Bug: Unhandled errors aren't translated to WebSocket error event

[sfc-gh-jtjhin]

The impact is that it's not possible to write tests for websocket.onerror.

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 18 or higher

Node.js version

v22

Reproduction repository

https://github.com/joshua-tj/msw-ws-onerror-test/

test("Connection should error", () =>
  new Promise((done, fail) => {
    const api = ws.link("wss://example.com/path");

    const server = setupServer(
      api.addEventListener("connection", () => {
        console.log("connecting");
        throw new Error("Connection error");
      })
    );
    server.listen();

    const connection = new WebSocket("wss://example.com/path");
    connection.onerror = (e) => {
      console.log("onerror", e);
      done();
    };

    connection.onclose = (e) => {
      console.log("onclose", e);
    };

    connection.onopen = () => {
      console.log("onopen");
      fail("Connection should not be opened");
    };
  }));

Reproduction steps

yarn
yarn test

Current behavior

Errors and fails tests as onerror handler is not called

Expected behavior

Should not fail since we handle errors and call done() in test

[kettanaito]

Hi, @sfc-gh-jtjhin. Thanks for opening this one.

I think it's a bug in MSW. We currently don't support erroring the connection so your test fails unexpectedly. This should be handled in Interceptors.

1. If an error is thrown before the connection is established, dispatch the error event and never open the connection.
2. If an error is thrown while the connection is open, consult the spec because I have no idea what the client expects in this case.

The check if the connection has errored is likely to be placed here.

[kettanaito]

Note that you can still close the pending connection via client.close(), but that never dispatches the error event, which is correct per spec.

[kettanaito]

Update

Hold on, I can see that we do have the try/catch for unhandled exceptions over here:

https://github.com/mswjs/interceptors/blob/51924bd01bf9e517b2bf123f34b5d992d4c27622/src/interceptors/WebSocket/index.ts#L137

Technically, that should result in the error event being dispatched. So it seems the thrown error isn't caught by this try/catch?

We are definitely missing tests for this, so adding some test cases would be much appreciated!

[sfc-gh-jtjhin]

Yes so I tried putting some console logs before and after line 102 and it doesn't error when the error is thrown from the listener.
So I believe the error is being eaten by emit? Are all the listeners run synchronously?

[kettanaito]

@sfc-gh-jtjhin, thanks for confirming that. Yes, the listeners should run synchronously as usual.

Could you please also try logging the error in the catch closure here https://github.com/mswjs/interceptors/blob/51924bd01bf9e517b2bf123f34b5d992d4c27622/src/interceptors/WebSocket/index.ts#L129? This is where I'd expect it to turn up.