Execute batch of promises in series. Once Promise.all is done go to the next batch

An answer from October 2020. Async/await makes it short: only 10 code lines+JSDoc.

/**
 * Same as Promise.all(items.map(item => task(item))), but it waits for
 * the first {batchSize} promises to finish before starting the next batch.
 *
 * @template A
 * @template B
 * @param {function(A): B} task The task to run for each item.
 * @param {A[]} items Arguments to pass to the task for each call.
 * @param {int} batchSize
 * @returns {Promise<B[]>}
 */
async function promiseAllInBatches(task, items, batchSize) {
    let position = 0;
    let results = [];
    while (position < items.length) {
        const itemsForBatch = items.slice(position, position + batchSize);
        results = [...results, ...await Promise.all(itemsForBatch.map(item => task(item)))];
        position += batchSize;
    }
    return results;
}