JavaScript Standard Style

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This is a summary of the standard JavaScript rules.

The best way to learn about standard is to just install it and give it a try on your code.

Rules

Use 2 spaces for indentation.

eslint: indent

function hello (name) {
  console.log('hi', name)
}

Use single quotes for strings except to avoid escaping.

eslint: quotes

console.log('hello there')    // ✓ ok
console.log("hello there")    // ✗ avoid
console.log(`hello there`)    // ✗ avoid

$("<div class='box'>")        // ✓ ok
console.log(`hello ${name}`)  // ✓ ok

No unused variables.

eslint: no-unused-vars

function myFunction () {
  var result = something()   // ✗ avoid
}

Add a space after keywords.

eslint: keyword-spacing

if (condition) { ... }   // ✓ ok
if(condition) { ... }    // ✗ avoid

Add a space before a function declaration's parentheses.

eslint: space-before-function-paren

function name (arg) { ... }   // ✓ ok
function name(arg) { ... }    // ✗ avoid

run(function () { ... })      // ✓ ok
run(function() { ... })       // ✗ avoid

Always use === instead of ==.

Exception: obj == null is allowed to check for null || undefined.

eslint: eqeqeq

if (name === 'John')   // ✓ ok
if (name == 'John')    // ✗ avoid

if (name !== 'John')   // ✓ ok
if (name != 'John')    // ✗ avoid

Infix operators must be spaced.

eslint: space-infix-ops

// ✓ ok
var x = 2
var message = 'hello, ' + name + '!'

// ✗ avoid
var x=2
var message = 'hello, '+name+'!'

Commas should have a space after them.

eslint: comma-spacing

// ✓ ok
var list = [1, 2, 3, 4]
function greet (name, options) { ... }

// ✗ avoid
var list = [1,2,3,4]
function greet (name,options) { ... }

Keep else statements on the same line as their curly braces.

eslint: brace-style

// ✓ ok
if (condition) {
  // ...
} else {
  // ...
}

// ✗ avoid
if (condition) {
  // ...
}
else {
  // ...
}

For multi-line if statements, use curly braces.

eslint: curly

// ✓ ok
if (options.quiet !== true) console.log('done')

// ✓ ok
if (options.quiet !== true) {
  console.log('done')
}

// ✗ avoid
if (options.quiet !== true)
  console.log('done')

Always handle the err function parameter.

eslint: handle-callback-err

// ✓ ok
run(function (err) {
  if (err) throw err
  window.alert('done')
})

// ✗ avoid
run(function (err) {
  window.alert('done')
})

Declare browser globals with a /* global */ comment.

Exceptions are: window, document, and navigator.

Prevents accidental use of poorly-named browser globals like open, length, event, and name.
js
```
/* global alert, prompt */

alert('hi')
prompt('ok?')
```
Explicitly referencing the function or property on window is okay too, though such code will not run in a Worker which uses self instead of window.

eslint: no-undef
js
```
window.alert('hi')   // ✓ ok
```

Multiple blank lines not allowed.

eslint: no-multiple-empty-lines

// ✓ ok
var value = 'hello world'
console.log(value)

// ✗ avoid
var value = 'hello world'
// blank line
// blank line
console.log(value)

For the ternary operator in a multi-line setting, place ? and : on their own lines.

eslint: operator-linebreak

// ✓ ok
var location = env.development ? 'localhost' : 'www.api.com'

// ✓ ok
var location = env.development
  ? 'localhost'
  : 'www.api.com'

// ✗ avoid
var location = env.development ?
  'localhost' :
  'www.api.com'

For var declarations, write each declaration in its own statement.

eslint: one-var

// ✓ ok
var silent = true
var verbose = true

// ✗ avoid
var silent = true, verbose = true

// ✗ avoid
var silent = true,
    verbose = true

Wrap conditional assignments with additional parentheses. This makes it clear that the expression is intentionally an assignment (=) rather than a typo for equality (===).

eslint: no-cond-assign
js
```
// ✓ ok
while ((m = text.match(expr))) {
  // ...
}

// ✗ avoid
while (m = text.match(expr)) {
  // ...
}
```

