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

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;
    }
}