javascriptoperators

What does this syntax means -----> " !! " in javascript?

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

Solution

  • 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