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

You are in misconception “!!” is not an operator, it is just the “!” operator twice. See the example below:-

let isIE8 = false;

isIE8 = !! navigator.userAgent.match(/MSIE 8.0/); console.log(isIE8);

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