Extra parentheses on function [duplicate]

Question

Possible Duplicate:
What do parentheses surrounding a JavaScript object/function/class declaration mean?
What does this “(function(){});”, a function inside brackets, mean in javascript?
A Javascript function

I encountered markup similar to this:

var something = (function(){
    //do stuff
    return stuff;
})()
document.ondblclick = function(e) { alert(something(e)) };

I don't understand the opening ( and closing )() in the something variable. Could you explain the difference to writing it like this?

var something = function(){
    //do stuff
    return stuff;
};

Thanks!

Answer 1

It's probably easier to understand if you leave the redundant parens out because they serve no purpose:

var something = function() {
    return 3;
} // <-- a function. 
(); // now invoke it and the result is 3 (because the return value is 3) assigned to variable called something 

console.log(something) //3 because the function returned 3 

var something = function() {
    return 3;
}; // a function is assigned to a variable called something

console.log(something) //logs the function body because it was assigned to a function
console.log(something()) //invoke the function assigned to something, resulting in 3 being logged to the console because that's what the function returns

Answer 2

(function(){ ... }) is a (anonymous) function expression, you could e.g. assign that value to a variable.

The brackets behind it will immidiately execute the function expression, resulting in the return value of the function (in here: stuff). The construct is called IIFE.

When stuff is a function (which I assume, because you invoke something lateron), this is called a closure - the returned function (stuff, assigned to something) still has access to the variables in the execution context of that anonymous function.