Chunk async tasks into batches and process in parallel

batch-async-tasks.js

// dummy async task
function asyncTask(n) {
  return new Promise(resolve => setTimeout(() => resolve(n), 500));
}

// takes a flat array and returns a nested one, eg. chunkArray([1,2,3], 1) ==> [[1],[2],[3]]
function chunkArray(arr, chunkSize) {
  return arr.reduce((aggr, item) => {
    let lastArr = aggr[aggr.length - 1];
    if (lastArr.length < chunkSize) {
      lastArr.push(item);
    } else {
      aggr.push([item]);
    }
    return aggr;
  }, [[]])
}

async function runTasks(arr, concurrency) {
  // chunks it into "jobs"
  let chunks = chunkArray(arr, concurrency);
  
  // process chunks in parallel, proceed to next when done
  for (let chunk of chunks) {
    let taskPromises = chunk.map(n => asyncTask(n).then(console.log));
    // when this is complete, it'll proceed to the next chunk
    await Promise.all(taskPromises);
  }
}

runTasks([1,2,3,4,5,6,7,8,10], 3);