
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.
// bad const AirbnbStyleGuide = require('./AirbnbStyleGuide'); module.exports = AirbnbStyleGuide.es6; // ok import AirbnbStyleGuide from './AirbnbStyleGuide'; export default AirbnbStyleGuide.es6; // best import { es6 } from './AirbnbStyleGuide'; export default es6;10.2 Do not use wildcard imports. Why? This makes sure you have a single default export.
// bad import * as AirbnbStyleGuide from './AirbnbStyleGuide'; // good import AirbnbStyleGuide from './AirbnbStyleGuide';10.3 And 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.js export { es6 as default } from './airbnbStyleGuide'; // good // filename es6.js import { es6 } from './AirbnbStyleGuide'; export default es6;
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.
const numbers = [1, 2, 3, 4, 5]; // bad let sum = 0; for (let num of numbers) { sum += num; } sum === 15; // good let sum = 0; numbers.forEach((num) => sum += num); sum === 15; // best (use the functional force, Luke) const sum = 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.
const luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi;12.2 Use subscript notation
[] when accessing properties with a variable.
const luke = { jedi: true, age: 28, }; function getProp(prop) { return luke[prop]; } const isJedi = getProp('jedi');
constorlet 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 const superPower = new SuperPower();13.2 Use one
constorlet 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.
// bad const items = getItems(), goSportsTeam = true, dragonball = 'z'; // bad // (compare to above, and try to spot the mistake) const items = getItems(), goSportsTeam = true; dragonball = 'z'; // good const items = getItems(); const goSportsTeam = true; const dragonball = '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.
// bad let i, len, dragonball, items = getItems(), goSportsTeam = true; // bad let i; const items = getItems(); let dragonball; const goSportsTeam = true; let len; // good const goSportsTeam = true; const items = getItems(); let dragonball; let i; let length;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.
// good function() { test(); console.log('doing stuff..'); //..other stuff.. const name = getName(); if (name === 'test') { return false; } return name; } // bad - unnecessary function call function(hasName) { const name = getName(); if (!hasName) { return false; } this.setFirstName(name); return true; } // good function(hasName) { if (!hasName) { return false; } const name = getName(); this.setFirstName(name); return true; }Note that referencing a variable declared by
letorconst before they are set results in a reference error, including typeof (see Why typeof is no longer "safe")
if (condition) { console.log(typeof value); // ReferenceError! let value = "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:
// bad const { first, // ignored second, // ignored third } = winners; // good const { first: ignoredFirst, // ignored second: 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 unused function (first, second) { return first; } // good -- `first` is listed before the used `second` argument function (first, second) { return second; }
var 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) 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() { let declaredButNotAssigned; console.log(declaredButNotAssigned); // => undefined declaredButNotAssigned = true; } // using const and let function example() { console.log(declaredButNotAssigned); // => throws a ReferenceError console.log(typeof declaredButNotAssigned); // => throws a ReferenceError const declaredButNotAssigned = true; }14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
function example() { console.log(anonymous); // => undefined anonymous(); // => TypeError anonymous is not a function var anonymous = function() { console.log('anonymous function expression'); }; }14.3 Named function expressions hoist the variable name, not the function name or the function body.
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'); } }14.4 Function declarations hoist their name and the function body.
function example() { superPower(); // => Flying function superPower() { 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 works foo(); 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 & HoistingbyBen Cherry.
=== 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 ifstatement 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 falseif+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.
// bad if (name !== '') { // ...stuff... } // good if (name) { // ...stuff... } // bad if (collection.length > 0) { // ...stuff... } // good if (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.
// bad switch (foo) { case 1: let x = 1; break; case 2: const y = 2; break; case 3: function f() {} break; default: class C {} } // good switch (foo) { case 1: { let x = 1; break; } case 2: { const y = 2; break; } case 3: { function f() {} break; } case 4: bar(); break; default: { class C {} } }15.6 Indent one full level for case statements.
// bad switch (foo) { case 1: break; default: break; } // bad switch (foo) { case 1: break; default: break; } // good switch (foo) { case 1: break; default: break; }15.7 Ternaries should not be nested and generally be single line expressions.
