Proxy handler.has() does not have a receiver argument?

Michael Theriot michael.lee.theriot at gmail.com
Fri Mar 18 13:54:14 UTC 2016


I'm trying to make the proxy-as-a-prototype pattern work but I've just
discovered the `ownKeys` trap is never called on traps on the prototype. So
even if the `has` trap is allowed to see the `receiver`, and thus verify
the properties "0", "1" exist, this pattern would fail to return the
properties "0", "1" exist on an `Object.getOwnPropertyNames` call.
Disappointing! I'd rather use a proxy on the prototype than create one for
each instance but without a correct `ownKeys` return it just doesn't come
full circle. Is there a trick to make this work or am I out of luck here? I
can only think of actually defining the properties to make it work, which
defeats the idea of using a proxy on the prototype to begin with.

Regardless I agree that traps called on a prototype chain should always
receive the `receiver` as an argument. I think the only trap other than
`set`, `get`, and `has` that can do this is the `getPrototypeOf` trap
(currently does not have a `receiver`) when the `instanceof` check needs to
climb the prototype chain.

On Thu, Mar 17, 2016 at 6:29 PM, Tom Van Cutsem <tomvc.be at gmail.com> wrote:

> The rationale for not having a `receiver` argument to `has` is that the
> value produced by the "in" operator is not normally dependent on the
> receiver. This is in contrast with `get` and `set` which may find an
> accessor up the proto chain that needs to run with a `this` bound to the
> receiver.
>
> That said, I follow your line of reasoning and it is true that `has`,
> `get` and `set` are the three traps that can be called on a
> proxy-used-as-prototype (now that `enumerate` is considered deprecated), so
> it would be consistent to allow all of them to  refer back to the original
> receiver. This enables the general pattern that you illustrate.
>
> As you note, the weirdness of this is apparent because it doesn't normally
> make sense to pass a `receiver` argument to Reflect.has(). However, if
> `receiver` would be made visible in a Proxy handler's `has` trap, then
> `Reflect.has` should nevertheless be likewise extended so that one can
> faithfully forward the `receiver` argument.
>
> Spec-wise, I think the only significant change is that 7.3.10 HasProperty
> <http://www.ecma-international.org/ecma-262/6.0/#sec-hasproperty>, step 3
> must be changed to `O.[[HasProperty]](P, O)` and all [[HasProperty]]
> internal methods must likewise be extended with an extra argument (which
> they ignore). Only the Proxy implementation in 9.5.7 would then actually
> refer to that argument.
>
> Cheers,
> Tom
>
> 2016-03-17 11:46 GMT+01:00 Michael Theriot <michael.lee.theriot at gmail.com>
> :
>
>> I feel like it should, or I am misunderstanding something fundamental. I
>> made a basic scenario to explain:
>>
>> ```js
>> var arrays = new WeakMap();
>>
>> function ArrayView(array) {
>>   arrays.set(this, array);
>>
>>   return new Proxy(this, {
>>     set: (target, property, value) => (arrays.has(this) && property in
>> arrays.get(this))  ? arrays.get(this)[property] = value : target[property]
>> = value,
>>     get: (target, property)        => (arrays.has(this) && property in
>> arrays.get(this))  ? arrays.get(this)[property]         : target[property],
>>     has: (target, property)        => (arrays.has(this) && property in
>> arrays.get(this)) || property in target
>>   });
>> }
>>
>> ArrayView.prototype = Object.create(Array.prototype, {
>>   arrayLength: {
>>     get() {
>>       return arrays.get(this).length;
>>     }
>>   }
>> });
>> ```
>>
>> When `new ArrayView(somearray)` is called the reference to `somearray` is
>> stored in the `arrays` weak map and a proxy is returned that allows you to
>> manipulate indices on it, or fallback to the object for other properties.
>>
>> This could be simplified by putting the proxy on the prototype chain to
>> reduce overhead and actually return a genuine `ArrayView` object instead:
>>
>> ```js
>> var arrays = new WeakMap();
>>
>> function ArrayView2(array) {
>>   arrays.set(this, array);
>> }
>>
>> var protoLayer = Object.create(Array.prototype, {
>>   arrayLength: {
>>     get() {
>>       return arrays.get(this).length;
>>     }
>>   }
>> });
>>
>> ArrayView2.prototype = new Proxy(protoLayer, {
>>   set: (target, property, value, receiver) => (arrays.has(receiver) &&
>> property in arrays.get(receiver))  ? arrays.get(receiver)[property] = value
>> : Reflect.set(target, property, value, receiver),
>>   get: (target, property, receiver)        => (arrays.has(receiver) &&
>> property in arrays.get(receiver))  ? arrays.get(receiver)[property]
>> : Reflect.get(target, property, receiver),
>>   has: (target, property)                  => (arrays.has(target)   &&
>> property in arrays.get(target))   || Reflect.has(target, property)
>> });
>> ```
>>
>> Under this setup `target` refers to the protoLayer object which is
>> useless here, but we can use the `receiver` argument in its place to access
>> the weak map, and replace our set/get operations with
>> Reflect.set/Reflect.get calls to the target (protoLayer) using a receiver
>> (the instance) to pass the correct `this` value to the `arrayLength` getter
>> and prevent infinite recursion.
>>
>> One problem - handler.has() lacks a receiver argument. So in this
>> scenario when using the `in` operator it will always fail on array
>> properties because we cannot check the weak map by passing in the instance.
>>
>> ```js
>> var arr = [0, 1];
>>
>> var a = new ArrayView(arr);
>> a.arrayLength; // 2
>> 'arrayLength' in a; // true
>> '0' in a; // true
>> '1' in a; // true
>> '2' in a; // false
>>
>> var b = new ArrayView2(arr);
>> b.arrayLength; // 2
>> 'arrayLength' in b; // true
>> '0' in b; // false
>> '1' in b; // false
>> '2' in b; // false
>> ```
>>
>> Without a receiver argument on handler.has(), it is practically useless
>> for proxies used as a prototype. You can't reference the instance calling
>> it and your target is simply the parent prototype.
>>
>> Is there a reason the handler.has() trap should not obtain the receiver
>> when used on the prototype chain? I can understand why Reflect.has()
>> wouldn't have a receiver argument (that wouldn't make sense) but this seems
>> like a legitimate use for it. Otherwise I don't see a reason to use the
>> handler.has() trap at all on prototype proxies except for bizarre behaviors
>> that have nothing to do with the instance. It will always have the same
>> behavior across all instances since you can't differentiate them.
>>
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>>
>>
>
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