Why reuse of `undefined`?

Question

In John Rezig 's slideshow on how he builds jQuery 1.4, he mentioned that he added the undefined variable to the jQuery closure because "we can reuse (the variable)".

undefined not a regular variable:

 > var undefined = 4 undefined > undefined undefined 

Therefore, we know that undefined is not a variable. So why would undefined be re undefined in jQuery source?

Answer 1

Because on some JavaScript machines, you can set the value to undefined . This means that undefined really undefined .

Answer 2

In addition to the + Rocket Hazmat answer, you can reduce the file size after compression a bit when your code often uses undefined . This is because the local undefined variable may have its name distorted by the compressor, while the global undefined may not be:

 foo === undefined; // ^----- don't touch this, put "undefined" in the compressed result (function (undefined) { foo === undefined; })(); // may however be mangled to (function(u){foo===u})();