Why does !{}[true] evaluate to true in JavaScript?

Question

{}[true] is [true] and ![true] should be false.

So why does !{}[true] evaluate to true

Answer 1

Because {}[true] does not return true, but undefined, and undefined is evaluated as false:

http://jsfiddle.net/67GEu/

'use strict';
var b = {}[true];
alert(b); // undefined
b = !{}[true];
alert(b); // true