Possible Duplicate:
What is the purpose of a self executing function in javascript?
Explain JavaScript's encapsulated anonymous function syntax
for instance:
(function($) { document.getElementById("foo").innerHTML = 'bar'; })(); I understand that we want to create our own scope to prevent variable collision but why does javascript need to have ()()?
That makes it a self-invoking anonymous function.
(function() { /* function body */ }) /* <-- end of function definition */ (); // <-- invoke that function immediately 
someFunction(); It only looks weird because the function itself is anonymous/isn't named. More info: 2007-2010.lovemikeg.com/2008/08/17/…That doesn't have ()(), it has (function(){})().
The function syntax is function(){} and the function call operator is (). The parentheses that wrap the function are not technically special and you could instead replace them with !:
!function(){}() Doing this wouldn't work:
function(){}() Because this in a statement context, the function starts a function declaration rather than expression. The syntax then fails because a function declaration must have a name.
If we have !function(){} (or (function(){}), then it couldn't be a statement because ! (or () already expects an expression, so it will be treated as an expression.
So you could do this without any extra:
var a = function() { return false; }(); Because var a = is already expecting an expression, function cannot possibly be the start of a function declaration.
An easy way to litmus test whether your function will be seen as an expression or declaration is to ask yourself, could I use var x here?
For instance:
var x; //all is fine, so if I said function here, it would be a start of a function declaration (var x) //Gives an error, so replacing var x with a function would be a function expression var myVar = var x; //Gives an error, so replacing var x with a function would be a function expression And so on
The first results in the function, in the same way that (2) results in 2. The second calls it.
