Strange JavaScript syntax like this: (function(){//code}) ();?

Question

What does the below JavaScript mean? Why is the function embedded inside ()?

(function() {
    var b = 3;
    a += b;
}) ();

Answer 1

It's an anonymous function, that is called directly after creation, then thrown away.

It's inside a parenthesis to prevent a syntax error. Without the parentheses the keyword function needs to be followed by an identifier.

You can also do like this to put the function in a variable, and then call it:

var x = function() {
   var b = 3;
   a += b;
}
x();

Notice that the parentheses are not needed when the function keyword is not first.