README
Smartprix JavaScript Style Guide() {
A mostly reasonable approach to JavaScript
![Gitter](https://badges.gitter.im/Join Chat.svg)
Other Style Guides
Table of Contents
- Types
- References
- Objects
- Arrays
- Destructuring
- Strings
- Functions
- Arrow Functions
- Classes & Constructors
- Modules
- Iterators and Generators
- Properties
- Variables
- Hoisting
- Comparison Operators & Equality
- Blocks
- Comments
- Whitespace
- Commas
- Semicolons
- Type Casting & Coercion
- Naming Conventions
- Accessors
- Events
- jQuery
- ECMAScript 5 Compatibility
- ECMAScript 6 Styles
- Testing
- Performance
- Resources
- License
Types
1.1 Primitives: When you access a primitive type you work directly on its value.
string
number
boolean
null
undefined
const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 9
1.2 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
References
2.1 Use
const
for all of your references; avoid usingvar
. eslint:prefer-const
,no-const-assign
Why? This ensures that you can't reassign your references, 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 reassign references, use
let
instead ofvar
. eslint:no-var
jscs:disallowVar
Why?
let
is block-scoped rather than function-scoped likevar
.// 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
andconst
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
Objects
3.1 Use the literal syntax for object creation. eslint:
no-new-object
// 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. jscs:
disallowIdentifierNames
// 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. jscs:
disallowIdentifierNames
// 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: 'San Francisco', }; obj[getKey('enabled')] = true; // good const obj = { id: 5, name: 'San Francisco', [getKey('enabled')]: true, };
3.5 Use object method shorthand. eslint:
object-shorthand
jscs:requireEnhancedObjectLiterals
// 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. eslint:
object-shorthand
jscs:requireEnhancedObjectLiterals
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 Only quote properties that are invalid identifiers. eslint:
quote-props
jscs:disallowQuotedKeysInObjects
Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```javascript
// bad
const bad = {
'foo': 3,
'bar': 4,
'data-blah': 5,
};
// good
const good = {
foo: 3,
bar: 4,
'data-blah': 5,
};
```
- 3.9 Do not call
Object.prototype
methods directly, such ashasOwnProperty
,propertyIsEnumerable
, andisPrototypeOf
.
Why? These methods may be shadowed by properties on the object in question - consider
{ hasOwnProperty: false }
- or, the object may be a null object (Object.create(null)
).
// bad
console.log(object.hasOwnProperty(key));
// good
console.log(Object.prototype.hasOwnProperty.call(object, key));
// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
/* or */
const has = require('has');
…
console.log(has.call(object, key));
Arrays
4.1 Use the literal syntax for array creation. eslint:
no-array-constructor
// 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);
4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint:
array-callback-return
// good [1, 2, 3].map((x) => { const y = x + 1; return x * y; }); // good [1, 2, 3].map(x => x + 1); // bad const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = flatten; }); // good const flat = {}; [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => { const flatten = memo.concat(item); flat[index] = flatten; return flatten; }); // bad inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } else { return false; } }); // good inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } return false; });
Destructuring
5.1 Use object destructuring when accessing and using multiple properties of an object. jscs:
requireObjectDestructuring
Why? Destructuring saves you from creating temporary references for those properties.
// bad function getFullName(user) { const firstName = user.firstName; const lastName = user.lastName; return `${firstName} ${lastName}`; } // good function getFullName(user) { const { firstName, lastName } = user; return `${firstName} ${lastName}`; } // best function getFullName({ firstName, lastName }) { return `${firstName} ${lastName}`; }
5.2 Use array destructuring. jscs:
requireArrayDestructuring
const arr = [1, 2, 3, 4]; // bad const first = arr[0]; const second = arr[1]; // good const [first, second] = arr;
5.3 Use object destructuring for multiple return values, not array destructuring. jscs:
disallowArrayDestructuringReturn
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, top } = processInput(input);
Strings
6.1 Use single quotes
''
for strings. eslint:quotes
jscs:validateQuoteMarks
// bad const name = "Capt. Janeway"; // good const name = 'Capt. Janeway';
- 6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.
