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.

  /* 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

  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

  // ✓ 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

  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

  class Dog {}
  Dog = 'Fido'    // ✗ avoid
  

• Avoid modifying variables declared using const.

  eslint: no-const-assign

  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

  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

  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

  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

  RegExp('[a-z')    // ✗ avoid
  RegExp('[a-z]')   // ✓ ok
  

• No irregular whitespace.

  eslint: no-irregular-whitespace

  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

  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

  const foo = new Symbol('foo')   // ✗ avoid
  

• No using primitive wrapper instances.

  eslint: no-new-wrappers

  const message = new String('hello')   // ✗ avoid
  

• No calling global object properties as functions.

  eslint: no-obj-calls

  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

  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

  name = name   // ✗ avoid
  

• Avoid comparing a variable to itself.

  eslint: no-self-compare

  if (score === score) {}   // ✗ avoid
  

• Avoid using the comma operator.

  eslint: no-sequences

  if (doSomething(), !!test) {}   // ✗ avoid
  

• Restricted names should not be shadowed.

  eslint: no-shadow-restricted-names

  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

  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

  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

  user .name      // ✗ avoid
  user.name       // ✓ ok
  

• No using with statements.

  eslint: no-with

  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

  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

  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

  // ✓ ok
  ;(function () {
    window.alert('ok')
  }())
  
  // ✗ avoid
  (function () {
    window.alert('ok')
  }())
  

  // ✓ ok
  ;[1, 2, 3].forEach(bar)
  
  // ✗ avoid
  [1, 2, 3].forEach(bar)
  

  // ✓ 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:

  ;[1, 2, 3].forEach(bar)
  

  This is strongly preferred:

  var nums = [1, 2, 3]
  nums.forEach(bar)
  

Helpful reading

• An Open Letter to JavaScript Leaders Regarding Semicolons
• JavaScript Semicolon Insertion – Everything you need to know

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: 1. 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 ,.) 2. The line is -- or ++ (in which case it will decrement/increment the next token.) 3. It is a for(), while(), do, if(), or else, and there is no { 4. 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:

foo();
[1,2,3].forEach(bar);

you could do this:

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.