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]

evaluates to undefined, and !undefined is true.

From @schlingel:

true is used as key and {} as hash map. There doesn't exist an property with the key true so it returns undefined. Not undefined is true, as expected.

Console session (Node.js [0.10.17]):

> {}[true]
undefined
> !{}[true]
true
> [true]
[ true ]
> ![true]
false
>

However, in the Google Chrome console:

> !{}[true]
true

So, no inconsistencies. You're probably using an old version of the JavaScript VM. For those who need further evidence:

UPDATE

With Firefox, it also evaluates to true: