Semicolon Usage

Semicolons ; are not required in JavaScript. The following code is a valid JavaScript code:

const hello = 'Hello'

function greet() {
  return hello
}

This is because JavaScript has a feature called Automatic Semicolon Insertion aka ASI.
ASI puts semicolons in your JavaScript for you. It’s a part of the language and can not be disabled, it's always active.

ASI has a set of rules to insert semicolons. If there is already a semicolon, it won’t change anything.

Our previous code will be transformed into this one:

const hello = 'Hello';  // <-- ASI

function greet() {
  return hello;         // <-- ASI
}

However, this source

a = b + c
( d + e ).print()

is not transformed, because the expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c( d + e ).print()

Rules of ASI

The JavaScript parser will automatically add a semicolon when, during the parsing of the source code when:

1 - The next line starts with a }, closing the current block.
2 - There is a return, break, throw or continue statement on its own line.
3 - The end of the source code file is reached.

What does the following function return?

function foo() {
  return
  {
    a: 1
  }
  ;
}

You forgot to put a semicolon, but doesn't matter. ASI kicked in and changed your code to:

function foo() {
  return;         // <-- ASI
  {
    a: 1
  }
  ;
}

Because of ASI, the compiler places a semicolon after the return keyword and therefore it returns undefined without an error being thrown.

Special characters

If you don't use semicolons, never start a line with [, (, ,, *, /, ,, ., +, -.

// ✖︎︎ problem
(function() {
  console.log( 'hey, yo!' )
}())

// ✔ solution
;(function() {
  console.log( 'hey, yo!' )
}())

// ✖︎︎ problem
[1, 2, 3].forEach( dude )

// ✔ solution
;[1, 2, 3].forEach( dude )

// ✖︎︎ problem
`hey bro`.indexOf( 'o' )

// ✔ solution
;`hey bro`.indexOf( 'o' )