What is the !! operator in JavaScript?

Question

I've seen 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?

Answer 1

I think that would just be !(!a), yes?

That is a logical NOT against another logical NOT.

As in:

true = !false == !!true

Answer 2

Converts it to boolean!

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

Answer 3

It's just the logical NOT operator, twice - it's used to convert something to boolean, e.g.:

true === !!10

false === !!0

Answer 4

It seems that the !! operator results in a double negation.

var foo = "Hello World!";

!foo // Result: false
!!foo // Result: true

Steve

Answer 5

I suspect this is a leftover from C++ where people override the ! operator but not the bool operator.

So to get a negative(or positive) answer in that case you would first need to use the ! operator to get a boolean, but if you wanted to check the positive case would use !!.