async-iterables.js

const pipe = (...fns) => {
  let [res] = fns;
  for (let i = 1, n = fns.length; i < n; i++) res = fns[i](res);
  return res;
};

const concat = iter2 => async function* (iter1) {
  yield* await iter1;
  yield* await iter2;
};

const take = n => async function* (iter) {
  let i = 0;
  for await (let v of iter) {
    if (++i < n) {
      yield v;
    } else {
      yield v;
      break;
    }
  }
};

async function* range(from, to, step = 1) {
  let i = from;
  while (i !== to) {
    yield i;
    i += step;
  }
}

function fromEvent(target, eventName) {
  let resolve;
  function setResolve(res) {
    resolve = res;
  }
  function listener(event) {
    resolve({ done: false, value: event });
  }
  return {
    [Symbol.asyncIterator]() {
      target.addEventListener(eventName, listener);
      return {
        next() {
          return new Promise(setResolve);
        },
        return() {
          target.removeEventListener(eventName, listener);
          return Promise.resolve({});
        }
      };
    }
  };
}

const subscribe = (observer = {}) => iter => {
  let terminate = false;

  (async () => {
    try {
      for await (let v of iter) {
        if (terminate) break;
        observer.next && observer.next(v);
      }
      !terminate && observer.complete && observer.complete();
    } catch (err) {
      !terminate && observer.error && observer.error(err);
    }
  })();

  return {
    unsubscribe() {
      terminate = true;
    }
  }
};

const subscription = pipe(
  fromEvent(document, 'click'),
  take(4),
  concat(range(0, 5)),
  subscribe({
    next(val) {
      console.log(val);
    },
    complete() {
      console.log('done!');
    },
    error(err) {
      console.log(err);
    }
  })
);