How do I redefine a property from a prototype?

Question

How do I remove the property p from the object's prototype?

var Test = function() {};
Object.defineProperty(Test.prototype, 'p', {
  get: function () { return 5; }
});

Object.defineProperty(Test.prototype, 'p', {
  get: function () { return 10; }
});

This produces TypeError: Cannot redefine property: p. Is there a way I can remove the property and re-add it? Or is it possible to set the configurable attribute after the property was created?

The original definition isn't mine, so I can't do that. The property is defined in an external library. — stackular, Jan 29 '15 at 15:31
Can you extend the prototype chain one level deeper (inherit from Test and put your property in the middle) or override it on the instance (that is do `function MyTest(){ t = new Test(); t.p = ...; return t; }` — Benjamin Gruenbaum, Jan 29 '15 at 15:32
No. I am trying to inject a bugfix in a library, after it's loaded and before it's execution. — stackular, Jan 29 '15 at 15:37
Are you able to run code before that code? Or must it be after? — Oriol, Jan 29 '15 at 15:37
@stackular by design - unless you can change the original code or override `Object.defineProperty` itself this is impossible. — Benjamin Gruenbaum, Jan 29 '15 at 15:42
The point of having configurable being toggle-able and in defineProperty is to prevent what you're trying to do. It exists to prevent a property from being changed or removed. — Bjorn, Jan 29 '15 at 15:43
Of course like I said you can do something like `var oldTest = Test; Test = function(){ var t = new oldTest; t.p = 10; return t;}` — Benjamin Gruenbaum, Jan 29 '15 at 15:43

Oriol · Accepted Answer · 2015-01-29T15:50:18.113

If you are able to run code before the code you want to avoid, you can try hijacking Object.defineProperty to prevent adding that property:

var _defineProperty = Object.defineProperty;
Object.defineProperty = function(obj, prop, descriptor) {
    if(obj != Test.prototype || prop != 'p')
        _defineProperty(obj, prop, descriptor);
    return obj;
};

Or you can make it configurable, to be able to modify it later:

var _defineProperty = Object.defineProperty;
Object.defineProperty = function(obj, prop, descriptor) {
    if(obj == Test.prototype && prop == 'p')
        descriptor.configurable = true;
    return _defineProperty(obj, prop, descriptor);
};

At the end, you can restore the original one:

Object.defineProperty = _defineProperty;

This is a *terrible* solution, but it is the only one I've found and it seems to work fine, so I'll go with it. — stackular, Jan 30 '15 at 11:25

plalx · Answer 2 · 2015-01-29T16:35:37.720

1

Have you tried something like this? It would have to run before new Test instances are created though.

var Test = function () {};

Object.defineProperties(Test.prototype, {
    p: {
        get: function () {
            return 5;
        }
    },

    a: {
        get: function () {
            return 5;
        }
    }
});

Test.prototype = Object.create(Test.prototype, {
    p: {
        get: function () {
            return 10;
        }
    }
});

var t = new Test();

console.log(t.a, t.p);

edited Jan 29 '15 at 16:35

answered Jan 29 '15 at 15:45

plalx

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This should work, and you could define the property as an argument of [`Object.create`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create) rather than call `Object.defineProperty` afterwards. http://jsfiddle.net/Xotic750/j66peg0z/ – Xotic750 Jan 29 '15 at 15:59
@Xotic750 Yes, I was just too lazy not to copy/paste so once again I just copy/pasted yours ;) – plalx Jan 29 '15 at 16:35

How do I redefine a property from a prototype?

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