这个语法是什么意思----->“ !! ”中的JavaScript？

Question

I am reading "Discover Meteor" at the moment, In chapter 7 is has code:

Posts.allow({
  insert: function(userId, doc) {
    // only allow posting if you are logged in
    return !! userId;                        ///// <<==== what does "!!" means?
  }
});

Thanks

Answer 1

Beautifully summed up by Tom Ritter as

// Maximum Obscurity:
val.enabled = !!userId;

// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;

// And finally, much easier to understand:
val.enabled = (userId != 0);

therefore doing casting to a boolean and then doing double negation

Answer 2

! will turn any positive value, true, or existing variable(such as strings and arrays) into a false, and any negative, undefined, null, or false into a true. !! applies it twice.

In this context it would be returning true if the variable userId exists and is not empty, null, or false.

Answer 3

It just likes you change variable type to boolean

!! userId;

// same as

userId ? true:false;