Async Generator Wrapper. Can be used with normal generator functions

ag.js

export function ag(f) {
  const promises = [];
  const g = f();
  return {
    [Symbol.asyncIterator]() {
      return this;
    },
    next(input) {
      if (promises.length > 0) {
        return new Promise((resolve, reject) => {
          promises.push({ input, resolve, reject });
        });
      }
      try {
        const { value, done } = g.next(input);
        if (done) {
          return Promise.resolve({ done: true });
        } else if (value && typeof value.then === "function") {
          value.then(this.resolve.bind(this), this.reject.bind(this));
          return new Promise((resolve, reject) => {
            promises.push({ resolve, reject });
          });
        } else {
          return Promise.resolve({ done, value });
        }
      } catch (e) {
        return Promise.reject(e);
      }
    },
    resolve(input) {
      this.step(() => g.next(input));
    },
    reject(err) {
      this.step(() => g.throw(err));
    },
    step(resume) {
      try {
        const { value, done } = resume();
        if (done) {
          promises.splice(0).forEach(p => p.resolve({ done: true }));
        } else if (value && typeof value.then === "function") {
          value.then(this.resolve.bind(this), this.reject.bind(this));
        } else {
          promises.shift().resolve({ done, value });
          if (promises.length > 0) {
            Promise.resolve(promises[0].input).then(this.resolve.bind(this));
          }
        }
      } catch (e) {
        promises.shift().reject(e);
        promises.splice(0).forEach(p => p.resolve({ done: true }));
      }
    },
    return() {
      g.return();
    },
  };
}