Nuking misleading properties in `Object.getOwnPropertyDescriptor`

Herby Vojčík herby at mailbox.sk
Sun Mar 10 04:37:47 PDT 2013


Yes, this was in this list some three-four months ago. There were 
answers like "JS is this way dynamic, playing with Object.prototype hurts".

I proposed that return value from Object.getOwnPropertyDescriptor should 
inherit from well well-known frozen object with all the default set. 
Back, then no-one seemed to be interested.

Herby

Nathan Wall wrote:
> Given that `defineProperty` uses properties on the prototype of the descriptor[1] and `getOwnPropertyDescriptor` returns an object which inherits from `Object.prototype`, the following use-case is volatile:
>
>      function copy(from, to) {
>          for (let name of Object.getOwnPropertyNames(from))
>              Object.defineProperty(to, name,
>                  Object.getOwnPropertyDescriptor(from, name));
>      }
>
> If a third party script happens to add `get`, `set`, or `value` to `Object.prototype` the `copy` function breaks.
>
>      Object.prototype.get = function() { };
>
>      var x = { foo: 1 };
>      var y = { };
>      copy(x, y); // TypeError: property descriptors must not specify a value or
>                  // be writable when a getter or setter has been specified
>
> It's misleading that you can't trust `getOwnPropertyDescriptor` to give you a descriptor which you can use to define a property with `defineProperty`.
>
> A script which doesn't use `defineProperty` could very well define a `get` property on `Object.prototype`, unknowingly breaking other scripts which do use `defineProperty`. This is a hazard in the language.
>
> I think one of the following would be an appropriate solution:
>
> (1) "Nuke" the special properties (`get`, `set`, and `value` when any of them is not defined) on a descriptor returned by `getOwnPropertyDescriptor` which shouldn't be inherited through the descriptor's prototype. By "nuke" them, I mean specify that they be defined as `undefined`, much like `callee` and `caller` in strict mode. (I don't think strict mode is required in this case, though, since I highly doubt anyone could be relying on this behavior.)
>
> (2) Use the introduction of `Reflect.defineProperty` as an opportunity to "fix" this function to only inspect own properties on the property descriptor so that the descriptor's prototype doesn't matter. This is similar to the addition of `Number.isNaN` to fix the broken `isNaN`.
>
> Nathan
>
>
> [1] https://mail.mozilla.org/pipermail/es-discuss/2012-November/026705.html 		 	   		
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