What is the !! (not not) operator in JavaScript?

http://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript

Question:

I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so:

!!

.
Can someone please tell me what this operator does?

The context in which I saw this was,

this.vertical = vertical !== undefined ? !!vertical : this.vertical;

Answer:

Coerces oObject to boolean. If it was falsey (e.g. 0,

null

,

undefined

,
etc.), it will be

false

,
otherwise,

true

.

!oObject  //Inverted boolean
!!oObject //Non inverted boolean so true boolean representation

So !! is not an operator, it's just the ! operator twice.
It converts a nonboolean to
an inverted boolean (for instance, !5 would be false, since 5 is a non-false value in JS), then boolean-inverts that so you get the original value as a boolean (so !!5 would be true).