When to use a semicolon

Question

Specifically in JavaScript, when does one use a semicolon and when do you not.

Here is an example snippet of code;

function Drawable() {
   this.init = function(x, y) {
      this.x = x;
      this.y = y;
   }

   this.speed = 0;
   this.canvasWidth = 0;
   this.canvasHeight = 0;

   this.draw = function() {
   };
}

Can someone enlighten me as to why

this.init = function(x,y) {}

does not end with a semicolon, however

this.draw = function(){};

does end with a semicolon in the above snippet of code?

Answer 1

This a matter of personal style since JavaScript sports automatic semicolon insertion.:

When, as the program is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if... The offending token is separated from the previous token by at least one LineTerminator.

The first one does not end in a semicolon because the above code is inconsistent.

The consistent way would have been:

this.init = function(x, y) {
   this.x = x;
   this.y = y;
};

The issue of whether or not you should use them has been discussed in SO before.

Answer 2

From Google JavaScript Style Guide:

// 1.
MyClass.prototype.myMethod = function() {
  return 42;
}  // No semicolon here.

(function() {
  // Some initialization code wrapped in a function to create a scope for locals.
})();


var x = {
  'i': 1,
  'j': 2
}  // No semicolon here.

// 2.  Trying to do one thing on Internet Explorer and another on Firefox.
// I know you'd never write code like this, but throw me a bone.
[normalVersion, ffVersion][isIE]();


var THINGS_TO_EAT = [apples, oysters, sprayOnCheese]  // No semicolon here.

// 3. conditional execution a la bash
-1 == resultOfOperation() || die();

1 - JavaScript error - first the function returning 42 is called with the second function as a parameter, then the number 42 is "called" resulting in an error.

2 - You will most likely get a 'no such property in undefined' error at runtime as it tries to call x[ffVersion]isIE.

3 - die is called unless resultOfOperation() is NaN and THINGS_TO_EAT gets assigned the result of die().