Why does one reuse `undefined`?
javascript, jquery
Solution
Because in some JavaScript engines it's possible to set `undefined` to a value. This is to make sure `undefined` is really `undefined`.
Problem
In John Resig's slideshow on how he was building jQuery 1.4, he mentioned a point where he added an `undefined` variable to the jQuery closure because "we can re-use (the variable)". `undefined` is not an ordinary variable: ``` > var undefined = 4 undefined > undefined undefined ``` Therefore, we know that undefined is not a variable. So why would an `undefined` be re-undefined in the jQuery source?