Javascript ! ! : Explain please

JavaScript
1

Continuing the discussion from freeCodeCamp Algorithm Challenge Guide: Falsy Bouncer:

    function bouncer(arr) {
// Don't show a false ID to this bouncer.
    return arr.filter(function(val) {
        return !!val ;
});
}

Can you explain the part with ! !val;
How does it work?
(This is catbegemot’s solution).

2

!! transitions value to boolean. If value was “falsy” (0, null, undefined, NaN etc) it’s gonna be false, otherwise it’s gonna be true.

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3

Technically though, he didn’t need to use !! at all. Simply returning the value is going to evaluate to true or false regardless. That could could be written:

function bouncer(arr) {
  // Don't show a false ID to this bouncer.
  return arr.filter(function(val) { return val; }) ;
}
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