How to understand "return obj === void 0" in the source of underscore? [duplicate]

Question

Possible Duplicate:
What does “javascript:void(0)” mean?
What does void 0 mean?

In the file http://underscorejs.org/underscore.js, you can see such a code block:

// Is a given variable undefined?
_.isUndefined = function(obj) {
  return obj === void 0;
};

I don't understand the void 0 part, it's very strange. How to understand it?

Answer 1

This is the console output

>typeof void 0
"undefined"
>void 0 === undefined
true
>"undefined".length
9
>"void 0".length
6

I think they are trying to save 3 bytes of file size ;)

Edit: This SO answer makes more sense of using void 0, as undefined is just a property of window object and is mutable. Hence void 0 is a trusted way to generate undefined across browsers

Answer 2

In javascript (since 1.1), the void operator is used to evaluate an expression and return undefined.

See ECMAScript Language Specification of the void operator

So void 0 is a correct and standard way to produce undefined.

As it is an operator, no parenthesis are needed.