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

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

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

Solution 1:

I believe that's because plain {}[true] is parsed as an empty statement block (not an object literal) followed by an array containing true, which is true.

On the other hand, applying the ! operator makes the parser interpret {} as an object literal, so the following {}[true] becomes a member access that returns undefined, and !{}[true] is indeed true (as !undefined is true).

Solution 2:

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