How do I monkey patch an object's constructor function?

Question

I'd like to monkey patch the constructor for this 'Controller' object. But how do I monkey patch the constructor function so I can still call the original? This is what I've tried.

// original
function Controller() {
    this._tag = 'div';
}
Controller.prototype.tag = function() {
    console.log(this._tag);
}

var c = new Controller(); 
c.tag(); // -> 'div', as expected


// patch attempt
var original = Controller;
Controller = function() {
    original.apply(this);
    this._tag = 'patched'; // patch
}

var c = new Controller();
c.tag(); // no method tag, prototype appears wiped...

Answer 1

You seem to want to do something like:

Constructor.prototype.oldTag = Constructor.prototype.tag;

Constructor.prototype.tag = function() {/* whatever */};

Now all instances get the new tag method and you can still call oldTag if you want (or put it back).

Or perhaps you want to do something like:

var oldConstructor = Constructor;

 var Constructor = function () { /* new constructor */ };
 Constructor.prototype = oldConstructor.prototype;

So now you have a new constructor with all the old methods. Or do both the above. Just use plain English to say what you want to do.