javascript - V8: ES6 proxies don't support iteration protocol when targeting custom objects? - Stack Overflow

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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?

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?

• javascript
• proxy
• ecmascript-6
• v8

Share Improve this question Follow edited Dec 8, 2016 at 20:09 Bergi 666k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Dec 8, 2016 at 18:09 AndromedaryAndromedary 3,55811 gold badge1818 silver badges2121 bronze badges 2

• What would you expect to iterate over if you've wrapped it in a null proxy ? – 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 Commented Dec 8, 2016 at 18:12

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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);
      }
  });
  

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