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

# Michael Theriot (4 years ago)

I feel like it should, or I am misunderstanding something fundamental. I made a basic scenario to explain:

var arrays = new WeakMap();

function ArrayView(array) {
  arrays.set(this, array);

  return new Proxy(this, {
    set: (target, property, value) => {
      if(arrays.has(this) && property in arrays.get(this)) {
        arrays.get(this)[property] = value;
      } else {
        target[property] = value;
      }
    },
    get: (target, property) => {
      if(arrays.has(this) && property in arrays.get(this)) {
        return arrays.get(this)[property];
      } else {
        return target[property];
      }
    },
    has: (target, property) => {
      return (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:

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) => {
    if(arrays.has(receiver) && property in arrays.get(receiver)) {
      arrays.get(receiver)[property] = value;
    } else {
      Reflect.set(target, property, value, receiver);
    }
  },
  get: (target, property, receiver) => {
    if(arrays.has(receiver) && property in arrays.get(receiver)) {
      return arrays.get(receiver)[property];
    } else {
      return Reflect.get(target, property, receiver);
    }
  },
  has: (target, property) => {
    return Reflect.has(target, property); // no receiver to check weak map!!
  }
});

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.

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
[...a]; // [0, 1]
a[0] = 1;
a[1] = 0;
[...a]; // [1, 0]

var b = new ArrayView2(arr);
b.arrayLength; // 2
'arrayLength' in b; // true
'0' in b; // false
'1' in b; // false
'2' in b; // false
// we can still get/set indices despite 'in' check failing
[...b]; // [1, 0]
b[0] = 0;
b[1] = 1;
[...b]; // [0, 1]

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.

# Tom Van Cutsem (4 years ago)

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 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>:

# Michael Theriot (4 years ago)

I'm trying to make the proxy-as-a-prototype pattern work but I've just discovered the ownKeys trap is never called on proxies 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.

# Andrea Giammarchi (4 years ago)

Agreed with everybody else the receiver is always needed and Proxy on the prototype makes way more sense than per instance.

Also the getPrototypeOf trap is really pointless right now

function Yak() {}
Yak.prototype = new Proxy(Yak.prototype, {
  getPrototypeOf: (target) => console.log('lulz')
});

var yup = new Yak;
Object.getPrototypeOf(yup);

The target is actually the original Yak.prototype which is already the yup prototype: useless trap if used in such way.

Being also unable to distinguish between getOwnPropertyNames vs keys is a bit weird.

Proxy looks so close to be that powerful but these bits make it kinda useless for most real-world cases I've been recently dealing with.

Thanks for any sort of improvement.

# Allen Wirfs-Brock (4 years ago)

On Mar 18, 2016, at 9:24 AM, Andrea Giammarchi <andrea.giammarchi at gmail.com> wrote:

Agreed with everybody else the receiver is always needed and Proxy on the prototype makes way more sense than per instance.

I don’t agree. While you certainly can imagine a language where each object’s “prototype” determines that object’s fundamental behaviors and provides the MOP intercession hooks(in fact that’s how most class-based languages work). But that’s not the JS object model. Each JS object is essentially a singleton that defines it’s own fundamental behaviors. Whether or this model is better or worse than the class-based model isn't really relevant, but in the context of JS there are advantage to consistently adhering to that model,

For example, in Michael’s desired approach, the instance objects of his ArrayView abstraction are “ordinary objects”. One of the fundamental behavioral characteristics of ordinary objects is that all of there own properties are defined and available to the implementation in a standard way. Implementations certainly make use of that characteristic for optimization purposes. Michael’s approach would make such optimizations invalid because every time an own property needed to be access a prototype walk would have to be performed because there might be an exotic object somewhere on the prototype chain that was injecting own property into the original “receiver”.

Michael’s preferred approach also introduces observable irregularity into the standard JS inheritance model for ordinary objects.

Consider an object created using Michael’s preferred approach:

var arr = [0, 1];
console.log(Reflect.has(arr,”0”));  //true, arr has “0” as an own property
var subArr = Object.create(arr);
console.log(Reflect.has(subArr,”0”));  //true, all unshadowed properties of proto visible from ordinary objects

var b = new ArrayView2(arr);
console.log(Reflect.has(b,”0”));  //true, prototype proxy makes array elements visible as if properties of b
var subB= Object.create(b);
console.log(Reflect.has(subB,”0”));  //false, some unshadowed properties of proto is not visible from subB

Note the his original Proxy implementation does not have this undesirable characteristic.

