Purpose of (fn...)() brackets in Self-Executing Anonymous Functions? [JavaScript]

Question

Possible Duplicate:
Location of parenthesis for auto-executing anonymous JavaScript functions?

Question is a duplicate of http://stackoverflow.com/questions/3384504/location-of-parenthesis-for-auto-executing-anonymous-javascript-functions and http://stackoverflow.com/questions/440739/what-do-parentheses-surrounding-a-javascript-object-function-class-declaration-me

Just curious really, what are the purposes of the brackets in this code:

(function() {})();

This looks like I could just as easily write:

var x=function(){};
(x)();

With jQuery plugins we would do something like...

(function($) {})(jQuery);

What's the deal with the brackets?

Answer 1

With the parentheses (surrounding the function), you don't have to declare a name, littering the namespace. And, they serve to alert readers of your code to the fact that you're using a self-invoking function.

The second set of parentheses actually invoke/call the (anonymous) function you just created. Since Javascript functions are actually just variables (or "first-class objects" in CS-speak), you've just created a variable (in the first set of parentheses), which you call using the second set.

Here's an example:

function callFunc(f) {
    return f("test");
}

callFunc(alert);

In the example, f actually references the function alert, which you call in the function code with the parentheses.

Answer 2

In case of the jQuery example you define an anonymous function that takes a parameter called $ and then pass the jQuery object to it. It will stay inside that scope and not conflict with other frameworks that have $ globally defined.

Other than that, personally it looks cleaner to me.