Understanding promise.race() usage

Question

As far as I know, there are two options about promise:

• promise.all()

• promise.race()

Ok, I know what promise.all()<

Answer 1

I've used it for request batching. We had to batch tens of thousands of records into batches for a long running execution. We could do it in parallel, but didn't want the number of pending requests to get out of hand.

Race lets us keep a fixed number of parallel promises running and add one to replace whenever one completes

const _ = require('lodash')

async function batchRequests(options) {
    let query = { offset: 0, limit: options.limit };

    do {
        batch = await model.findAll(query);
        query.offset += options.limit;

        if (batch.length) {
            const promise = doLongRequestForBatch(batch).then(() => {
                // Once complete, pop this promise from our array
                // so that we know we can add another batch in its place
                _.remove(promises, p => p === promise);
            });
            promises.push(promise);

            // Once we hit our concurrency limit, wait for at least one promise to
            // resolve before continuing to batch off requests
            if (promises.length >= options.concurrentBatches) {
                await Promise.race(promises);
            }
        }
    } while (batch.length);

    // Wait for remaining batches to finish
    return Promise.all(promises);
}

batchRequests({ limit: 100, concurrentBatches: 5 });