Closure in Javascript (Simple)

Question

I'm very new to programming and have a quick question regarding a demonstration of closure given here: http://youtu.be/hQVTIJBZook?t=27m19s.

The code is:

var digit_name = function () { var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; return function (n) { return names[n]; }; } (); alert(digit_name(3)); 

My question is: Why is there an additional (); outside of the function digit_name? Are we calling the anonymous secondary function? I ran it without the (); and it came back with " function (n) {return names[n];} ". I would really appreciate a description of what happens to the parameter 3 when you pass it to the function digit_name, since that function doesn't have a specified parameter.

I apologize if I'm asking a basic question or using incorrect terminology. I've looked into a few related questions but alas, to no avail! Thank you in advance to those kind enough to provide a well-fashioned answer.

Answer 1

In that code, it makes the outer function execute immediately, thus returning the inner function as digit_name. But since the returned function carries a reference to names, the scope of the outer function does not terminate like it normally should. Thus, the scope lives and it becomes a closure.

It's like doing:

function anon() { var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; return function (n) { return names[n]; }; } var digit_name = anon(); alert(digit_name(3)); 

So when you call digit_name(), you are actually calling the returned inner function which carries it's own names.

I suggest you read about closures at MDN. I also have an article discussing about closures in simple terms.