So what about the use of receiver in [[Get]]/[[Set]]. That’s a different situation. [[Get]]/[[Set]] are not fundamental, rather they are derived (they work by applying other more fundamental MOP operations). The receiver argument is not used by them to perform property lookup (they use [[GetOwnProperty]] and [[GetPrototypeOf]]) for the actual property lookup). receiver is only used in the semantics of what happens after the property lookup occurs. Adding a receiver argument to the other MOP operations for the purpose of changing property lookup semantics seems like a step too far. The ES MOP design is a balancing act between capability, implementability, and consistency. I think adding receiver to every MOP operation would throw the design out of balance.

Finally,

Note that we are not really talking about a new capability here. Michael’s first design shows that ES proxies already have the capability to implement the object level semantics he desires. So, we are only talking about exactly how he goes about using Proxy to implement that semantics. He would prefer a different Proxy design than what was actually provided by ES6. But that isn’t what was specified or what has now been implemented. We can all imagine how many JS features might be “better” if they worked somewhat differently. But that generally isn’t an option. The existing language features and their implementations are what they are and as JS programmers we need to work within that reality.

# Mark S. Miller (4 years ago)

I agree with Allen. I am certainly willing -- often eager -- to revisit and revise old design decisions that are considered done, when I think the cost of leaving it alone exceeds the cost of changing it. In this case, the arguments that this extra parameter would be an improvement seem weak. Even without the revising-old-decision costs, I am uncertain which decision I would prefer. Given these costs, it seems clear we should leave this one alone.

Unless it turns out that the cost of leaving it alone is much greater than I have understood. If so, please help me see what I'm missing.

# Andrea Giammarchi (4 years ago)

AFAIK the reason there is a receiver is to deal with prototype cases ... if that was a good enough reason to have one, every prototype case should be considered for consistency sake.

We've been advocating prototypal inheritance for 20 years and now it's an obstacle or "not how JS is"?

class Magic extends new Proxy(unbe, lievable) {
  // please make it happen
  // as it is now, that won't work at all
}

Best

# Michael Theriot (4 years ago)

Michael’s preferred approach also introduces observable irregularity into the standard JS inheritance model for ordinary objects. Consider an object created using Michael’s preferred approach:

var arr = [0, 1];
console.log(Reflect.has(arr,”0”));  //true, arr has “0” as an own property
var subArr = Object.create(arr);
console.log(Reflect.has(subArr,”0”));  //true, all unshadowed properties
of proto visible from ordinary objects
var b = new ArrayView2(arr);
console.log(Reflect.has(b,”0”));  //true, prototype proxy makes array
elements visible as if properties of b
var subB= Object.create(b);
console.log(Reflect.has(subB,”0”));  //false, some unshadowed properties
of proto is not visible from subB

I think this relates to the original concern; if you could pass the receiver this could be resolved. That still leaves getOwnPropertyNames reporting wrong values though and I see no foreseeable way to resolve that without returning an actual proxy itself.

The reason I'm trying this approach is because I read on the MDN that when used in the prototype a receiver argument is passed that references the instance, so I assumed this was the intent behind it. The only other explanation I could think of is that proxies have a receiver to mimic the Reflect.set/Reflect.get methods which need a receiver for getters/setters to work properly, not so you can use them on the prototype chain.

The other case I would make is every instance would have an identical proxy, and it just makes sense to put that on the prototype for the same reasons you put shared methods/properties there.

Note that we are not really talking about a new capability here. Michael’s first design shows that ES proxies already have the capability to implement the object level semantics he desires.

