What is the portable way of setting prototype property after an object is created?

Question

I have a JavaScript object created from JSON. I really need to set its prototype property to be a different object because of JS5 getters and setters. Here is an example of what I need that works on Chrome:

function MyObj() { }
MyObj.prototype = { 
  get myProp : function () { return this._myProp; },
  set myProp : function (arg) { this._myProp = arg; }
}

... stuff ...

var instance = JSON.parse(result);
instance.constructor = MyObj;
instance.__proto__ = MyObj.prototype;

With this code, I can get and set properties on instance using the getters and setters defined in the prototype. However, this is not portable and will not work on IE, (nor node, I think).

What is the proper and portable way of doing this?

possible duplicate of [Setting prototype for Object Literal](http://stackoverflow.com/questions/15472005/setting-prototype-for-object-literal) — Dagg Nabbit, Mar 18 '13 at 20:17
@GGG Missed that question when I searched earlier. It doesn't exactly answer what I am trying to do. It is suggesting that I use Object.create(...), but since JSON is instantiating the object, I cannot. — Andrew Eisenberg, Mar 18 '13 at 20:22
@BrianVanderbusch Should work on any ECMA5 compatible runtime. — Andrew Eisenberg, Mar 18 '13 at 20:23
@AndrewEisenberg if you have `get` and `set`, I'm pretty sure you have `__proto__`, no? — Dagg Nabbit, Mar 18 '13 at 20:27
so you're trying to extend the JSON object by implementing the interface of your own custom object? — Brian Vanderbusch, Mar 18 '13 at 20:28
He's trying to assign a prototype to an existing object, a subject that has been discussed here ad nauseum. We have `__proto__` and we narrowly missed having a horribly awkward triangle operator. We have `Object.create()` as a workaround, and its predecessor, the "prototype dance." Those are pretty much the only options, it's not going to change. — Dagg Nabbit, Mar 18 '13 at 20:36
@GGG unfortunately, no. get and set are ECMA5 compliant. `__proto__` is not. — Andrew Eisenberg, Mar 18 '13 at 20:36
@AndrewEisenberg ah, I see, IE 9 and 10 have get and set don't they... I realize `__proto__` is not in ES5, but I was thinking that `get` and `set` only worked in some subset of browsers that are not IE. — Dagg Nabbit, Mar 18 '13 at 20:39

score 2 · Accepted Answer · answered Mar 18 '13 at 20:33

2

Think the other way around:

Create a new instance of MyObj and then copy the properties of instance to it. Or give MyObj a constructor that does that, when you provide it with an argument:

function MyObj(instance) {
 if (instance) {
   //copy properties

This way you may be even able to do

var instance = new MyObj(JSON.parse(result));

answered Mar 18 '13 at 20:33

a better oliver

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I like this idea. I will try it out. However, I also need to be able to support a different kind of constructor, like this: new MyObj(prop1, prop2, prop3); But I should be able to overload the constructor. – Andrew Eisenberg Mar 18 '13 at 20:38
this was my thought as well.. beat me to the punch! – Brian Vanderbusch Mar 18 '13 at 20:45
@AndrewEisenberg: if prop1 is not an object you can check its type. If it is an object, then you know it's the instance. – a better oliver Mar 18 '13 at 20:48
In the end, my solution most closely looks like yours and so I am giving you the correct answer. I decided to go with a separate revive() function defined on MyObj. I don't like the idea of overloading constructors and functions in JS. – Andrew Eisenberg Mar 18 '13 at 23:55

Dagg Nabbit · Answer 2 · 2013-03-18T21:36:46.077

1

You could try using a JSON reviver function.

function revive(json) {

  var m = new MyObj(), y;

  return JSON.parse(json, function(k, v){ 
    if (!y) y = this; 
    return k == "" ? m : this == y ? m[k] = v : v 
  });

}

Call revive('... a json string ...'), it will spit out an instance of MyObj with the properties defined in the JSON string.

Caveat: this will only work if the first item in your JSON object is a primitive value. If that's not possible in your case, here's a rather ugly workaround:

function revive(json) {

  var m = new MyObj(), c = json.charAt(0), y, a;

  if (c == '[') {
    a = json = '[0,' + json.substring(1);
  } else if (c == '{') {
    json = '{"@":0,' + json.substring(1);
  }

  return JSON.parse(json, function(k, v){ 
    if (!y) { y = this; return; } 
    return k == "" ? m : this == y ? m[a ? k - 1 : k] = v : v 
  });

}

edited Mar 18 '13 at 21:36

answered Mar 18 '13 at 20:53

Dagg Nabbit

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Nice. I had no idea about reviver functions. – Andrew Eisenberg Mar 18 '13 at 21:39
Yeah, this sounded good when I started writing it, but really you might as well just do the usual `JSON.parse` and then copy the properties over. This is more of a curiosity than anything, I guess. – Dagg Nabbit Mar 18 '13 at 21:42
In the end, I didn't want to use a revive function. Although a neat idea, I wanted to encapsulate the converting to MyObj in the MyObj code, and not where we get the json string. Thanks for the answer anyway. – Andrew Eisenberg Mar 18 '13 at 23:56

What is the portable way of setting prototype property after an object is created?

2 Answers2