`RpcTarget` properties resolved in runtime is not what the type suggests

[EqualMa]

import { RpcTarget, RpcStub, newMessagePortRpcSession } from "capnweb";

interface MyApi extends RpcTarget {
  myProp: number;
}

class MyApiImpl extends RpcTarget implements MyApi {
  myProp: number = 1;
}

const { port1, port2 } = new MessageChannel();

using stub: RpcStub<MyApi> = newMessagePortRpcSession<MyApi>(port1);

using _sessionFrom2To1 = newMessagePortRpcSession(port2, new MyApiImpl());

const myProp: number = await stub.myProp;

console.log(myProp);
// Expect: 1
//    Got: undefined

const myPropStr: string = await stub.myProp.toString();
console.log(myPropStr);
// Expect: "1"
//    Got: [object Function]

You can test this by running deno test.ts or node test.ts with Node >= 24

[kentonv]

console.log(myProp);
// Expect: 1
//    Got: undefined

Per the docs:

When you send someone an RpcTarget reference, they will be able to call any class method over RPC, including getters. They will not, however, be able to access "own" properties. In precise JavaScript terms, they can access prototype properties but not instance properties. This policy is intended to "do the right thing" for typical JavaScript code, where private members are typically stored as instance properties.

In your code, myProp is an "own" property, and so per the policy, it is hidden. If you want it to be revealed over RPC, you can declare a getter, like:

class MyApiImpl extends RpcTarget implements MyApi {
  myProp_: number = 1;
  get myProp() { return this.myProp_; }
}

You may have noticed that the auto-generated TypeScript types are not following this rule correctly. Unfortunately, I'm not sure if it's possible to express this rule in TS, though if someone has an idea how, I'd love to hear it. In the absence of that, perhaps we could at least provide some sort of runtime warning/error that makes it clearer what's wrong.

const myPropStr: string = await stub.myProp.toString();
console.log(myPropStr);
// Expect: "1"
//    Got: [object Function]

Unfortunately, we cannot support toString() over RPC, because toString() is not allowed to return a promise -- it must return a plain string. This method is special since it is called implicitly whenever you coerce a value to a string. What is happening here is, stub.myProp is an RpcPromise, and you are stringifying the promise itself, which returns [object Function] becasue RpcPromise is callable. (The await in your code does nothing, since toString() isn't returning a promise.)

Perhaps we should override toString() on stubs and promises to at least return some indication that it's a stub or a promise, to at least make it a little clearer what's going on here.