Add spaces inside single line blocks.

eslint: block-spacing

  function foo () {return true}    // ✗ avoid
  function foo () { return true }  // ✓ ok

Use camelcase when naming variables and functions.

eslint: camelcase

  function my_function () { }    // ✗ avoid
  function myFunction () { }     // ✓ ok

  var my_var = 'hello'           // ✗ avoid
  var myVar = 'hello'            // ✓ ok

Trailing commas not allowed.

eslint: comma-dangle

  var obj = {
    message: 'hello',   // ✗ avoid
  }

Commas must be placed at the end of the current line.

eslint: comma-style

  var obj = {
    foo: 'foo'
    ,bar: 'bar'   // ✗ avoid
  }

  var obj = {
    foo: 'foo',
    bar: 'bar'   // ✓ ok
  }

Dot should be on the same line as property.

eslint: dot-location

  console.
    log('hello')  // ✗ avoid

  console
    .log('hello') // ✓ ok

Files must end with a newline.

eslint: eol-last
No space between function identifiers and their invocations.

eslint: func-call-spacing
js
```
console.log ('hello') // ✗ avoid
console.log('hello')  // ✓ ok
```

Add space between colon and value in key value pairs.

eslint: key-spacing

var obj = { 'key' : 'value' }    // ✗ avoid
var obj = { 'key' :'value' }     // ✗ avoid
var obj = { 'key':'value' }      // ✗ avoid
var obj = { 'key': 'value' }     // ✓ ok

Constructor names must begin with a capital letter.

eslint: new-cap

function animal () {}
var dog = new animal()    // ✗ avoid

function Animal () {}
var dog = new Animal()    // ✓ ok

Constructor with no arguments must be invoked with parentheses.

eslint: new-parens

function Animal () {}
var dog = new Animal    // ✗ avoid
var dog = new Animal()  // ✓ ok

Objects must contain a getter when a setter is defined.

eslint: accessor-pairs

var person = {
  set name (value) {    // ✗ avoid
    this._name = value
  }
}

var person = {
  set name (value) {
    this._name = value
  },
  get name () {         // ✓ ok
    return this._name
  }
}

Constructors of derived classes must call super.

eslint: constructor-super

class Dog {
  constructor () {
    super()             // ✗ avoid
    this.legs = 4
  }
}

class Dog extends Animal {
  constructor () {      // ✗ avoid
    this.legs = 4
  }
}

class Dog extends Animal {
  constructor () {
    super()             // ✓ ok
    this.legs = 4
  }
}

Use array literals instead of array constructors.

eslint: no-array-constructor

var nums = new Array(1, 2, 3)   // ✗ avoid
var nums = [1, 2, 3]            // ✓ ok

Avoid using arguments.callee and arguments.caller.

eslint: no-caller

function foo (n) {
  if (n <= 0) return

  arguments.callee(n - 1)   // ✗ avoid
}

function foo (n) {
  if (n <= 0) return

  foo(n - 1)                // ✓ ok
}

Avoid modifying variables of class declarations.

eslint: no-class-assign
js
```
class Dog {}
Dog = 'Fido'    // ✗ avoid
```
Avoid modifying variables declared using const.

eslint: no-const-assign
js
```
const score = 100
score = 125       // ✗ avoid
```

Avoid using constant expressions in conditions (except loops).

eslint: no-constant-condition

if (false) {    // ✗ avoid
  // ...
}

if (x === 0) {  // ✓ ok
  // ...
}

while (true) {  // ✓ ok
  // ...
}

No control characters in regular expressions.

eslint: no-control-regex

var pattern = /\x1f/    // ✗ avoid
var pattern = /\x20/    // ✓ ok

No debugger statements.

eslint: no-debugger

function sum (a, b) {
  debugger      // ✗ avoid
  return a + b
}

No delete operator on variables.

eslint: no-delete-var
js
```
var name
delete name     // ✗ avoid
```

No duplicate arguments in function definitions.

eslint: no-dupe-args

function sum (a, b, a) {  // ✗ avoid
  // ...
}

function sum (a, b, c) {  // ✓ ok
  // ...
}

No duplicate name in class members.

