Functional event loops

eracer.ts

const target = new EventTarget();

setInterval(() => {
  target.dispatchEvent(new Event("event-a"));
}, 3333);

setInterval(() => {
  target.dispatchEvent(new Event("event-b"));
}, 4444);

setInterval(() => {
  target.dispatchEvent(new Event("event-c"));
}, 5555);

const main = (tick: number) => {
  const controller = new AbortController();
  Promise.race([
    ofEvent("event-a", target, controller.signal),
    ofEvent("event-b", target, controller.signal),
    ofEvent("event-c", target, controller.signal)
  ])
    .then((event: Event) => {
      console.log(`Tick ${tick}: ${event.type}`);
    })
    .finally(() => {
      controller.abort();
      main(tick + 1);
    });
};
main(0);

function ofEvent(type: string, target: EventTarget, signal: AbortSignal) {
  return new Promise<Event>((resolve) => {
    target.addEventListener(type, resolve, { once: true, signal });
  });
}

ereader.ts

const target = new EventTarget();

setInterval(() => {
  target.dispatchEvent(new Event("event-a"));
}, 3333);

setInterval(() => {
  target.dispatchEvent(new Event("event-b"));
}, 4444);

setInterval(() => {
  target.dispatchEvent(new Event("event-c"));
}, 5555);

const eStream = new ReadableStream<Event>(
  eventSource([
    ["event-a", target],
    ["event-b", target],
    ["event-c", target],
  ])
);

const main = (tick: number, reader: ReadableStreamDefaultReader<Event>) => {
  reader
    .read()
    .then((result) => {
      console.log(`Tick ${tick}: ${result.value?.type}`);
    })
    .finally(() => {
      main(tick + 1, reader);
    });
};
main(0, eStream.getReader());

function eventSource(
  sources: [type: string, target: EventTarget][]
): UnderlyingSource<Event> {
  const abortController = new AbortController();
  return {
    cancel() {
      abortController.abort();
    },
    start(controller: ReadableStreamDefaultController) {
      const listener = (event: Event) => {
        controller.enqueue(event);
      };
      sources.forEach(([type, target]) => {
        target.addEventListener(type, listener, {
          signal: abortController.signal,
        });
      });
    },
  };
}