var a = 1;

var six = (function() {
    var foo = 6;

    return function() {
        // JavaScript "closure" means I have access to foo in here,
        // because it is defined in the function in which I was defined.
    alert(foo);
  };
})

();

six();      // alerts 6
alert(a);   // alerts 1
alert(foo); // ReferenceError: foo is not defined