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

Question

I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: !!. Can someone please tell me what this operator does?

The context in which I saw this was,

this.vertical = vertical !== undefined ? !!vertical : this.vertical;

Answer 1

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