obs-async-iterator.js

function createAsyncIterator() {
  const promises = [];
  const values = [];
  let done = false;

  const subscription = this.subscribe({
    next(value) {
      if (promises.length > 0) {
        promises.shift().resolve({ value, done: false });
      } else {
        values.push(value);
      }
    },
    complete() {
      done = true;
      promises.splice(0).forEach(p => p.resolve({ done }));
    }
  });
  return {
    next() {
      if (values.length > 0) {
        return { value: values.shift(), done: false };
      }
      if (done) {
        return { done };
      }
      return new Promise((resolve, reject) => {
        promises.push({ resolve, reject });
      });
    },
    return() {
      subscription.unsubscribe();
    }
  };
}