[javascript] Double exclamation points?

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What is the !! (not not) operator in JavaScript?
What does the !! operator (double exclamation point) mean in JavaScript?

So I was debuging some code and ran across this:

var foo.bar = 0; // this is actually passed from another function, adding it for context

function(foo) {
    var someVar = !!foo.bar;

    if (foo.bar) {
      // ..stuff happens
    } else {
      // .. something else happens

Okay my questions is what is the point of !!? All that is doing is making the 0 === false.

  1. Is there any benefit to using that compared to boolean(foo.bar)?

  2. foo.bar can be evaluated in an if as is because 0 === false already, so why go through the conversion? (someVar is not reused anywhere else)

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