// bad const foo = maybe1 > maybe2?"bar" : value1 > value2?"baz" : null; // better const maybeNull = value1 > value2?'baz' : null; const foo = maybe1 > maybe2?'bar' : maybeNull; // best const maybeNull = value1 > value2?'baz' : null; const foo = maybe1 > maybe2?'bar' : maybeNull;15.8 Avoid unneeded ternary statements.
// bad const foo = a?a : b; const bar = c?true : false; const baz = c?false : true; // good const foo = a || b; const bar =!!c; const baz =!c;15.9 Use any of the following styles for multi-line ternary statements:
// good const foo = thisisasuperlongexpression?value : otherValue; // good const foo = thisisasuperlongexpression?value : otherValue; // good const foo = thisisasuperlongexpression?value : otherValue;
// bad if (test) return false; // good if (test) return false; // good if (test) { return false; } // bad function() { return false; } // good function() { return false; }16.2 If you're using multi-line blocks with
ifand else, put else on the same line as your ifblock's closing brace.
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
/** ... */ 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} 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; }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.
// bad const active = true; // is current tab // good // is current tab const active = true; // bad function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this._type || 'no type'; return type; } // good function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this._type || 'no type'; return type; }17.3 Always put a single space between where your comment starts (ie.
/*, /**, or //) and the comment.
17.4 Prefixing your comments with FIXMEorTODO 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 outorTODO -- need to implement.
17.5 Use // FIXME: to annotate problems.
class Calculator extends Abacus { constructor() { super(); // FIXME: shouldn't use a global here total = 0; } }17.6 Use
// TODO: to annotate solutions to problems.
class Calculator extends Abacus { constructor() { super(); // TODO: total should be configurable by an options param this.total = 0; } }
// good function() { ∙∙∙∙const name; } // bad function() { ∙const name; } // bad function() { ∙∙const name; }18.2 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', });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.
// bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function() { console.log('Anonymous'); } // good -- easier to tell this is a function decarlation rather than function call function () { console.log('Anonymous'); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); }18.4 Set off operators with spaces.
// bad const x=y+5; // good const x = 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(); // bad const request = fetch('/users').then(...).catch(...).finally(...); // good const request = fetch('/users') .then(...) .catch(...) .finally(...);18.7 Leave a blank line after blocks and before the next statement.
// bad if (foo) { return bar; } return baz; // good if (foo) { return bar; } return baz; // bad const obj = { foo() { }, bar() { }, }; return obj; // good const obj = { foo() { }, bar() { }, }; return obj; // bad const arr = [ function foo() { }, function bar() { }, ]; return arr; // good const arr = [ function foo() { }, function bar() { }, ]; return arr;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.
// bad if (aReallyReallyLongExpr && anotherSuperLongExpr && wowSoManyExpr && longExprToCheckTheWorldIsOk) { ... } // good if (aReallyReallyLongExpr && anotherSuperLongExpr && wowSoManyExpr && longExprToCheckTheWorldIsOk) { ... } // good while (aReallyReallyLongExpr && anotherSuperLongExpr && wowSoManyExpr && longExprToCheckTheWorldIsOk) { ... }18.9 Do not pad your blocks with blank lines.
// bad function bar() { console.log(foo); } // also bad if (baz) { console.log(qux); } else { console.log(foo); } // good function bar() { console.log(foo); } // good if (baz) { console.log(qux); } else { console.log(foo); }18.10 Do not add spaces inside parentheses.
// bad function bar( foo ) { return foo; } // good function bar(foo) { return foo; } // bad if ( foo ) { console.log(foo); } // good if (foo) { console.log(foo); }18.11 Do not add spaces inside brackets.
// bad const foo = [ 1, 2, 3 ]; console.log(foo[ 0 ]); // good const foo = [1, 2, 3]; console.log(foo[0]);18.12 Add spaces inside curly braces.
// bad const foo = {clark: 'kent'}; // good const foo = { clark: 'kent' };18.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Why? This ensures readability and maintainability.
