How to convert Node.js async streaming callback into an async generator?

Question

I have a function that streams data in batches via a callback.

Each batch will await the callback function before fetching another batch and the entire function retu

Answer 1

Would it work if there will be typescript solution?

It should handle condition when callback is called faster then promise is resolved a couple of times. Callback can be a method that has this signature callback(error, result, index) It is set to finish when callback is called with no arguments. Usage:

asAsyncOf(this.storage, this.storage.each);

Solution:

function asAsyncOf<T1, T2, T3, T4, Y>(c, fn: { (a: T1, a1: T2, a2: T3, a3: T4, cb: { (err?, res?: Y, index?: number): boolean }): void }, a: T1, a1: T2, a2: T3, a3: T4): AsyncGenerator<Y>
function asAsyncOf<T1, T2, T3, Y>(c, fn: { (a: T1, a1: T2, a2: T3, cb: { (err?, res?: Y, index?: number): boolean }): void }, a: T1, a1: T2, a3: T3): AsyncGenerator<Y>
function asAsyncOf<T1, T2, Y>(c, fn: { (a: T1, a1: T2, cb: {(err?, res?: Y, index?: number): boolean}): void}, a: T1, a1: T2): AsyncGenerator<Y>
function asAsyncOf<T, Y>(c, fn: { (a: T, cb: { (err?, res?: Y, index?: number): boolean }): void }, a: T): AsyncGenerator<Y>
function asAsyncOf<Y>(c, fn: { (cb: {(err?, res?: Y, index?: number): boolean}): void}): AsyncGenerator<Y>
async function* asAsyncOf(context, fn, ...args) {
    let next = (result?) => { };
    let fail = (err) => { };
    let finish = {};
    const items = [];
    let started = true;
    try {
        fn.apply(context, [...args, function (err, result, index) {
            const nextArgs = [].slice.call(arguments, 0);
            if (nextArgs.length === 0) {
                started = false;
                next(finish);
                return true;
            }
            if (err) {
                fail(err);
                return true;
            }
            items.push(result);
            next(result);
        }]);
    } catch (ex) {
        fail(ex);
    }
    while (true) {
        const promise = started ? new Promise((resolve, error) => {
            next = resolve;
            fail = error;
        }) : Promise.resolve(finish);
        const record = await promise;
        if (record === finish) {
            while (items.length) {
                const item = items.shift();
                yield item;
            }
            return;
        }
        while (items.length) {
            const item = items.shift();
            yield item;
        }
    }
}
export { asAsyncOf };

Answer 2

No, I don't think there's a way to implement this transformation in a way that's easy to understand and easy to follow. However, I would recommend to drop the deferreds (you're never rejecting anyway) and just use the promise constructor. Also I'd rather implement an asynchronous generator right away.

function queue() {
    let resolve = () => {};
    const q = {
        put() {
            resolve();
            q.promise = new Promise(r => { resolve = r; });
        },
        promise: null,
    }
    q.put(); // generate first promise
    return q;
}
function toAsyncIterator(callbackStream) {
    const query = queue();
    const result = queue();
    const end = callbackStream(batch => {
        result.put(batch);
        return query.promise;
    }).then(value => ({value, done: true}));
    end.catch(e => void e); // prevent unhandled promise rejection warnings
    return {
        [Symbol.asyncIterator]() { return this; },
        next(x) {
            query.put(x);
            return Promise.race([
                end,
                result.promise.then(value => ({value, done:false})
            ]);
        }
    }
}
async function* batchToAsyncIterator(batchCallbackStream) {
    for await (const batch of toAsyncIterator(batchCallbackStream)) {
        // for (const val of batch) yield val;
        // or simpler:
        yield* batch;
    }
}

Answer 3

You need a event bucket, here is an example:

function bucket() {
  const stack = [],
    iterate = bucket();

  var next;

  async function* bucket() {
    while (true) {
      yield new Promise((res) => {
        if (stack.length > 0) {
          return res(stack.shift());
        }

        next = res;
      });
    }
  }

  iterate.push = (itm) => {
    if (next) {
      next(itm);
      next = false;
      return;
    }

    stack.push(itm);
  }

  return iterate;
}

;
(async function() {
  let evts = new bucket();

  setInterval(() => {
    evts.push(Date.now());
    evts.push(Date.now() + '++');
  }, 1000);

  for await (let evt of evts) {
    console.log(evt);
  }
})();