javascript: What is a NOT NOT? (!! operator ) [duplicate]

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);
}

It's a duplicate, but that duplicate question barely mentions why it's used. In your case, it's the bottom answer: it's an idiom to check 'is defined'. — Rup
– Rup, Commented Mar 23, 2011 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
– Val, Commented Mar 23, 2011 at 12:05
The title of this question is much easier to find then the duplicate one — Ido Ran
– Ido Ran, Commented Sep 10, 2011 at 11:17

sharpner · Accepted Answer · 2011-03-23 11:45:52Z

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, 2011 at 11:45

sharpner

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2 Comments

This is horri-bad. Try if(!!"false" == true) { alert("false"); } and see what happens.

I did not say you should use it

n0rd · Accepted Answer · 2011-03-23 11:44:17Z

5

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

answered Mar 23, 2011 at 11:44

n0rd

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