eslint: no-dupe-class-members

class Dog {
  bark () {}
  bark () {}    // ✗ avoid
}

No duplicate keys in object literals.

eslint: no-dupe-keys

var user = {
  name: 'Jane Doe',
  name: 'John Doe'    // ✗ avoid
}

No duplicate case labels in switch statements.

eslint: no-duplicate-case

switch (id) {
  case 1:
    // ...
  case 1:     // ✗ avoid
}

Use a single import statement per module.

eslint: no-duplicate-imports

import { myFunc1 } from 'module'
import { myFunc2 } from 'module'          // ✗ avoid

import { myFunc1, myFunc2 } from 'module' // ✓ ok

No empty character classes in regular expressions.

eslint: no-empty-character-class

const myRegex = /^abc[]/      // ✗ avoid
const myRegex = /^abc[a-z]/   // ✓ ok

No empty destructuring patterns.

eslint: no-empty-pattern

const { a: {} } = foo         // ✗ avoid
const { a: { b } } = foo      // ✓ ok

No using eval().

eslint: no-eval

eval( "var result = user." + propName ) // ✗ avoid
var result = user[propName]             // ✓ ok

No reassigning exceptions in catch clauses.

eslint: no-ex-assign

try {
  // ...
} catch (e) {
  e = 'new value'             // ✗ avoid
}

try {
  // ...
} catch (e) {
  const newVal = 'new value'  // ✓ ok
}

No extending native objects.

eslint: no-extend-native
js
```
Object.prototype.age = 21     // ✗ avoid
```

Avoid unnecessary function binding.

eslint: no-extra-bind

const name = function () {
  getName()
}.bind(user)    // ✗ avoid

const name = function () {
  this.getName()
}.bind(user)    // ✓ ok

Avoid unnecessary boolean casts.

eslint: no-extra-boolean-cast

const result = true
if (!!result) {   // ✗ avoid
  // ...
}

const result = true
if (result) {     // ✓ ok
  // ...
}

No unnecessary parentheses around function expressions.

eslint: no-extra-parens

const myFunc = (function () { })   // ✗ avoid
const myFunc = function () { }     // ✓ ok

Use break to prevent fallthrough in switch cases.

eslint: no-fallthrough

switch (filter) {
  case 1:
    doSomething()    // ✗ avoid
  case 2:
    doSomethingElse()
}

switch (filter) {
  case 1:
    doSomething()
    break           // ✓ ok
  case 2:
    doSomethingElse()
}

switch (filter) {
  case 1:
    doSomething()
    // fallthrough  // ✓ ok
  case 2:
    doSomethingElse()
}

No floating decimals.

eslint: no-floating-decimal

const discount = .5      // ✗ avoid
const discount = 0.5     // ✓ ok

Avoid reassigning function declarations.

eslint: no-func-assign

function myFunc () { }
myFunc = myOtherFunc    // ✗ avoid

No reassigning read-only global variables.

eslint: no-global-assign
js
```
window = {}     // ✗ avoid
```

No implied eval().

eslint: no-implied-eval

setTimeout("alert('Hello world')")                   // ✗ avoid
setTimeout(function () { alert('Hello world') })     // ✓ ok

No function declarations in nested blocks.

eslint: no-inner-declarations

if (authenticated) {
  function setAuthUser () {}    // ✗ avoid
}

No invalid regular expression strings in RegExp constructors.

eslint: no-invalid-regexp
js
```
RegExp('[a-z')    // ✗ avoid
RegExp('[a-z]')   // ✓ ok
```
No irregular whitespace.

eslint: no-irregular-whitespace
js
```
function myFunc () /*<NBSP>*/{}   // ✗ avoid
```

No using __iterator__.

eslint: no-iterator

Foo.prototype.__iterator__ = function () {}   // ✗ avoid

No labels that share a name with an in scope variable.

eslint: no-label-var

var score = 100
function game () {
  score: while (true) {      // ✗ avoid
    score -= 10
    if (score > 0) continue score
    break
  }
}

No label statements.

eslint: no-labels

label:
  while (true) {
    break label     // ✗ avoid
  }

No unnecessary nested blocks.