// bad const foo = '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.')); // good const foo = '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:
// bad const foo = funcCall(this, is, a, really, reallllyyyyyyy, long, function, call); // good const foo = funcCall( this, is, a, really, reallllyyyyyyy, long, function,c all );
// bad const story = [ once , upon , aTime ]; // good const story = [ once, upon, aTime, ]; // bad const hero = { firstName: 'Ada' , lastName: 'Lovelace' , birthYear: 1815 , superPower: 'computers' }; // good const hero = { 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 comma const hero = { firstName: 'Florence', - lastName: 'Nightingale' + lastName: 'Nightingale', + inventorOf: ['coxcomb graph', 'modern nursing'] }; // good - git diff with trailing comma const hero = { firstName: 'Florence', lastName: 'Nightingale', + inventorOf: ['coxcomb chart', 'modern nursing'], }; // bad const hero = { firstName: 'Dana', lastName: 'Scully' }; const heroes = [ 'Batman', 'Superman' ]; // good const hero = { firstName: 'Dana', lastName: 'Scully', }; const heroes = [ 'Batman', 'Superman', ];
// bad (function() { const name = 'Skywalker'; return name; })(); // good (() => { const name = 'Skywalker' return name })()
// => this.reviewScore = 9; // bad const totalScore = this.reviewScore + ''; // good const totalScore = String(this.reviewScore);21.3 Numbers: Use
Number for type casting and parseInt always with a radix.
const inputValue = '4'; // bad const val = new Number(inputValue); // bad const val = +inputValue; // bad const val = inputValue >> 0; // bad const val = parseInt(inputValue); // good const val = Number(inputValue); // good const val = 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. */ const val = inputValue >> 0;21.5 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 //=> 2147483647 2147483648 >> 0 //=> -2147483648 2147483649 >> 0 //=> -214748364721.6 Booleans:
const age = 0; // bad const hasAge = new Boolean(age); // good const hasAge = Boolean(age); // good const hasAge =!!age;
// bad function q() { // ...stuff... } // good function query() { // ..stuff.. }22.2 Use camelCase when naming objects, functions, and instances.
// bad const OBJEcttsssss = {}; const this_is_my_object = {}; function c() {} // good const thisIsMyObject = {}; function thisIsMyFunction() {}22.3 Use PascalCase when naming constructors or classes.
// bad function user(options) { this.name = options.name; } const bad = new user({ name: 'nope', }); // good class UserPascalCase { constructor(options) { this.name = options.name; } } const good = new UserPascalCase({ name: 'yup', });22.4 Use a leading underscore
_ when naming private properties.
// bad this.__firstName__ = 'Panda'; this.firstName_ = 'Panda'; // good this._firstName = 'Panda';22.5 Don't save references to
this. Use arrow functions or Function#bind.
// bad function foo() { const self = this; return function() { console.log(self); }; } // bad function foo() { const that = this; return function() { console.log(that); }; } // good function foo() { return () => { console.log(this); }; } // good function foo() { 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 contents class CheckBox { // ... } export default CheckBox; // in some other file // bad import CheckBox from './checkBox'; // bad import CheckBox from './CheckBox'; // good import CheckBox from './check_box';22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
function makeStyleGuide() { } export default makeStyleGuide;22.8 Use PascalCase when you export a singleton / function library / bare object.
const AirbnbStyleGuide = { es6: { } }; export default AirbnbStyleGuide;
// bad dragon.age(); // good dragon.getAge(); // bad dragon.age(25); // good dragon.setAge(25);23.3 If the property is a
boolean, use isVal()orhasVal().
// bad if (!dragon.age()) { return false; } // good if (!dragon.hasAge()) { return false; }23.4 It's okay to create get() and set() functions, but be consistent.
class Jedi { constructor(options = {}) { const lightsaber = options.lightsaber || 'blue'; this.set('lightsaber', lightsaber); } set(key, val) { this[key] = val; } get(key) { return this[key]; } }
// 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 });
$.
// bad const sidebar = $('.sidebar'); // good const $sidebar = $('.sidebar'); // good const $sidebarBtn = $('.sidebar-btn');25.2 Cache jQuery lookups.
// bad function setSidebar() { $('.sidebar').hide(); // ...stuff... $('.sidebar').css({ 'background-color': 'pink' }); } // good function setSidebar() { 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();
function() { return true; }