Limiting Concurrrent Execution

parallelPromises.js

async function concurrent(maxConcurrency = 4) {
  const thingsToOperateOn = [];
  const createConcurrentPromise = () => {
    const thing = thingsToOperateOn.shift();
    return new Promise((resolve, reject) => {
      // Do something to thing
      console.log(thing);
    })
      .then(() => {
        createConcurrentPromise();
      })
      .catch(err => {
        console.error(err);
      });
  };

  // Spawn array of concurrent promises
  Array(maxConcurrency)
    .fill(null)
    .map(() => createConcurrentPromise());
}