PriorityJobQueue.js

const { EventEmitter } = require('events')

class PriorityJobQueue extends EventEmitter {
  constructor (levels, ...rest) {
    super(...rest)
    this.queues = new Array(levels).fill([])
    this.currentJob = null
    this.totalJobs = 0
  }

  get length () {
    return this.queues.reduce((ax, dx) => ax + dx.length, 0)
  }

  push (job, priority = 0) {
    const id = ++this.totalJobs
    this.queues[priority].push({ id, job })
    this.checkQueue()
    return id
  }
  
  checkQueue () {
    if (!this.shouldRun) return
    for (const queue of this.queues) {
      if (queue.length > 0) {
        this.currentJob = queue.shift()
        break
      }
    }
    this.run()
  }

  get shouldRun () {
    return !this.currentJob && this.length > 0
  }

  async run () {
    const { id, job } = this.currentJob

    try {
      const data = await job()
      this.emit('jobSuccess', id, data)
    } catch (error) {
      this.emit('jobError', id, error)
    }

    this.currentJob = null
    this.checkQueue()
  }
}