Cloudflare Workers and TransformStream: "The script will never generate a response"

runtime.md

I'm not sure if this a bug per-se, but I encountered a Worker runtime behavior that wasn't reproducible in Miniflare.

tl;dr Using TransformStream will often trigger a runtime error: "The script will never generate a response"

---

My workers library, Keywork, has a JSXResponse class that accepts some React JSX and returns an instance of Response. This works fine when the JSX is first rendered to a string and then fed to Response's body, since this is really just a convenience wrapper around a few functions:

const app = new WorkerRouter()

app.get('/', () => {
  return new JSXResponse(
    <div>
      <h1>Look! I'm rendering JSX</h1>
    </div>
  )
})

However, refactoring JSXResponse to use React's streaming renderer was not so obvious. JS constructors must be synchronous, and renderToReadableStream is async.

Ideally, a Keywork developer wouldn't have to change their code to take advantage of streaming. So, being the very clever web developer I am, I wrote something like this:

class JSXResponse extends Response {
  constructor(jsx: React.ReactElement, ...omittedArgs: any[]) {
    const passThroughStream = new TransformStream()

    super(passThroughStream.readable)

    pipeReactRenderToStream(passThroughStream, jsx)
  }
}

function pipeReactRenderToStream(passThroughStream: TransformStream, jsx: React.ReactElement) {
  const readableStreamFromReact = await renderToReadableStream(jsx)
  readableStreamFromReact.pipeTo(passThroughStream.writable)
}

This works fine in Miniflare, but throws an exception once deployed: "The script will never generate a response." Maybe it's the case that the Workers runtime isn't checking if TransformStream is still pending. I couldn't find an example of this behavior on the docs or in Discord, so this might help if someone else later runs into this bug