Make a revocable function that takes a nice function and returns a revoke function that denies the access to the nice function, and an invoke function it is invokes the nice function until be revoked

exercise_10.js

var revoke = (function(func) {
  var tmp = func;
  return {
    invoke: function(p) {
      tmp(p);
    },
    revoke: function() {
      tmp = null;
    }
  }
});

var r = revoke(alert);
r.invoke("I am invoked"); // Alert
r.revoke(); // nothing
r.invoke("I am invoked"); // throw