promise_batcher.ts

type PromiseFactory = () => Promise<any>;
function promiseAllBatched(
  promiseFactories: PromiseFactory[],
  batchSize: number
): Promise<any> {
  let completed = 0;
  let upNext = 0;
  let results: any[] = [];

  const initialBatchSize = Math.min(batchSize, promiseFactories.length);

  return new Promise((resolve, reject) => {
    function onPromiseCompletion(index: number, result: any): void {
      completed += 1;
      results[index] = result;

      if (completed == promiseFactories.length) {
        // Done Case
        resolve(results);
        return;
      }

      if (upNext < promiseFactories.length) {
        promiseFactories[upNext]().then(
          onPromiseCompletion.bind(null, upNext),
          onPromiseCompletion.bind(null, upNext)
        );

        upNext += 1;
      }
    }
    if (batchSize <= 0 || typeof batchSize !== "number") {
      reject(new Error("Invalid batch size, please supply a positive number"));
    } else {
      for (let i = 0; i < initialBatchSize; i++) {
        promiseFactories[i]().then(
          onPromiseCompletion.bind(null, i),
          onPromiseCompletion.bind(null, i)
        );
      }

      upNext = initialBatchSize;
    }
  });
}

function waitRandomTimeSecondsReturnFive(): Promise<number> {
  return new Promise((resolve) => {
    setTimeout(() => resolve(5), 1000 + Math.floor(Math.random() * 1000));
  });
}

promiseAllBatched(Array(1000).fill(waitRandomTimeSecondsReturnFive), 100).then(
  console.log
);