javascript: What is a NOT NOT? (!! operator )

Question

Possible Duplicate:
What is the !! operator in JavaScript?

What is a not not in javascript I have seen this a few times :)

function foo(){
    return !!(window.history);
}

Exact duplicate: http://stackoverflow.com/questions/784929/what-is-the-operator-in-javascript — ig0774, Mar 23 '11 at 11:44
It's a duplicate, but that duplicate question barely mentions why it's used. In your case, it's [the bottom answer](http://stackoverflow.com/questions/784929/what-is-the-operator-in-javascript/4339463#4339463): it's an idiom to check 'is defined'. — Rup, Mar 23 '11 at 11:48
@rup I agree the answers there pretty wack, I didnt understand half of them answers and their technical B-S, they should be more friendly with their answer and not assume everyone is on their level :) especially when someone asks :) — Val, Mar 23 '11 at 12:05
The title of this question is much easier to find then the duplicate one — Ido Ran, Sep 10 '11 at 11:17

score 12 · Accepted Answer · answered Mar 23 '11 at 11:45

12

i believe it is used for enforcing boolean types...

for example

if("true" == true){
    alert("1");
}else{
    if(!!"true" == true){
        alert("2");
    }
}

alerts 2 not 1

answered Mar 23 '11 at 11:45

sharpner

1

This is horri-bad. Try `if(!!"false" == true) { alert("false"); }` and see what happens. – Randolpho Feb 28 '13 at 19:48
9

I did not say you should use it – sharpner Mar 01 '13 at 14:39

score 5 · Answer 2 · answered Mar 23 '11 at 11:44

5

I think it used to [not so] implicitly cast value to boolean type

answered Mar 23 '11 at 11:44

n0rd

2 Answers2