What does the !! operator do?

Question

In JavaScript, what does the '!!' operator do? Is it a NOT NOT statement?

For example:

someFunc(foo) {
    return !! foo;
}
// Return foo only if foo exists?

It's not an operator itself, it's two `!` operators in a row. In this case it uses javascripts type coersion to turn whatever foo is into a boolean. — asawyer, May 18 '13 at 03:57
My apologies for the duplicate. I did search for this prior to posting but didn't get any results — monners, May 18 '13 at 04:03

PSR · Accepted Answer · 2013-05-18T04:02:44.620

2

First this is not an operator.converts it to boolean in javascript

EXAMPLE:

var test = true;

!test = false; //It will converted to false
!!test = true; //Again it will converted to true

edited May 18 '13 at 04:02

answered May 18 '13 at 03:57

PSR

score 2 · Answer 2 · answered May 18 '13 at 03:57

2

! is the "not" operator, casting its one argument to a boolean and negating it.

The second ! negates it back, so effectively !! casts the value to a boolean.

answered May 18 '13 at 03:57

score 0 · Answer 3 · answered May 18 '13 at 03:59

0

Converts foo to boolean.

var foo = "TEST";

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

answered May 18 '13 at 03:59

Rong Nguyen

3 Answers3