Why would one write global code inside a function definition-call pair?

Question

I see examples where JavaScript code including jQuery and jslint use the notation below:

(function(){
  // do something
})();

instead of:

Answer 1

As everyone else has said, it's pretty much entirely to do with creating local scope. Another benefit is that you can use it to (for want of a better word) "rename" variables. Take for instance, how several javascript frameworks use $ as a shorthand for their main library function. If you create a closure like in your example, it doesn't matter what $ is outside, you can use it as a parameter and inside it can be whatever you want:

// out here $ might be Prototype, something else, or even undefined
(function($) {
    // in here, $ is jQuery
})(jQuery);

Another little tip for eking an extra couple of milliseconds of your script is to use this same technique to create an undefined variable. Most people think that undefined is a special keyword in javascript, but it's actually just treated as a normal variable, which you'd hope no one would define. The somewhat standard practice of checking for a undefined variable:

if (x == undefined)

...is actually rather wasteful, since it checks the entire scope chain for a variable named "undefined". To shortcut this, you can use this method:

(function($, undefined) {
    // code here
})(jQuery);  // note that there's just one parameter passed

Now that undefined is actually in a scope (with an undefined value), checking up the scope chain can stop at that point. Micro-optimisation, yes, but it doesn't hurt to know.