eslint: no-lone-blocks

function myFunc () {
  {                   // ✗ avoid
    myOtherFunc()
  }
}

function myFunc () {
  myOtherFunc()       // ✓ ok
}

Avoid mixing spaces and tabs for indentation.

eslint: no-mixed-spaces-and-tabs

Do not use multiple spaces except for indentation.

eslint: no-multi-spaces

const id =    1234    // ✗ avoid
const id = 1234       // ✓ ok

No multiline strings.

eslint: no-multi-str

const message = 'Hello \
                 world'     // ✗ avoid

No new without assigning object to a variable.

eslint: no-new

new Character()                     // ✗ avoid
const character = new Character()   // ✓ ok

No using the Function constructor.

eslint: no-new-func

var sum = new Function('a', 'b', 'return a + b')    // ✗ avoid

No using the Object constructor.

eslint: no-new-object
js
```
let config = new Object()   // ✗ avoid
```

No using new require.

eslint: no-new-require

const myModule = new require('my-module')    // ✗ avoid

No using the Symbol constructor.

eslint: no-new-symbol
js
```
const foo = new Symbol('foo')   // ✗ avoid
```
No using primitive wrapper instances.

eslint: no-new-wrappers
js
```
const message = new String('hello')   // ✗ avoid
```
No calling global object properties as functions.

eslint: no-obj-calls
js
```
const math = Math()   // ✗ avoid
```

No octal literals.

eslint: no-octal

const octal = 042         // ✗ avoid
const decimal = 34        // ✓ ok
const octalString = '042' // ✓ ok

No octal escape sequences in string literals.

eslint: no-octal-escape
js
```
const copyright = 'Copyright \251'  // ✗ avoid
```

Avoid string concatenation when using __dirname and __filename.

eslint: no-path-concat

const pathToFile = __dirname + '/app.js'            // ✗ avoid
const pathToFile = path.join(__dirname, 'app.js')   // ✓ ok

Avoid using __proto__. Use getPrototypeOf instead.

eslint: no-proto

const foo = obj.__proto__               // ✗ avoid
const foo = Object.getPrototypeOf(obj)  // ✓ ok

No redeclaring variables.

eslint: no-redeclare

let name = 'John'
let name = 'Jane'     // ✗ avoid

let name = 'John'
name = 'Jane'         // ✓ ok

Avoid multiple spaces in regular expression literals.

eslint: no-regex-spaces

const regexp = /test   value/   // ✗ avoid

const regexp = /test {3}value/  // ✓ ok
const regexp = /test value/     // ✓ ok

Assignments in return statements must be surrounded by parentheses.

eslint: no-return-assign

function sum (a, b) {
  return result = a + b     // ✗ avoid
}

function sum (a, b) {
  return (result = a + b)   // ✓ ok
}

Avoid assigning a variable to itself

eslint: no-self-assign
js
```
name = name   // ✗ avoid
```
Avoid comparing a variable to itself.

eslint: no-self-compare
js
```
if (score === score) {}   // ✗ avoid
```
Avoid using the comma operator.

eslint: no-sequences
js
```
if (doSomething(), !!test) {}   // ✗ avoid
```
Restricted names should not be shadowed.

eslint: no-shadow-restricted-names
js
```
let undefined = 'value'     // ✗ avoid
```

Sparse arrays are not allowed.

eslint: no-sparse-arrays

let fruits = ['apple',, 'orange']       // ✗ avoid

Tabs should not be used

eslint: no-tabs

Regular strings must not contain template literal placeholders.

eslint: no-template-curly-in-string

const message = 'Hello ${name}'   // ✗ avoid
const message = `Hello ${name}`   // ✓ ok

super() must be called before using this.

eslint: no-this-before-super

class Dog extends Animal {
  constructor () {
    this.legs = 4     // ✗ avoid
    super()
  }
}

Only throw an Error object.

eslint: no-throw-literal

throw 'error'               // ✗ avoid
throw new Error('error')    // ✓ ok

Whitespace not allowed at end of line.

eslint: no-trailing-spaces

Initializing to undefined is not allowed.

