V8: ES6 proxies don't support iteration protocol when targeting custom objects?

Question

I'm using the V8 API to create JavaScript objects. Some of these objects support iteration by setting up a native (intercepted) function at the Symbol.iterator property.

Iterating such an object via for...of works perfectly. However, if I wrap it in a null proxy (e.g., let x = new Proxy(obj, {});), the resulting object is not iterable and throws a TypeError with the message "Illegal invocation" if an attempt is made to iterate over it.

Wrapping a standard array doesn't exhibit this issue. Is this a V8 bug?

What would you expect to iterate over if you've wrapped it in a null proxy ? — Pogrindis
– Pogrindis, Commented Dec 8, 2016 at 18:11
@Pogrindis I'd expect the proxy to iterate over its target; that's what happens if the target is a standard array. — Andromedary
– Andromedary, Commented Dec 8, 2016 at 18:12

Bergi · Accepted Answer · 2016-12-08 20:04:04Z

8

Wrapping a standard array doesn't exhibit this issue.

Yes, that's how array iterators work. They don't care about the kind of the object they are iterating - they simply access its .length and indexed properties (which are routed normally through the proxy).

However, other standard exotic objects don't behave that nice either. If you try to invoke [Symbol.iterator]() on a typed array, map or set that is wrapped in a proxy, they'll bitch about being invoked on the wrong object.

Is this a V8 bug?

No, it's a bug in the application. You've got three choices:

Create an iterator that does not depend on the internal slots of your custom objects, but rather uses their public (proxy-interceptable) property interface. Make sure your [Symbol.iterator] method does not typecheck its receiver.
Check the type of the receiver in your iterator method, and if it is a proxy (i.e. has a [[ProxyTarget]] internal slot) then use that value. I would strongly advise against this, as it does not match the standard behaviour and breaches the proxy when bypassing the handler.

Don't use a null proxy:

let x = new Proxy(obj, {
    get(target, key, receiver) {
       if (key === Symbol.iterator)
           return target[Symbol.iterator].bind(target);
       else
           return Reflect.get(target, key, receiver);
    }
});

answered Dec 8, 2016 at 20:04

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Andromedary Over a year ago

Thanks for your answer, @Bergi. You're right; the problem is that my [Symbol.iterator] method expects its holder to have certain private fields, which aren't accessible when the holder is a proxy. Quick follow-up question: Do you know why proxy[Symbol.iterator]() throws an exception without invoking my interceptor, whereas it = proxy[Symbol.iterator]; it(); invokes the interceptor and crashes as expected?

Bergi Over a year ago

Both of those should invoke the get handler if that is what you mean by "interceptor"? I can't imagine why it would not.

Andromedary Over a year ago

Sorry, let me try to clarify. I'm not specifying a custom get handler; the method is stored as a normal property on the target and appears to be retrieved correctly in both cases. What I don't understand is why the first syntax throws an exception without calling the method, while the second syntax calls the method and crashes as expected. The method itself is a native function whose invocation causes a callback into native (C++) code, but that should be irrelevant AFAICT.

Bergi Over a year ago

The obvious difference is the receiver (proxy vs undefined), which would be the value of the this keyword in a js function, but I have no idea how native functions are affected by it. Apparently there's some guard that prevents your custom function from running, but I don't know about V8 details. Does it happen on non-proxy objects as well?

Andromedary Over a year ago

Right again; the difference appears to be somewhere deep within V8's function-call machinery. It looks like x.call(y) fails early if x is a native function and y is a proxy.

Collectives™ on Stack Overflow

V8: ES6 proxies don't support iteration protocol when targeting custom objects?

1 Answer 1

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related