The Double NOT (!!) operator magic in JavaScript

Yes, it is possible to use a couple of NOT (!) operators together in JavaScript. JavaScript double not operator returns a Boolean value that depends on the expression’s truthfulness. So, the double not operator !! is not an operator, it’s just the “Logical NOT ” ! operator twice. And there are no restrictions on using more than twice you can use it 2, 3, 4 as many times as you need.

If we say in short:- JavaScript double not operator converts a non-boolean to an inverted boolean.

The double not operator is a horribly obscure way to do type conversion, but beautifully concise to eyes. So when we actually use it? Okay, I got that I’ll give you a real-world example below.

“Test IE version”:

let isIE8 = false;  
isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);  
console.log(isIE8); // returns true or false 

If you do console.log(navigator.userAgent.match(/MSIE 8.0/)); it will return either an Array or null.

But if you do console.log(!!navigator.userAgent.match(/MSIE 8.0/)); it will return either true or false. Boom you convert it to the boolean so, you can easily compare it.

Some More Examples

          !!false === false
           !!true === true

              !!0 === false
!!parseInt("foo") === false // NaN is falsy
              !!1 === true
             !!-1 === true  // -1 is truthy

             !!"" === false // empty string is falsy
          !!"foo" === true  // non-empty string is truthy
        !!"false" === true  // ...even if it contains a falsy value

     !!window.foo === false // undefined is falsy
           !!null === false // null is falsy

             !!{} === true  // an (empty) object is truthy
             !![] === true  // an (empty) array is truthy;

MDN

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