eslint: no-undef-init

let name = undefined    // ✗ avoid

let name
name = 'value'          // ✓ ok

No unmodified conditions of loops.

eslint: no-unmodified-loop-condition

for (let i = 0; i < items.length; j++) {...}    // ✗ avoid
for (let i = 0; i < items.length; i++) {...}    // ✓ ok

No ternary operators when simpler alternatives exist.

eslint: no-unneeded-ternary

let score = val ? val : 0     // ✗ avoid
let score = val || 0          // ✓ ok

No unreachable code after return, throw, continue, and break statements.

eslint: no-unreachable

function doSomething () {
  return true
  console.log('never called')     // ✗ avoid
}

No flow control statements in finally blocks.

eslint: no-unsafe-finally

try {
  // ...
} catch (e) {
  // ...
} finally {
  return 42     // ✗ avoid
}

The left operand of relational operators must not be negated.

eslint: no-unsafe-negation

if (!key in obj) {}       // ✗ avoid
if (!(key in obj)) {}     // ✓ ok

Avoid unnecessary use of .call() and .apply().

eslint: no-useless-call
js
```
sum.call(null, 1, 2, 3)   // ✗ avoid
```

Avoid using unnecessary computed property keys on objects.

eslint: no-useless-computed-key

const user = { ['name']: 'John Doe' }   // ✗ avoid
const user = { name: 'John Doe' }       // ✓ ok

No unnecessary constructor.

eslint: no-useless-constructor

class Car {
  constructor () {      // ✗ avoid
  }
}

No unnecessary use of escape.

eslint: no-useless-escape
js
```
let message = 'Hell\o'  // ✗ avoid
```

Renaming import, export, and destructured assignments to the same name is not allowed.

eslint: no-useless-rename

import { config as config } from './config'     // ✗ avoid
import { config } from './config'               // ✓ ok

No whitespace before properties.

eslint: no-whitespace-before-property
js
```
user .name      // ✗ avoid
user.name       // ✓ ok
```
No using with statements.

eslint: no-with
js
```
with (val) {...}    // ✗ avoid
```

Maintain consistency of newlines between object properties.

eslint: object-property-newline

const user = {
  name: 'Jane Doe', age: 30,
  username: 'jdoe86'            // ✗ avoid
}

const user = { name: 'Jane Doe', age: 30, username: 'jdoe86' }    // ✓ ok

const user = {
  name: 'Jane Doe',
  age: 30,
  username: 'jdoe86'
}                                                                 // ✓ ok

No padding within blocks.

eslint: padded-blocks

if (user) {
                            // ✗ avoid
  const name = getName()

}

if (user) {
  const name = getName()    // ✓ ok
}

No whitespace between spread operators and their expressions.

eslint: rest-spread-spacing
js
```
fn(... args)    // ✗ avoid
fn(...args)     // ✓ ok
```

Semicolons must have a space after and no space before.

eslint: semi-spacing

for (let i = 0 ;i < items.length ;i++) {...}    // ✗ avoid
for (let i = 0; i < items.length; i++) {...}    // ✓ ok

Must have a space before blocks.

eslint: space-before-blocks

if (admin){...}     // ✗ avoid
if (admin) {...}    // ✓ ok

No spaces inside parentheses.

eslint: space-in-parens

getName( name )     // ✗ avoid
getName(name)       // ✓ ok

Unary operators must have a space after.

eslint: space-unary-ops

typeof!admin        // ✗ avoid
typeof !admin        // ✓ ok

Use spaces inside comments.

eslint: spaced-comment

//comment           // ✗ avoid
// comment          // ✓ ok

/*comment*/         // ✗ avoid
/* comment */       // ✓ ok

No spacing in template strings.

eslint: template-curly-spacing

const message = `Hello, ${ name }`    // ✗ avoid
const message = `Hello, ${name}`      // ✓ ok

Use isNaN() when checking for NaN.

eslint: use-isnan

if (price === NaN) { }      // ✗ avoid
if (isNaN(price)) { }       // ✓ ok

typeof must be compared to a valid string.

eslint: valid-typeof

typeof name === 'undefimed'     // ✗ avoid
typeof name === 'undefined'     // ✓ ok

