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

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

!oObject  // inverted boolean!!oObject // non inverted boolean so true boolean representation

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

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.

! is 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, then inverting it, 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);

!!expr (two ! operators followed by an expression) returns a Boolean value (true or false) depending on the truthiness of the expression. It makes more sense when used on non-boolean types. Consider these examples, especially the 3rd example and onward:

          !!false === false           !!true === true              !!0 === false!!parseInt("foo") === false // NaN is falsy              !!1 === true             !!-1 === true  // -1 is truthy          !!(1/0) === true  // Infinity 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 value is falsy      !!undefined === false // undefined primitive is falsy           !!null === false // null is falsy             !!{} === true  // an (empty) object is truthy             !![] === true  // an (empty) array is truthy; PHP programmers beware!