Sometimes I see such links

& lt; a href = "javascript: void (0)" & gt; Login & lt; / a & gt;

and void 0 are also used in the Backbone.js library, for example:

if (obj == null) return void 0;

What does void 0 mean and what is it for?

Answer 1, authority 100%

Operator void ^{[MDN] [specification]} evaluates the passed expression and always returns undefined .

Examples:

void 0 // returns undefined
void (0) // this also returns undefined
void "hello" // also returns undefined
void new Date () // always returns undefined

Why is this operator needed at all?

If void 0 always returns undefined , you can just use undefined , no?

You can. In fact, void is not a very useful operator and I rarely use it. But it comes in handy in some situations:

• In older browsers (I’m not sure which ones. In my opinion, in IE 6 and below and in Netscape) it was possible to change the value of undefined :

  undefined = 5; // in older browsers will change the value to undefined,
           // does nothing in new ones
  5 === undefined; // true in older browsers, false in newer
  

  It is not always known that undefined means undefined . Therefore, some libraries use void 0 .

• Even in newer browsers undefined is a valid variable name inside a function:

  function f () {
   var undefined = 5;
   return undefined;
  }
  f (); // returns 5
  

  But if you write code like this, it’s your fault.

• If you want to save your energy: void 0 is three characters shorter than undefined . Although 0 [0] is even shorter.

A link starting with javascript: usually sends the user to a page with the text that the code returns. For example:

Press & lt; a href = "javascript: 'Hello!'" & gt; HERE & lt; / a & gt ;!