Why does !{}[true] evaluate to true in JavaScript?
{}[true] is [true] and ![true] should be false .
So why does !{}[true] evaluate to true ?
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 ).
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
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 :
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