promisequeue.js

class PromiseQueue {
  constructor (limit = 20, cb = () => {}) {
    /** @type {{promiseFn: () => Promise, cb: void}[]} */
    this.queue = []
    this.running = 0
    this.limit = limit

    this.cb = cb
  }

  /** 
   * @param {() => Promise} promiseFn 
   * @param {void} cb
  */
  add (promiseFn, cb) {
    if (this.running >= this.limit) {
      this.queue.push({promiseFn, cb})
    } else {
      ++this.running
      promiseFn().then(this.updateQueue(cb))
    }
  }

  printInfo () {
    console.log(`running: ${this.running} queue: ${this.queue.length}`)
  }

  updateQueue (cb) {
    this.printInfo()
    return (data) => {
      --this.running
      cb(data)
      this.printInfo()

      const first = this.queue.shift()
      if (!first && this.running === 0) {
        this.cb()
      }
      if (first && first.promiseFn) {
        ++this.running
        first.promiseFn().then(this.updateQueue(first.cb))
      }
    }
  }
}

// пример использования

const queue = new PromiseQueue(20, () => {
  console.log('finished')
})

for (let i = 0; i < 50; i++) {
  const promise = async function () {
    const a = i;
    return new Promise (res => {
      setTimeout(() => res(a), Math.random()*1000)
    })
  }

  queue.add(promise, (data) => {
    console.log(data)
  })
}