Javascript: resetting object definition with hook added (hooking construction)

I'm trying to hook the creation of objects in Javascript. This is easy for methods, but afaik not for constructors. I cannot change the way to create objects as this has to work for current libraries.

Anyway, this is what I have now:

Function.prototype.afterConstruction = function(hookFunc) {
    var oldObj = this;
    var newObj = function() {
        oldObj.apply(oldObj, arguments);
        hookFunc.apply(oldObj, arguments);
    }
    newObj.prototype = oldObj.prototype; //copy prototype fields of oldObj
    for (var key in oldObj) { //copy static fields of oldObj
        if (oldObj.hasOwnProperty(key)) newObj[key] = oldObj[key];
    }
    return newObj;
};

This is how it's used:

SomeObj = SomeObj.afterConstruction(function() {
   //executed after SomeObj construction
});

The problem is that it seems to create problems with prototype methods, even though I copy the prototype.

This issue is shown in the following jsfiddle: http://www.jsfiddle.net/6rBTZ/1/

If you run it with firebug开发者_如何学Python, it'll give you a _thisInstance.onTextChange is not a function, because of the afterConstruction call earlier on this object.

Any help or improvements to my construction hooking is appreciated.

The problem is this line here:

oldObj.apply(oldObj, arguments);

In your code, you never actually create an instance of oldObj - you're creating an instance of newObj. Since you're technically replacing the old constructor with a new one, you need to be applying the old constructor function to your replacement's instance:

oldObj.apply(this, arguments);

http://www.jsfiddle.net/AndyE/6rBTZ/9/