difference between "void 0 " and "undefined"

I'm using "Closure Compiler", when compiling my scripts I spend the following:

Before compiling:

// ==ClosureCompiler==
// @compilation_level SIMPLE_OPTIMIZATIONS
// @output_file_name default.js
// @formatting pretty_print,print_input_delimiter
// ==/ClosureCompiler==

var myObj1 = (function() {

  var undefined;   //<----- declare undefined

  this.test = function(value, arg1) {

    var exp = 0;
    arg1 = arg1 == undefined ? true : arg1;  //<----- use declare undefined
    exp = (arg1) ? value * 5 :  value * 10;

    return exp;
  };

  return this;
}).call({});

var myObj2 = (function() {

  this.test = function(value, arg1) {

    var exp = 0;
    arg1 = arg1 == undefined ? true : arg1;  //<----- without declare undefined
    exp = (arg1) ? value * 5 :  value * 10;

    return exp;
  };

  return this;
}).call({});

Compiled:

// Input 0
var myObj1 = function() {
  this.test = function(b, a) {
    a = a == void 0 ? true : a;  //<-----
    var c = 0;
    return c = a ? b * 5 : b * 10
  };
  return this
}.call({}), myObj2 = function() {
  this.test = function(b, a) {
    a = a == undefined ? true : a; //<-----
    var c = 0;
    return c = a ? b * 5 : b * 10
  };
  return this
}.call({});

With this I believe the question of the use of "void 0 " and "undefined", is there any difference in the use or the two cases are well?.

Edit

if I define "var undefined" compiled with "void 0 ", if I did not define "undefined" compiled with "undedined. " then not a matter of number of characters between "undefined" and "void 0"

Test

Edit II: performance, based on this link

Code and Test

IE 8:
typeof: 228ms
undefined: 62ms
void 0: 57ms

Firefox 3.6:
typeof: 10ms
undefined: 3ms
void 0: 3ms

Opera 11:
typeof: 67ms
undefined: 19ms
void 0: 20ms

Chrome 8:
typeof: 3ms
undefined: 5ms
void 0: 3ms

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!