Question

I’m pretty new to JavaScript, which I am learning on my own. I’m currently creating and tweaking GreaseMonkey scripts. I’ve noticed that most simple scripts (i.e. those with no need for named functions) go straight into the main code, but some instead are set up like this:

(function() {  
    //main code here  
})();

What is the significance of this type of coding? I’ve commented out both the top and bottom, and the script still runs exactly the same.

Is it just a coding standard, or does it actually have a function? And, as my title asks, is it something specific to GreaseMonkey, or something that I should do all the time?

Answer 1

Here’s a micro-tutorial for those who never heard of closures:

Suppose we have a function

function sayHi() { alert('Hello'); }

We would call this function by doing this:

sayHi();

The above is whats called a named function, if you don’t know what that means, then it’s what you think of when you hear the word function

In some languages you don’t have to name a function, you can leave it blank like so:

alert('Sup');
function() {alert('Hello'); }
3 + 1;
alert('Peace');

Line 2 is perfectly valid. Of course line 2 won’t really do anything in your page, just like line 3 doesn’t do anything. That is called an anonymous function. Now just like we can assign a variable to line 3 like:

result = 3 + 1;

We can do the same with line 2, like this:

myFunc = function() { alert('Hello'); };

And we can use myFunc as a function like the sayHi() function before. We call it just like we call sayHi()

sayHi();
myFunc();

Now since javascript is written to be versatile, where stuff like [0, 1, 2].indexOf(1) works, we can do the following:

func = function() { alert('hello'); };
func();
(function() { alert('hello'); })();

And line 1 & 2 will accomplish the same thing as line 3 since line 3 is just an expanded version of line 1 and 2. The advantage of line 3 is that if someone later on in the code uses a func variable it wouldn’t cause a problem with your own code, also any variables declared in line 3’s function (with a var keyword) won’t be a valid variable outside of your function, which in this case is what a closure is.