jerome wrote:
(function(){
...
})();
How is this basic to understanding how jQuery works, and how
JavaScript works?
The basic idea is that global variables are evil. This technique moves
what would otherwise be global variables into a closure [1], removing
them from the global scope. For example, in the following code
var count = 0;
function createClickHandler(nbr) {
return function() {
alert("You clicked dead link " + (1 + nbr));
}
}
var links = document.getElementsByTagName("A");
for (var i = 0; i < links.length; i++) {
var link = links[i];
if (link.getAttribute("href") == "#") {
link.onclick = createClickHandler(count++);
}
}
all the variables used are now available in the global scope, including
"count", "links", the function "createClickHandler", and, perhaps
surprisingly, "i" and "link". You can see for yourself by clicking the
button on this page:
http://scott.sauyet.com/Javascript/Demo/2009-06-22a/
If, however, you surround the code with "(function(){" and "})();", as
you can see at
http://scott.sauyet.com/Javascript/Demo/2009-06-22b/
the variables are no longer defined when you click the button. You've
removed the variables from the global scope.
As to the syntax, what you are seeing is an anonymous function created
and then immediately applied. You could do this with a named function
like this:
function myFunc() {
// code here
}
myFunc();
but then you've added "myFunc" to the global namespace. By removing the
function name, you are adding nothing at all to the global namespace.
Ideally, this would be accomplished with syntax more like this:
function() {
// code here
}();
i.e., we create the function and apply it. But that syntax is not
legal. For technical reasons, you need to surround the anonymous
function with parentheses to be able to apply it with the "()" operator,
and we end up with
(function() {
// code here
})();
As to how closures manage to keep the global namespace clean, you will
need to read up on closures. [1]
This has become a very common idiom in JavaScript programming, and it
really is important to understand it if you want to work with any but
the most basic of modern scripts.
Cheers,
-- Scott
[1] http://www.google.com/search?q=javascript+closure
