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

i saw code seems use operator don't recognize, in form of 2 exclamation points, so: !!. can please tell me operator does?

the context in saw was,

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

coerces oobject boolean. if falsey (e.g. 0, null, undefined, etc.), false, otherwise, true.

!oobject  //inverted boolean !!oobject //non inverted boolean true boolean representation

so !! not operator, it's ! operator twice.

real world example "test ie version":

let isie8 = false;   isie8 = !! navigator.useragent.match(/msie 8.0/);   console.log(isie8); // returns true or false

if =>

console.log(navigator.useragent.match(/msie 8.0/));   // returns null

but if =>

console.log(!!navigator.useragent.match(/msie 8.0/));   // returns true or false

shell