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

This question is similar to: What does the !! operator do in JavaScript?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — Donald Duck is with Ukraine
– Donald Duck is with Ukraine, Commented Nov 7, 2024 at 14:22

Scary Wombat · Accepted Answer · 2013-10-02 05:36:23Z

3

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

answered Oct 2, 2013 at 5:36

Scary Wombat

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Grallen · Accepted Answer · 2013-10-02 05:33:43Z

1

! 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.

answered Oct 2, 2013 at 5:33

Grallen

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Comments

Chokchai · Accepted Answer · 2013-10-02 05:37:44Z

0

It just likes you change variable type to boolean

!! userId;

// same as

userId ? true:false;

answered Oct 2, 2013 at 5:37

Chokchai

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