React Native Support

[aaronholla]

First off just wanted to say i love this package and how easy to understand the code is for something so complex. 👏🏽

I was experimenting a bit with running capnweb inside a react-native app.

Surprisingly there was not very much that didn't work from what i could tell.

I did however come across a couple minor issues that i had to patch. Wanted to share those here in hopes that we can upstream something to allow running capnweb in react-native with no patches.

1. Hermes (react-native's JS runtime) does not support Promise.withResolvers
2. Metro gets tripped up on the dynamic import.

Promise.withResolvers

To fix the withResolvers issue I was able to fix this by adding a polyfill (Although a more robust polyfill may be needed for better memory management??, Not entirely sure as i didn't profile):

import { Platform } from 'react-native'

if (Platform.OS !== "web") {
  globalThis.Promise.withResolvers = function () {
    let resolve, reject;
    const promise = new Promise((res, rej) => {
      resolve = res;
      reject = rej;
    });
    return { resolve, reject, promise };
  };
}

Dynamic Imports

For the dynamic imports this PR seems to fix the bundling issues #82

[cmsparks]

I also noticed that Promise.withResolvers also has issues in relatively recent versions of Safari, as it was only introduced in Safari in 2024. That's compounded by the fact that Safari's version is bound to the OS

[kentonv]

I'm adding code to polyfill Promise.withResolvers since that's easy enough, and the dynamic import is already gone in #82. Will have a release with both in the next 24 hours.

Unfortunately I couldn't easily extend the test suite to cover React Native, so I can't really say for sure that these are the only issues. There's a vitest-react-native package, but it seems unmaintained, and it wasn't clear if it actually uses Hermes. An LLM told me that people usually use Jest with React Native, but such tests don't normally actually run under Hermes, so even if the tests were written for Jest, this wouldn't be helpful.

I'd love to accept a PR that extends the test suite to run under Hermes, if someone knows how to do that!

[aaronholla]

I'm also not sure of a easy way to add tests for Hermes here either. The most common setup in react native apps is to test JavaScript against v8 in Jest. To test against Hermes usually e2e tests in a simulator are the norm (which seems a bit overkill for this packages use case).

After a tiny bit of research though i did stumble across a new integration testing tool that react native is using internally that seems like a great candidate for this package.

https://github.com/facebook/react-native/blob/main/private/react-native-fantom/__docs__/README.md

Although it seems its still a bit experimental as they are still working on getting it running inside GitHub CI (only runs inside Meta CI at the moment).

[mhofman]

@leotm added support for Hermes in our stack (endojs/endo#2334) and might have tips on how to test under that engine, but as far as I know, we do only limited smoke testing and do not have a runner.

[kentonv]

Can anyone confirm manually whether 0.2.0 now works in React Native?

[kentonv]

Closing since there's no response. If it still doesn't work in React Native, feel free to open a new issue with details.

[aaronholla]

Sorry for no response that's my fault.

I did create a quick test recently with the latest Expo and the core features worked without any patching needed! Websockets and callbacks called on the server as well.

Thank you for the support on this.