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

Have you ever taken a look under the hood at the JQuery 1.4 source code and noticed how it's encapsulated in the following way:

(function( window, undefined ) {

  //All the JQuery code here 
  ...

})(window);

I've read an article on JavaScript Namespacing and another one called "An Important Pair of Parens," so I know some about what's going on here.

But I've never seen this particular syntax before. What is that undefined doing there? And why does window need to be passed and then appear at the end again?

A.

The undefined is a normal variable and can be changed simply with undefined = "new value";. So jQuery creates a local "undefined" variable that is REALLY undefined.

The window variable is made local for performance reasons. Because when JavaScript looks up a variable, it first goes through the local variables until it finds the variable name. When it's not found, JavaScript goes through the next scope etc. until it filters through the global variables. So if the window variable is made local, JavaScript can look it up quicker. Further information: Speed Up Your JavaScript - Nicholas C. Zakas

source - http://stackoverflow.com/questions/2716069/how-does-this-javascript-jquery-syntax-work-function-window-undefined