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

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?

Solution

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);
    }
});