What's the purpose of two exclamation marks before a variable?

Question

I'm studying the TodoMVC source and came across this helper:

// addEventListener wrapper:
window.$on = function (target, type, callback, useCapture) {
  target.addEventListener(type, callback, !!useCapture);
};

What's up with !!useCapture?

I'd suggest to read about false and falsy values in javascript. You'd know what it is — Akshay Khandelwal, Jul 21 '16 at 16:31

score 0 · Answer 1 · answered Jul 21 '16 at 16:32

0

The double exclamation mark forces a truthy/falsey value into a boolean. Think of it like !(!useCapture), or not (not use Capture).

answered Jul 21 '16 at 16:32

iampueroo

1 Answers1