Why does jQuery file have two parameters in its function but receives only one parameter [duplicate]

Question

Possible Duplicate:
How does this JavaScript/JQuery Syntax work: (function( window, undefined ) { })(window)?

I am trying to learn about js scopes and anonymous functions. I have tried to read the jQuery.js file and it looks like this:

(function( window, undefined ) {

...

}(window));

Why does it have in the function params undefined when no parameter is being passed to it when it is executed?

Answer 1

This method is used so you can be sure that no one has previously redefined undefined value with something like

var undefined = true; 

or with other tricky/evil assignments outside the jQuery scoped function. So, inside that function every comparison done against undefined is safe.