p3-stream-echo-qjs

QuickJS Preview 3 stream echo

This example uses Jco's componentize-qjs backend to build a JavaScript component that echoes a Preview 3 Component Model stream of messages.

The guest receives and returns Component Model streams as JavaScript async iterables. Each input message is yielded immediately, without buffering the complete stream. A Component Model future resolves with the message count once the input stream is exhausted.

Quickstart

Use Node.js 24 or newer, install dependencies, and run the assertion-backed example:

pnpm install
pnpm run all

The all script:

1. Builds echo.js with jco componentize --backend qjs.
2. Transpiles the component for Node.js.
3. Demonstrates output arriving before input finishes, then cancels another stream and verifies its completion future and input cleanup.
4. Runs test.js for empty input and cancellation before reading or partway through a stream.

The WIT result contains independently consumable stream and future handles:

enum echo-error {
  cancelled,
}

type message-stream = stream<string>;
type completion = future<result<u32, echo-error>>;

record echo-output {
  messages: message-stream,
  completion: completion,
}

The guest adapts an async generator with wit.Stream.from() and uses wit.Future.from(). The types are inferred because the world has only one stream type and one future type. The adapters release their writable endpoints in finally blocks, after pending writes settle. Their readable handles transfer to the host on return.