To be fair I had several obstacles with inheritance using the first version.

var wm1 = new WeakMap();

function A() {
  wm1.set(this, {});
  return new Proxy(this, {});
}

var wm2 = new WeakMap();

function B() {
  A.call(this);
  wm2.set(this, {});
  return new Proxy(this, {});
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // false
wm2.has(b); // true

As you can see storing a reference to this can't work anymore, since we actually return a proxy. You can try to work around this...

var wm1 = new WeakMap();

function A() {
  let self = this;
  if(new.target === A) {
    self = new Proxy(this, {});
  }
  wm1.set(self, {});
  return self;
}

var wm2 = new WeakMap();

function B() {
  let self = this;
  if(new.target === B) {
    self = new Proxy(this, {});
  }
  A.call(self);
  wm2.set(self, {});
  return self;
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // true
wm2.has(b); // true

But then problems arise because the new proxy doesn't go through the old proxy. So anything guaranteed by A()'s proxy is not guaranteed by B()'s proxy.

var wm1 = new WeakMap();

function A() {
  let self = this;
  if(new.target === A) {
    self = new Proxy(this, {
      get: (target, property, receiver) => property === 'bacon' || target[property]
    });
  }
  wm1.set(self, {});
  return self;
}

var wm2 = new WeakMap();

function B() {
  let self = this;
  if(new.target === B) {
    self = new Proxy(this, {
      get: (target, property, receiver) => property === 'ham' || target[property]
    });
  }
  A.call(self);
  wm2.set(self, {});
  return self;
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // true
wm2.has(b); // true

a.bacon; // true
a.ham; // undefined

b.bacon; // undefined
b.ham; // true

(I'm open to solutions on this particular case... One that doesn't require me to leak the handler of the A proxy)

Ultimately I can actually achieve both what I want with the ArrayView example and inheritance by using a lot of defineProperty calls on this in the constructor, but performance is a disaster as you might expect.

# Michael Theriot (4 years ago)

I think I figured out how to make inheritance work...

var wm1 = new WeakMap();

function A() {
  let proxy = new Proxy(this, {
    get: (target, property, receiver) => property === 'bacon' || target[property]
  });
  wm1.set(proxy, {});
  return proxy;
}

var wm2 = new WeakMap();

function B() {
  let proxy = A.call(new Proxy(this, {
    get: (target, property, receiver) => property === 'ham' || target[property]
  }));
  wm2.set(proxy, {});
  return proxy;
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // true
wm2.has(b); // true

a.bacon; // true
a.ham; // undefined

b.bacon; // true
b.ham; // true

I can't imagine this is good for optimizations though. But I guess it does what I need.

Edit:

Actually I seem to have it backwards... I believe the correct way would be the following:

function B() {
  let proxy = new Proxy(A.call(this), {
    get: (target, property, receiver) => property === 'ham' || target[property]
  });
  wm2.set(proxy, {});
  return proxy;
}
# Tom Van Cutsem (4 years ago)

I was on board with potentially making has() receiver-dependent but you lost me when you expected getOwnPropertyNames to trigger a trap on a proxy-as-prototype. Adding receiver to every MOP method is a no-go. It fundamentally changes the meaning of these operations and it would destroy the pay-only-when-you-use-it performance model of proxies, since operations that used to be local to only the 'own' object would now need to search the proto-chain for a potential proxy trap.

Using a proxy-as-prototype was never intended as a way to be able to intercept arbitrary operations on whatever object happened to inherit from the proxy. A proxy-as-prototype still emulates only a single object. It just happens to be an object that serves as a prototype for other objects.

Cheers, Tom

2016-03-18 23:18 GMT+01:00 Michael Theriot <michael.lee.theriot at gmail.com>:

# Michael Theriot (4 years ago)

To be clear, I'm not suggesting behavior like getOwnPropertyNames be overridden by anything on the prototype, just a way to use proxies without having to instantiate identical copies that all use the same handler.

I still believe a proxy on the prototype should always have a receiver sent to each trap, but if I need to return a proxy for each object it doesn't really matter for the example I made.

# Tom Van Cutsem (4 years ago)

2016-03-19 0:15 GMT+01:00 Michael Theriot <michael.lee.theriot at gmail.com>:

To be clear, I'm not suggesting behavior like getOwnPropertyNames be overridden by anything on the prototype, just a way to use proxies without having to instantiate identical copies that all use the same handler.

If you can already reuse the same handler object between multiple proxies, you're in good shape. There should be very little overhead in creating many proxy objects that share the same handler. A proxy object is just an empty object with a hidden reference to its handler.

Besides, if you want to reliably proxy multiple objects, you must instantiate a proxy per object anyway, otherwise you may get in trouble with object-identity. A single proxy object that proxies many different target objects would have only 1 object identity, so all the proxied targets would appear to be identity-equal.

# Michael Theriot (4 years ago)

That is good news then. I think I have the right expectations of proxies now.

