Sanchithasr

Posted on Dec 31, 2020

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

#javascript #webdev #beginners

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 associated value of true. We can use this to cast a variable to true or false using the double bang operator.

Let’s dive deep into what it is and how it works.

The ! in JavaScript, also called “bang”, is the logical “not” operator. If you place this operator in front of a boolean value, it will reverse the value, returning the opposite.

!true // returns false
!false // returns true

isTrue = true // variable which is boolean
!isTrue // returns false

If the single bang returns the opposite boolean value, imagine what double-bang would return?

The associated boolean value. In other words, true or false according to whether it is truthy or falsy values.

Values that are associated with boolean true are said to be truthy. Values that are associated with boolean false values are said to be falsy.

!!true // returns true
!!false // returns false

isTrue = true // variable which is boolean
!!isTrue // returns true

We can take advantage of this using double-bang on non-boolean values as well which is pretty cool.

isNumber = 34 // variable which is not boolean
!!isNumber // returns true

Truthy values:

In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context.

The following values are few examples that are considered by JavaScript to be truthys:

Object: {}
Array: []
Not empty string: "anything"
Number other than zero: 3.14
Date: new Date();

In the below example, variable something has the non-empty string value which has truthy value in JavaScript so the console will print the first message.

var something = ‘string’;
 if (!!something) {
   console.log('This is truthy')
 } else {
   console.log('This is falsey') 
 }

You can find more about it in the link here.

Falsy values:

A falsy value is a value that is considered false when encountered in a Boolean context.

The following values are few of the examples that are considered by JavaScript to be falsey:

Empty string: ""
0
null
undefined
NaN and the list of falsy values below.

In the below example, variable nothing has the 0 which has falsy value in JavaScript so the console will print the second message.

var nothing = 0;
 if (!!nothing) {
   console.log('This is truthy')
 } else {
   console.log('This is falsey') 
 }

You can find more about falsy values in the link here.

Let us see how we can use it for type casting.

function BankAccount(cash) {
this.cash = cash;
this.hasMoney = !!cash;
}

var myAccount = new BankAccount(80);
console.log(myAccount.cash); // expected result: 80
console.log(myAccount.hasMoney); // expected result: true

var emptyAccount = new BankAccount(0);
console.log(emptyAccount.cash); // expected result: 0
console.log(emptyAccount.hasMoney); // expected result: false

And that sums it up!
Thank You!

Top comments (14)

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)