JobsHoldThread.js

const pool = [0,1,2,3] // 4 threads available
while (pool.length && queue.length) {
    var id = pool.pop();
    promises.push(new Promise((...) => {
       ...
       worker.on("exit", (...) => {
          pool.push(id)
          resolve()
       })
       ...
    })
}