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?

Coronograph

Sure, you need to define it as configurable. – Photomap

The original definition isn't mine, so I can't do that. The property is defined in an external library. – Coronograph

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; } – Photomap

No. I am trying to inject a bugfix in a library, after it's loaded and before it's execution. – Coronograph

Are you able to run code before that code? Or must it be after? – Minetta

@Oriol: Any time. I just can't change the original code. – Coronograph

@Coronograph by design - unless you can change the original code or override Object.defineProperty itself this is impossible. – Photomap

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. – Jest

Of course like I said you can do something like var oldTest = Test; Test = function(){ var t = new oldTest; t.p = 10; return t;} – Photomap

Answer 1

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;

Minetta

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. – Coronograph

Answer 2

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);

Mitman

This should work, and you could define the property as an argument of Object.create rather than call Object.defineProperty afterwards. jsfiddle.net/Xotic750/j66peg0z – Habana

@Habana Yes, I was just too lazy not to copy/paste so once again I just copy/pasted yours ;) – Mitman