What does it mean "return !! userId"

Question

In Meteor tutorial project Microscope there is row with "!!" operator

Posts.allow({
  insert: function(userId, doc) {
  return !! userId; 
 }
});

What does it mean?

Answer 1

It returns userId as a boolean

Answer 2

It's a boolean cast, from a fasly or truethy value to false or true respectively.

You might see:

var string = ""; // empty string is falsy
var bool = !!string; // false

---

falsy by the way means that it follows: myvar == false as opposed to false which follows: myvar === false (triple comparison).

Similarly truethy is myvar == true.

Answer 3

!!userId is the same as Boolean(userId).

Answer 4

The ! operator inverts a boolean, so by inverting a value two times, you get the same boolean. So true === !!true.

This often is used to convert a value into a boolean, as ! is guaranteed to return a boolean.