Why does (![]+[])[+!![]+[]] produce “a”

Question

I am interested in understanding the internals of JavaScript. I\'ve tried to read the source for SpiderMonkey and Rhino but it\'s quite complex to wrap my head around.

Answer 1

If you want to understand why those wierd expressions work as they do, you can open firebug console and experiment yourself. I did and I got that ![] is false, !![] is true, adding an array to a boolean value (false+[] or true+[]) produces a string-version of this value (false+[]="false").

That way the expression boils down to:

"false"["1"] 

which is obviously "a"