What is the significance of this function definition? [duplicate]

Question

I understand how to define functions like this:

function myfunc(x,y,z) {
   alert("Just an example " + x + y + z)
}

But not this:

    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

    <script>
        (function ($) {
        $.fn.idle = function (x, y, z) {
            alert("Just an example " + x + y + z)
    }(jQuery));
    </script>

The above is a part of a library I'm using, but I simply can't understand the $.fn.idle bit.

What is it doing? It's defining a function called 'idle', somehow, but what about the $.fn? And what about the (function ($) { part? Again, I understand $(document).ready(function() { but (function ($) { is completely alien. Is it a short hand?

And what is the significance of the (jQuery)); at the bottom?

Answer 1

An immediately invoked function expression which aliases jQuery to $ inside its scope:

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

It creates and executes a function immediately. This way you can use $ inside the function to reference jQuery regardless of what the global $ references to. Useful for pages using jQuery.noConflict(). Also the variables declared inside that IIFE don't leak to the global scope.

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As for the other part of the question, $.fn === jQuery.prototype. So by adding a method to the jQuery prototype, you may call it on any jQuery object. E.g.:

$.fn.doSomething = function(){}; //add a method to the jQuery prototype
$('selector').doSomething(); //then you may call it on any jQuery object

More on jQuery plugin authoring.