6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.
// 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 When programmatically building up strings, use template strings instead of concatenation. eslint:
prefer-template
template-curly-spacing
jscs:requireTemplateStrings
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(); } // bad function sayHi(name) { return `How are you, ${ name }?`; } // good function sayHi(name) { return `How are you, ${name}?`; }
- 6.5 Never use
eval()
on a string, it opens too many vulnerabilities.
6.6 Do not unnecessarily escape characters in strings. eslint:
no-useless-escape
Why? Backslashes harm readability, thus they should only be present when necessary.
// bad const foo = '\'this\' \i\s \"quoted\"'; // good const foo = '\'this\' is "quoted"'; const foo = `'this' is "quoted"`;
Functions
7.1 Use function declarations instead of function expressions. jscs:
requireFunctionDeclarations
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.
// bad const foo = function () { }; // good function foo() { }
7.2 Wrap immediately invoked function expressions in parentheses. eslint:
wrap-iife
jscs:requireParenthesesAroundIIFE
Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
// immediately-invoked function expression (IIFE) (function () { 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. eslint:
no-loop-func
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 thearguments
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. eslint:prefer-rest-params
Why?
...
is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like likearguments
.// 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(); // 3
7.9 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');
7.11 Spacing in a function signature.
Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.
// bad const f = function(){}; const g = function (){}; const h = function() {}; // good const x = function () {}; const y = function a() {};
7.12 Never mutate parameters. eslint:
no-param-reassign
Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
// bad function f1(obj) { obj.key = 1; }; // good function f2(obj) { const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1; };
7.13 Never reassign parameters. eslint:
no-param-reassign
Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the
arguments
object. It can also cause optimization issues, especially in V8.// bad function f1(a) { a = 1; } function f2(a) { if (!a) { a = 1; } } // good function f3(a) { const b = a || 1; } function f4(a = 1) { }
Arrow Functions
8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint:
prefer-arrow-callback
,arrow-spacing
jscs:requireArrowFunctions
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) { 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, omit the braces and use the implicit return. Otherwise, keep the braces and use a
return
statement. eslint:arrow-parens
,arrow-body-style
jscs:disallowParenthesesAroundArrowParam
,requireShorthandArrowFunctions
Why? Syntactic sugar. It reads well when multiple functions are chained together.
// bad [1, 2, 3].map(number => { const nextNumber = number + 1; `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map(number => `A string containing the ${number}.`); // good [1, 2, 3].map((number) => { const nextNumber = number + 1; return `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map((number, index) => ({ index: number }));
8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
Why? It shows clearly where the function starts and ends.
// bad [1, 2, 3].map(number => 'As time went by, the string containing the ' + `${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 takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint:
arrow-parens
jscs:disallowParenthesesAroundArrowParam
Why? Less visual clutter.
// bad [1, 2, 3].map((x) => x * x); // good [1, 2, 3].map(x => x * x); // good [1, 2, 3].map(number => ( `A long string with the ${number}. It’s so long that we’ve broken it ` + 'over multiple lines!' )); // bad [1, 2, 3].map(x => { const y = x + 1; return x * y; }); // good [1, 2, 3].map((x) => { const y = x + 1; return x * y; });
8.5 Avoid confusing arrow function syntax (
=>
) with comparison operators (<=
,>=
). eslint:no-confusing-arrow
// bad const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize; // bad const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize; // good const itemHeight = (item) => { return item.height > 256 ? item.largeSize : item.smallSize; };
Classes & Constructors
9.1 Always use
class
. Avoid manipulatingprototype
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(options = {}) { this.name = options.name || 'no name'; } getName() { return this.name; } toString() { return `Jedi - ${this.getName()}`; } }
9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint:
no-useless-constructor
// bad class Jedi { constructor() {} getName() { return this.name; } } // bad class Rey extends Jedi { constructor(...args) { super(...args); } } // good class Rey extends Jedi { constructor(...args) { super(...args); this.name = 'Rey'; } }
9.6 Avoid duplicate class members. eslint:
no-dupe-class-members
Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.
