send-to-iter.js

/* global setTimeout */

(async () => {
  const [iter, send, done] = (() => {
    let promise;
    let resolve;
    const queue = [];

    const setPromise = () => {
      promise = new Promise(res => { resolve = res; });
    };

    setPromise();

    const send = (v) => {
      queue.push(v);
      if (queue.length === 1) resolve();
    };

    const DONE = Symbol('done');
    const done = () => { send(DONE); };

    const iterator = async function*() {
      let v;
      while (true) {
        if (!queue.length) await promise;
        v = queue.shift();
        if (v === DONE) break;
        if (!queue.length) setPromise();
        yield v;
      }
    };

    return [iterator(), send, done];
  })();

  setTimeout(() => send(1), 1000);
  setTimeout(() => send(2), 2000);
  setTimeout(() => send(3), 3000);
  setTimeout(done, 4000);
  setTimeout(() => send(4), 5000); // won't print `x: 4` because it's already done

  for await (const x of iter) {
    console.log('x:', x);
  }
})();