What is the !! (not not) operator in JavaScript?

It converts Object to boolean. If it was falsey (e.g., 0, null, undefined, etc.), it would be false, otherwise, true.

!object  // Inverted Boolean
!!object // Noninverted Boolean, so true Boolean representation

So !! is not an operator; it's just the ! operator twice.

It may be simpler to do:

Boolean(object) // Boolean

Real World Example "Test IE version":

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

If you ⇒

console.log(navigator.userAgent.match(/MSIE 8.0/));
// Returns either an Array or null

But if you ⇒

console.log(!!navigator.userAgent.match(/MSIE 8.0/));
// Returns either true or false

It's a horribly obscure way to do a type conversion.

! means NOT. So !true is false, and !false is true. !0 is true, and !1 is false.

So you're converting a value to a Boolean, inverting it, and then inverting it again.

// Maximum Obscurity:
val.enabled = !!userId;

// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;

// And finally, much easier to understand:
val.enabled = (userId != 0);

// Or just
val.enabled = Boolean(userId);

Note: the latter two expressions aren't exactly equivalent to the first expression when it comes to some edge case (when userId is [], for example), due to the way the != operator works and what values are considered truthy.