gistfile1.js

// I needed a basis with which to mock.
function CanYouMockMe(){
  this.i_want_stubbed = function(){ return false; };
  this.say = function(){ console.log(arguments[0] || "hi!"); };
};

// new Object def.
function Foo() {
  this.do_something_interesting = function() {
    //hard coded dependency... like my ex-wife. :P 
    var dependency = new CanYouMockMe();
    if(dependency.i_want_stubbed() === true) {
      return "I've been stubbed!"
    } else {
      return "I 100% original!";
    }
  }
}

function Mock(){
  this.i_want_stubbed = function(){ return true; };
};


// inherit from the object you wish to mock
// this ensures that any non-stubbed/mocked 
// properties will exist on your mock as written;
Mock.prototype = new CanYouMockMe();

// see, we got all the cool stuff.
var myMock = new Mock();
myMock.say();
myMock.say("dude!");
console.log(myMock.i_want_stubbed());

// see nothing has changed yet.
var myConcrete = new CanYouMockMe();
myConcrete.say();
myConcrete.say("dude!");
console.log(myConcrete.i_want_stubbed());

// I want to be able to get it back.
ORIG_CanYouMockMe = CanYouMockMe;

// it's clobberin time!
CanYouMockMe = Mock;

// changes!
var myConcrete = new CanYouMockMe();
myConcrete.say();
myConcrete.say("dude!");
console.log(myConcrete.i_want_stubbed());

//AND FOR MY FINAL TRICK! 

// it's clobberin time!
CanYouMockMe = ORIG_CanYouMockMe;

// AND WE'RE BACK!!!
var myConcrete = new CanYouMockMe();
myConcrete.say();
myConcrete.say("dude!");
console.log(myConcrete.i_want_stubbed());

//original foo
var foo = new Foo();
console.log(foo.do_something_interesting());

CanYouMockMe = Mock;
//mocked dependency
var foo2 = new Foo();
console.log(foo2.do_something_interesting());

//bring it back.
CanYouMockMe = ORIG_CanYouMockMe
var foo3 = new Foo();
console.log(foo3.do_something_interesting());