difference between "void 0 " and "undefined"

From MDN:

The void operator evaluates the given expression and then returns undefined.

This operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.

The void operator is often used merely to obtain the undefined primitive value, usually using "void(0)" (which is equivalent to "void 0"). In these cases, the global variable undefined can be used instead (assuming it has not been assigned to a non-default value).

Closure Compiler swaps in void 0 because it contains fewer characters than undefined, therefore producing equivalent, smaller code.

Re: OP comment

yes, I read the documentation, but in the example I gave, "google closure" in a case using "void 0" and another "undefined"

I believe this is actually a bug in Google Closure Compiler!

The real only semantic difference between void expr and undefined is that on ECMAScript 3, the undefined property of the global object (window.undefined on browser environments) is writable, whereas the void operator will return the undefined value always.

A popular pattern that is often implemented, to use undefined without worries is simply declaring an argument, and not passing anything to it:

(function (undefined) {
  //...
  if (foo !== undefined) {
    // ...
  }

})();

That will allow minifiers to shrink the argument maybe to a single letter (even shorter than void 0 :), e.g.:

(function (a) {
  //...
  if (foo !== a) {
    // ...
  }
})();

Just a follow-up on all the answers before.

They look the same, but to the Compiler they are completely different.

The two code sections compile to different outputs because one is referring to a local variable (the var undefined), and the compiler simply in-lines it because it is used exactly once and is no more than one line. If it is used more than once, then this in-lining won't happen. The in-lining provides a result of "undefined", which is shorter to represent as "void 0".

The one without a local variable is referring to the variable called "undefined" under the global object, which is automatically "extern'ed" by the Closure Compiler (in fact, all global object properties are). Therefore, no renaming takes place, and no in-lining takes place. Voila! still "undefined".