[mattdw]

It's actually understandable, just tedious. Lot of dynamic typing/casting tricks.

First, we set var '$' to bitwise NOT of [], which is -1 ([] casts to 0 or false depending on the operator):

     $ = ~[];

Then we re-set $ to an object:

     $ = {___: ++$,
          $$$$: (![]+"")[$],
          ...

Which gives us:

     $.___ == 0 // ($ was still -1, now it's 0)
     $.$$$$ == "f" // I think

The "f" is a bit clever:

     ![] == false
     (![]+"") == "false"
     (![]+"")[$] is an index on the string, and $ is now 0

That's as far as I've bothered. (edited for formatting)