Duplicate a javascript prototype instance?

Question

I'm given an instance "aNode" of a prototype called "Node". Following this discussion, I attempt to:

var newNode = clone(aNode);

The clone function goes like:

function clone(obj) {
    if (null == obj || "object" != typeof obj) return obj;
    var copy = obj.constructor();
    for (var attr in obj) {
        if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
    }
    return copy;
}

Despite obj.consturctor() properly refers to aNode's constructor, copy is "underfined". Can't find why :(

Is this method wrong, or am I missing something :) ?

Thanks, J.

Edit: here is what the Node prototype looks like:

Node.prototype = new PhysicsNode();
Node.prototype.constructor=Node;

function Node() {

    PhysicsNode.call(this);

    this.setBounds(0, 0, 0, 0);
    this.createStaticBody();
}

This clone method is correct and is working well. Do you have more details about your aNode instance ?! — Seb T., Apr 10 '12 at 09:43
The code is fine, however using ES5 `Object.create(aNode);` is a preffered way to do this. Try using this method and tell us if it works. — freakish, Apr 10 '12 at 10:22
@Sebastien Thuilliez I've added the Node prototype definition — Jem, Apr 10 '12 at 15:55

Diode · Accepted Answer · 2012-04-11T03:48:00.507

2

Ok. Here

var copy = obj.constructor();

is just a function call and constructor function Node does not return anything. You have to use new keyword to create new object.

var copy = new obj.constructor();

edited Apr 11 '12 at 03:48

answered Apr 11 '12 at 03:42

Diode

24,570
8
40
51

All right! Thanks Diode :-) What's the best practice, have your constructor return something, or not? – Jem Apr 11 '12 at 08:19

score 1 · Answer 2 · answered Apr 10 '12 at 16:20

1

var copy = obj.constructor();

This line takes the constructor of obj, calls it and assigns the result to copy.

As obj is a Node, according to the line Node.prototype.constructor=Node; it holds that obj.constructor === Node.

Look at the function Node:

function Node() {

    PhysicsNode.call(this);

    this.setBounds(0, 0, 0, 0);
    this.createStaticBody();
}

This function is a constructor, it has to be called with the new keyword. If it is not called with the new keyword, it is just a regular function. As there is no return statement, this function returns nothing.

Therefore,

var copy = obj.constructor();

assigns nothing to obj, so obj is undefined.

Finally,

return copy;

you return this undefined value from your clone function.

answered Apr 10 '12 at 16:20

Imp

8,409
1
25
36

Thanks for your answer. Is it a good practice to have your constructor (prototype wise) return the actual object? Inheritance might make this tricky :( – Jem Apr 11 '12 at 08:21
1

No. If you call `var x = new MyConstructor();`, the `new` keyword creates a new object and sets `this` to this new object. So whatever `MyConstructor()` does (e.g. `this.property = value`) relates to the new object. You don't return the object from the constructor - the `new` keyword takes care of this. – Imp Apr 11 '12 at 09:12

Duplicate a javascript prototype instance?

2 Answers2