Waiting for an empty queue

promise-queue.ts

const queue: Set<Promise<unknown>> = new Set()

const queueEmpty = async (): Promise<void> => {
  await Promise.all(queue)

  if (queue.size) {
    await queueEmpty()
  }
}

const addPromiseToQueue = async (promise: Promise<unknown>): Promise<void> => {
  queue.add(promise)
  await promise
  queue.delete(promise)
}

const test = async () => {
  const asyncTask = (delay: number, duration: number): Promise<void> =>
    new Promise((resolve) =>
      setTimeout(() => {
        console.log(`${delay}+${duration}ms task complete`)
        resolve()
      }, duration),
    )

  const addTaskToQueue = (delay: number, duration: number): void => {
    if (delay) {
      setTimeout(() => addPromiseToQueue(asyncTask(delay, duration)), delay)
    } else {
      addPromiseToQueue(asyncTask(0, duration))
    }
  }

  addTaskToQueue(0, 400)
  addTaskToQueue(200, 400)
  addTaskToQueue(0, 800)
  addTaskToQueue(600, 2000)

  await queueEmpty()

  console.log('all tasks complete')
}

test()

// output:
// 0+400ms task complete
// 200+400ms task complete
// 0+800ms task complete
// 600+2000ms task complete
// all tasks complete