Copy and modify similar object for instantiation javascript

Question

I have an object that looks like

var customObject = function() {
    this.property = "value";
};

customObject.prototype = new otherObject();

customObject.prototype.property2 = function() {};

etc. - it's much bigger than this.

I can successfully instantiate the object by writing new customObject().

Now I would like to create a rather similar object, although a little different. This involves modifying certain properties and perhaps even adding or removing some. As in the above example, I would like it to be invokable by writing new customObject2().

I thought I could simply do:

var customObject2 = new customObject();
customObject2.prototype = customObject.prototype;
customObject2.property = "modified value";

etc.

However, when I try to instantiate it by doing new customObject2() I receive an error, stating that the customObject2 is not a function.

I hope I could illustrate well enough as to what pattern I desire to create. What approach should I take to create such a pattern?

Well looking at your code, you're doing `customObject2 = new customObject()`. That should explain why it's not a function — Jani Hartikainen, Jul 26 '13 at 22:15
Is there some quick way to get what I need? Or am I supposed to manually create a `var customObject2 = function() {};` and loop over all the direct properties of the original `customObject` to assign them to the `this` of `customObject2`? — user2180613, Jul 26 '13 at 22:18

Paul S. · Accepted Answer · 2013-07-26T22:31:46.680

1

If customObject is not a host object (i.e. won't give you an illegal invocation error if you try to call it differently to expected) you can apply the constructor to a different this Object;

var customObject2 = function () {
    customObject.call(this); // construct as if `customObject`
    // now do more stuff
    this.anotherProperty = 'foo';
};
customObject2.prototype = Object.create(customObject.prototype);
    // inherit prototype but keep original safe

new customObject2();

Backwards compatible Object.create

function objectWithProto(proto) {
    var f;
    if (Object.create) return Object.create(proto);
    f = function () {};
    f.prototype = proto;
    return new f();
}

edited Jul 26 '13 at 22:31

answered Jul 26 '13 at 22:24

Paul S.

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Do you have an alternative for the Object.create that is compatible with older browsers as well? Oh, hang on: http://stackoverflow.com/questions/5199126/javascript-object-create-not-working-in-firefox – user2180613 Jul 26 '13 at 22:29
Thanks, this is the short solution I was looking for and it avoids simply extending `customObject` – user2180613 Jul 26 '13 at 22:36
1

@user2180613 But this is also extending customObject, it's just a different technique. – bfavaretto Jul 27 '13 at 00:04
@bfavaretto iirc coffeescript uses this style with their `extend` keyword – Paul S. Jul 27 '13 at 02:25

score 0 · Answer 2 · edited May 23 '17 at 11:49

0

I think this should answer your question. Basically, the new keyword is returning an object and not a function.

edited May 23 '17 at 11:49

Community

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answered Jul 26 '13 at 22:18

Ryan George

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No it doesn't. I know very well what the keyword does, but I was wondering if there is a quick way to copy an object and have it remain "instantiatable". – user2180613 Jul 26 '13 at 22:20

bfavaretto · Answer 3 · 2013-07-26T22:29:31.453

0

Why are you not using the same formula you used the first time? For example:

var customObject2 = function(){};
customObject2.prototype = new customObject();
customObject2.property = "modified value";
new customObject2(); // works!

All properties of customObject will be inherited by the instances of customObject2 through the prototype chain.

edited Jul 26 '13 at 22:29

answered Jul 26 '13 at 22:22

bfavaretto

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I'm not looking for inheritance. – user2180613 Jul 26 '13 at 22:30

Copy and modify similar object for instantiation javascript

3 Answers3