Extend an existsing object method

Question

I have an object:

MyObj = new Class.create();
MyObj.prototype = {
    initialize: function () {
        this.myArg = null;
    },

    setArg: function(arg) {
        this.myArg = arg;

    },

    showArg: function () {
        alert(this.myArg);
    }
};

var x = new MyObj();
x.setArg('Something');
x.showArg();

x.setArg = function(arg) {
    //how to call here the original??
    alert('Here is my custom code')
}

It is created in a code what I can not overwrite.

What I want to do is to extend this existing object like this:

x.setArg = function(arg) {
        this.myArg = arg + ' a new cake';
}

I just do not want to copy the original code.

UPDATE:

I have nothing to do with the prototype. The object has already created.

Answer 1

You have two options here

Option One

Override only the method

var MyObj = function(){};
MyObj.prototype = {
    initialize: function () {
        this.myArg = null;
    },

    setArg: function(arg) {
        this.myArg = arg;

    },

    showArg: function () {
        alert(this.myArg);
    }
};

var oldMethod = MyObj.prototype.setArg;
MyObj.prototype.setArg = function(arg) {
    oldMethod.call(this,arg)
    console.log(arg)
}


var x = new MyObj();
x.setArg('Something');
x.showArg();

Option Two

Extend the object then override any method you want

var MyObj = function(){};
MyObj.prototype = {
    initialize: function () {
        this.myArg = null;
    },

    setArg: function(arg) {
        this.myArg = arg;

    },

    showArg: function () {
        alert(this.myArg);
    }
};


var MyNewObject = function() {
}

MyNewObject.prototype = new MyObj()
MyNewObject.prototype.setArg = function(arg) {
    MyObj.prototype.setArg.call(this,arg)
    console.log(arg)
}


var x = new MyNewObject();
x.setArg('Something');
x.showArg();