Can you write nested functions in JavaScript?

I am wondering if JavaScript supports writing a function within another function, or nested functions (I read it in a blog). Is this really possible?. In fact, I have used these but am unsure of this concept. I am really unclear on this -- please help!

Is this really possible.

Yes.

function a(x) {    // <-- function
  function b(y) { // <-- inner function
    return x + y; // <-- use variables from outer scope
  }
  return b;       // <-- you can even return a function.
}
console.log(a(3)(4));

The following is nasty, but serves to demonstrate how you can treat functions like any other kind of object.

var foo = function () { alert('default function'); }

function pickAFunction(a_or_b) {
    var funcs = {
        a: function () {
            alert('a');
        },
        b: function () {
            alert('b');
        }
    };
    foo = funcs[a_or_b];
}

foo();
pickAFunction('a');
foo();
pickAFunction('b');
foo();