// bad class Foo { bar() { return 1; } bar() { return 2; } } // good class Foo { bar() { return 1; } } // good class Foo { bar() { return 2; } }
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.
// 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;
10.4 Only import from a path in one place. eslint:
no-duplicate-imports
Why? Having multiple lines that import from the same path can make code harder to maintain.
// bad import foo from 'foo'; // … some other imports … // import { named1, named2 } from 'foo'; // good import foo, { named1, named2 } from 'foo'; // good import foo, { named1, named2, } from 'foo';
10.5 Do not export mutable bindings. eslint:
import/no-mutable-exports
Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
// bad let foo = 3; export { foo } // good const foo = 3; export { foo }
10.6 In modules with a single export, prefer default export over named export. eslint:
import/prefer-default-export
// bad export function foo() {} // good export default function foo() {}
10.7 Put all
import
s above non-import statements. eslint:import/imports-first
Why? Since
import
s are hoisted, keeping them all at the top prevents surprising behavior.// bad import foo from 'foo'; foo.init(); import bar from 'bar'; // good import foo from 'foo'; import bar from 'bar'; foo.init();
Iterators and Generators
11.1 Don't use iterators. Prefer JavaScript's higher-order functions like
map()
andreduce()
instead of loops likefor-of
. eslint:no-iterator
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) const sum = numbers.reduce((total, num) => total + num, 0); sum === 15;
11.2 Don't use generators for now.
Why? They don't transpile well to ES5.
11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint:
generator-star-spacing
Why?
function
and*
are part of the same conceptual keyword -*
is not a modifier forfunction
,function*
is a unique construct, different fromfunction
.// bad function * foo() { } const bar = function * () { } const baz = function *() { } const quux = function*() { } function*foo() { } function *foo() { } // very bad function * foo() { } const wat = function * () { } // good function* foo() { } const foo = function* () { }
Properties
12.1 Use dot notation when accessing properties. eslint:
dot-notation
jscs:requireDotNotation
const luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi;
12.2 Use bracket notation
[]
when accessing properties with a variable.const luke = { jedi: true, age: 28, }; function getProp(prop) { return luke[prop]; } const isJedi = getProp('jedi');
Variables
13.1 Always use
const
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
const
declaration per variable. eslint:one-var
jscs:disallowMultipleVarDecl
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. You can also step through each declaration with the debugger, instead of jumping through all of them at once.// 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
const
s and then group all yourlet
s.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
andconst
are block scoped and not function scoped.// bad - unnecessary function call function checkName(hasName) { const name = getName(); if (hasName === 'test') { return false; } if (name === 'test') { this.setName(''); return false; } return name; } // good function checkName(hasName) { if (hasName === 'test') { return false; } const name = getName(); if (name === 'test') { this.setName(''); return false; } return name; }
Hoisting
14.1
var
declarations get hoisted to the top of their scope, their assignment does not.const
andlet
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'); } }
For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.
Comparison Operators & Equality
15.2 Conditional statements such as the
if
statement evaluate their expression using coercion with theToBoolean
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 ([0] && []) { // true // an array (even an empty one) is an object, objects will 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
anddefault
clauses that contain lexical declarations (e.g.let
,const
,function
, andclass
).
Why? Lexical declarations are visible in the entire
switch
block but only get initialized when assigned, which only happens when itscase
is reached. This causes problems when multiplecase
clauses attempt to define the same thing.
eslint rules: no-case-declarations
.