Immediately Invoked Function Expressions (IIFEs) must be wrapped.

eslint: wrap-iife

const getName = function () { }()     // ✗ avoid

const getName = (function () { }())   // ✓ ok
const getName = (function () { })()   // ✓ ok

The * in yield*expressions must have a space before and after.

eslint: yield-star-spacing
js
```
yield* increment()    // ✗ avoid
yield * increment()   // ✓ ok
```

Avoid Yoda conditions.

eslint: yoda

if (42 === age) { }    // ✗ avoid
if (age === 42) { }    // ✓ ok

Semicolons

No semicolons. (see: 1, 2, 3)

eslint: semi

window.alert('hi')   // ✓ ok
window.alert('hi');  // ✗ avoid

Never start a line with (, [, `, or a handful of other unlikely possibilities.

This is the only gotcha with omitting semicolons, and standard protects you from this potential issue.

(The full list is: [, (, `, +, *, /, -, ,, ., but most of these will never appear at the start of a line in real code.)

eslint: no-unexpected-multiline
js
```
// ✓ ok
;(function () {
  window.alert('ok')
}())

// ✗ avoid
(function () {
  window.alert('ok')
}())
```
js
```
// ✓ ok
;[1, 2, 3].forEach(bar)

// ✗ avoid
[1, 2, 3].forEach(bar)
```
js
```
// ✓ ok
;`hello`.indexOf('o')

// ✗ avoid
`hello`.indexOf('o')
```
Note: If you're often writing code like this, you may be trying to be too clever.

Clever short-hands are discouraged, in favor of clear and readable expressions, whenever possible.

Instead of this:
js
```
;[1, 2, 3].forEach(bar)
```
This is strongly preferred:
js
```
var nums = [1, 2, 3]
nums.forEach(bar)
```

Helpful reading

And a helpful video:

Are Semicolons Necessary in JavaScript? - YouTube

All popular code minifiers in use today use AST-based minification, so they can handle semicolon-less JavaScript with no issues (since semicolons are not required in JavaScript).

Excerpt from "An Open Letter to JavaScript Leaders Regarding Semicolons":

[Relying on automatic semicolon insertion] is quite safe, and perfectly valid JS that every browser understands. Closure compiler, yuicompressor, packer, and jsmin all can properly minify it. There is no performance impact anywhere.

I am sorry that, instead of educating you, the leaders in this language community have given you lies and fear. That was shameful. I recommend learning how statements in JS are actually terminated (and in which cases they are not terminated), so that you can write code that you find beautiful.

In general, \n ends a statement unless:

The statement has an unclosed paren, array literal, or object literal or ends in some other way that is not a valid way to end a statement. (For instance, ending with . or ,.)

The line is -- or ++ (in which case it will decrement/increment the next token.)

It is a for(), while(), do, if(), or else, and there is no {

The next line starts with [, (, +, *, /, -, ,, ., or some other binary operator that can only be found between two tokens in a single expression.

The first is pretty obvious. Even JSLint is ok with \n chars in JSON and parenthesized constructs, and with var statements that span multiple lines ending in ,.

The second is super weird. I’ve never seen a case (outside of these sorts of conversations) where you’d want to do write i\n++\nj, but, point of fact, that’s parsed as i; ++j, not i++; j.

The third is well understood, if generally despised. if (x)\ny() is equivalent to if (x) { y() }. The construct doesn’t end until it reaches either a block, or a statement.

; is a valid JavaScript statement, so if(x); is equivalent to if(x){} or, “If x, do nothing.” This is more commonly applied to loops where the loop check also is the update function. Unusual, but not unheard of.

The fourth is generally the fud-inducing “oh noes, you need semicolons!” case. But, as it turns out, it’s quite easy to prefix those lines with semicolons if you don’t mean them to be continuations of the previous line. For example, instead of this:
js
foo();
[1,2,3].forEach(bar);
you could do this:
js
foo()
;[1,2,3].forEach(bar)
The advantage is that the prefixes are easier to notice, once you are accustomed to never seeing lines starting with ( or [ without semis.