Question

The first lines of code of the jQuery source (bit.ly/jqsource) are:

(function( window, undefined ) {

// Use the correct document accordingly with window argument (sandbox)
var document = window.document,

I think I understand what the code does, but not why it does it. What is the “sandbox” all about?

Answer 1

It helps prevent people from overriding the main window.document or document.

Things like this:

document = 27;

If jQuery called document.getElementById, it would fail miserably.

As for undefined, its a “hack” to get at the real value of undefined because one can do this:

undefined = 'haha';

then conditions like this would fail.

if (myObj == undefined) { 
  alert("don't use myObj"); 
}