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What is the Double bang (!!) operator in JavaScript?

Sanchithasr on December 31, 2020

Every value has truth or false values in JavaScript. For example, a null value has an associated boolean value of false. Similarly 34 has an associ...

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RVxLab • Dec 31 '20

This is very well explained. You can also call this the double NOT operator if you'd like.

While Boolean(someValue) the same things as !!someValue, using the double not operator is shorter to write and a tiny bit faster. The speed increase is nothing meaningful, but interesting nonetheless.

Benchmark I ran in case you want to see for yourself:

(function(){
    console.time('!!');

    for (let i = 0; i < 1000000; i++) {
        const number = 1;
        const bool = !!number;
    }

    console.timeEnd('!!');

    console.time('Boolean()');

    for (let i = 0; i < 1000000; i++) {
        const number = 1;
        const bool = Boolean(number);
    }

    console.timeEnd('Boolean()');
})();

Sanchithasr • Dec 31 '20 • Edited

This is interesting. Never thought about Boolean() . Thank you.

Jon Randy 🎖️ • Jan 1 '21

It isn't a double bang operator, it is using the ! operator twice. Also, since - as you rightly state - values already have truthy and falsey-ness, it is largely pointless to use this unless you really, really need a pure boolean (perhaps for building some JSON to send somewhere on another system that requires the correct type)

MrViK • Dec 31 '20

That rocks, I remember it as "Bang Bang, you're a boolean now".

Sanchithasr • Dec 31 '20

Bang Bang, you're a Boolean now

I totally had forgotten about this. :)

Warren Parad • Jan 9 '21

I feel like they article made this whole concept really complicated. It's actually simple, What is !!?. It's the same as Coerce to boolean. It would do the same as a method Convert.toBoolean(value) where value is truthy or falsy and the result is true or false.

You need this because some parameters are expected to be true or false and not truthy and falsy. Boolean(value) doesn't actually coerce a value to a boolean, it's unfortunately less useful than that. And so using !! (the double NOT) is a great way to coerce a value to a boolean.

Sergio Turpín • Dec 31 '20

Cool!! I didn't know 👍

Sanchithasr • Dec 31 '20

👍

amrishpandey • Sep 21 '21 • Edited

what i learned that if a value is true or false.
by default it does not return anything if it is not explicitly set with true or false boolean . that means by default it returns the value only.

if single bang (!) is added before the value, it returns false.
if double bang (!!) is added before the value, it returns true.

am i correct ?

david kane • Jul 19

@geometry dash lite Instead of using a double bang operator, it is twice using the! operator. As you properly point out, values already have truthiness and falseness, so using this is practically pointless unless you really, truly need a pure boolean (maybe for making some JSON to transmit someplace on another system that has the correct type)