PromiseLimit.js

export function PromiseLimit(concurrency, promises) {
  const count = promises.length;
  concurrency = Math.min(concurrency, count);
  let finished = 0;

  const next = (resolve, reject) => {
    let promise = promises.pop();
    if (typeof promise === "undefined") {
      if (finished === count) {
        resolve();
      }
    } else {
      promise().then(
        v => {
          finished++;
          next(resolve, reject);
        },
        reject => reject();
      );
    }
  };

  return new Promise((resolve, reject) => {
    for (let i = 0; i < concurrency; i++) {
      next(resolve, reject);
    }
  });
}