Demonstrates closures. Closure is nested function. Idea is to make inner function with vars accessible once outer function has returned.

gistfile1.js

function say667() {
    // Local variable that ends up within closure
    var num = 666;
    var sayAlert = function() { alert(num); }
    num++;
    return sayAlert;
}
var sayNumber = say667();
sayNumber(); // alerts 667