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.

i think this will be useful stackoverflow.com/questions/13745021/… — Naor Tedgi
– Naor Tedgi, Commented Apr 23, 2018 at 13:06
Are you using a third-party library here, EG Prototype? That first line does not look like vanilla JS. Maybe worth adding a tag for that if it's the case. And it would be good to indicate to people helping that you do or don't want to stick to using that lib and its methods. — Matt Morgan
– Matt Morgan, Commented Apr 23, 2018 at 13:10
Yes, "I am" using prototype, added the tag, thanks. Basically it is a magento2 thing. — vaso123
– vaso123, Commented Apr 23, 2018 at 13:12

Hyyan Abo Fakher · Accepted Answer · 2018-04-23 13:15:43Z

2

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();

edited Apr 23, 2018 at 13:15

answered Apr 23, 2018 at 13:08

Hyyan Abo Fakher

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1 Comment

vaso123 Over a year ago

The problem is that I have nothing to do with the prototype. The object has already created. I need to use an existing object.

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