Sharing one handler is easy too. All you need to do is map both the proxy and its target to the same data. receiver is actually the proxy but the argument is no longer important here.

var priv = new WeakMap();

var handler = {
  get: (target, property, receiver) => property in priv.get(target)
    ? priv.get(target)[property]
    : target[property],
  set: (target, property, value, receiver) => property in priv.get(target)
    ? priv.get(target)[property] = value
    : target[property] = value,
  has: (target, property) => property in priv.get(target) || property in target
};

function A() {
  let proxy = new Proxy(this, handler);
  let store = {secret: 4};
  priv.set(this, store).set(proxy, store);
  return proxy;
}

A.prototype.getSecret = function () {
  return priv.get(this).secret;
};

var a = new A();
a.getSecret(); // 4
a.secret; // 4
a.secret = 5;
a.secret; // 5
a.getSecret(); // 5
'secret' in a; // true

(sorry for any dupes, new to mailing lists...)

# #!/JoePea (24 days ago)

I really thing that because has is about detecting inherited properties, the receiver parameter should be included. For things like ownKeys, which are not about inheritance, then yeah, let's not add receiver there.

I'm trying to implement my own multiple inheritance, but now I stumbled on how to make it send back true for inherited keys when I need to fork the lookup based on instances that are WeakMaped to the receiver.

# #!/JoePea (24 days ago)

After messing with Proxy-on-prototypes for two days, I've just come to the conclusion that I probably need to have Proxies on this (the receiver) returned from constructors to achieve what I want. At least, it's much easier to code it that way. I think it'd be nice to have receiver on all inheritance-related traps. That might make some things easier.

# Cyril Auburtin (23 days ago)

It's not answering your issue with Proxy but more about multiple inheritance

It can be solved in a static way: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Mix-ins

Concrete example here: pepabo/gmopg/blob/master/src/gmopg.ts#L10

# #!/JoePea (23 days ago)

thanks for pointing that out! I know about those, I've been using class-factory mixins for a while now. But the problem with them is having to wrap all your classes in a function, which gets much more painful in TypeScript with all the type-annotation boilerplate that is required.

For example, here's a boilerplate-heavy mixin of mine:

Multiple inheritance with Proxies actually turns out very easy to type in TypeScript, and a lot more convenient than class factories. So, given a set of regular classes,

class One {
  doOne() { /* ... */ }
}

class Two {
  doTwo() { /* ... */ }
}

class Three {
  doThree() { /* ... */ }
}

a Proxy-based multiple inheritance system makes the composition super easy, clean, and simple. We can convert the previous example into:

import mix from './mix'

class One {
  doOne() { /* ... */ }
}

class Two {
  doTwo() { /* ... */ }
}

class Three extends mix(One, Two) {
  doThree() { /* ... */ }
  doAll() {
    this.doOne()
    this.doTwo()
    this.doThree()
  }
}

All without touching the original source of the One and Two classes. How convenient, and easier to read and look at!

My particular implementation will allow to call individual constructors with specific args, unlike the class-factory mixins. For example,

import mix from './mix'

class One {
  constructor(arg1, arg2) {/*...*/}
  doOne() { /* ... */ }
}

class Two {
  constructor(arg3) {/*...*/}
  doTwo() { /* ... */ }
}

class Three extends mix(One, Two) {
  constructor(arg1, arg2, arg3) {
    this.callConstructor(One, arg1, arg2)
    this.callConstructor(Two, arg3)
  }
  doThree() { /* ... */ }
  doAll() {
    this.doOne()
    this.doTwo()
    this.doThree()
  }
}

At the moment I'm looking to convert from my Proxies-on-prototypes implementation to Proxies-on-instances so that things like in operator will work, and implementation will be simpler and easier. Otherwise if the has trap had a receiver parameter, then I could stick with the Proxies-on-prototypes version which would be more efficient (the Proxy would be re-used on the prototype chain instead of being created once per instance).

Would it be worth adding a receiver parameter to the has trap in the specs? Seems like it would be totally backwards compatible, because current code that doesn't rely on it could continue not relying on it oblivious to the new parameter.

All the best,

# #!/JoePea (23 days ago)

I forgot to add, this even works with builtin scenarios like Custom Elements,

class MyEl extends mix(HTMLButtonElement, One, Two, Three) { /* ... */ }
customElements.define('my-el', MyEl)