```javascript
// 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 Ternaries should not be nested and generally be single line expressions.
eslint rules:
no-nested-ternary
.// 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;
15.7 Avoid unneeded ternary statements.
eslint rules:
no-unneeded-ternary
.// 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;
Blocks
16.1 Use braces with all multi-line blocks.
// bad if (test) return false; // good if (test) return false; // good if (test) { return false; } // bad function foo() { return false; } // good function bar() { return false; }
16.2 If you're using multi-line blocks with
if
andelse
, putelse
on the next line after yourif
block's closing brace. eslint:brace-style
jscs:disallowNewlineBeforeBlockStatements
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
Comments
17.1 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} 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 unless it's on the first line of a block.// 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; } // also good function getType() { // set the default type to 'no type' const type = this._type || 'no type'; return type; }
- 17.3 Prefixing your comments with
FIXME
orTODO
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 areFIXME: -- need to figure this out
orTODO: -- need to implement
.
17.4 Use
// FIXME:
to annotate problems.class Calculator extends Abacus { constructor() { super(); // FIXME: shouldn't use a global here total = 0; } }
17.5 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; } }
Whitespace
18.1 Use hard tabs set to 4 spaces. eslint:
indent
jscs:validateIndentation
// bad function foo() { ∙∙const name; } // bad function bar() { ∙∙∙∙const name; } // good function baz() { --->const name; }
18.2 Place 1 space before the leading brace. eslint:
space-before-blocks
jscs:requireSpaceBeforeBlockStatements
// 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.). Place no space between the argument list and the function name in function calls and declarations. eslint:keyword-spacing
jscs:requireSpaceAfterKeywords
// bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); }
18.4 Set off operators with spaces. eslint:
space-infix-ops
jscs:requireSpaceBeforeBinaryOperators
,requireSpaceAfterBinaryOperators
// 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 (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint:
newline-per-chained-call
no-whitespace-before-property
// 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 leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true) .attr('width', (radius + margin) * 2).append('svg:g') .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')') .call(tron.led); // good const leds = stage.selectAll('.led') .data(data) .enter().append('svg:svg') .classed('led', true) .attr('width', (radius + margin) * 2) .append('svg:g') .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')') .call(tron.led); // good const leds = stage.selectAll('.led').data(data);
18.7 Leave a blank line after blocks and before the next statement. jscs:
requirePaddingNewLinesAfterBlocks
// 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 Do not pad your blocks with blank lines. eslint:
padded-blocks
jscs:disallowPaddingNewlinesInBlocks
// 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); }
18.9 Do not add spaces inside parentheses. eslint:
space-in-parens
jscs:disallowSpacesInsideParentheses
// bad function bar( foo ) { return foo; } // good function bar(foo) { return foo; } // bad if ( foo ) { console.log(foo); } // good if (foo) { console.log(foo); }
18.10 Do not add spaces inside brackets. eslint:
array-bracket-spacing
jscs:disallowSpacesInsideArrayBrackets
// bad const foo = [ 1, 2, 3 ]; console.log(foo[ 0 ]); // good const foo = [1, 2, 3]; console.log(foo[0]);
18.11 Add spaces inside curly braces. eslint:
object-curly-spacing
jscs:requireSpacesInsideObjectBrackets
// bad const foo = {clark: 'kent'}; // good const foo = { clark: 'kent' };
18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint:
max-len
jscs:maximumLineLength
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.'));
Commas
19.1 Leading commas: Nope. eslint:
comma-style
jscs:requireCommaBeforeLineBreak
// 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. eslint:
comma-dangle
jscs:requireTrailingComma
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', ];
Semicolons
20.1 Yup. eslint:
semi
jscs:requireSemicolons
// bad (function () { const name = 'Skywalker' return name })() // good (function () { const name = 'Skywalker'; return name; }()); // good, but legacy (guards against the function becoming an argument when two files with IIFEs are concatenated) ;(() => { const name = 'Skywalker'; return name; }());
Type Casting & Coercion
- 21.1 Perform type coercion at the beginning of the statement.
21.2 Strings:
// => this.reviewScore = 9; // bad const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf() // bad const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string // good const totalScore = String(this.reviewScore);