detailed demo of event priority.

eventPriority.js

(function () {
  setImmediate(function () {
    console.log(6) // run macro-task
  })

  setTimeout(function () {
    console.log(8) // 4ms delay
  }, 0)

  Promise
    .resolve()
    .then(() => {console.log(3)}) // priority after nextTick
    .then(() => {console.log(4)}) // add to current micro-task & run
    .then(() => new Promise(resolve => { // add to current micro-task & run
      process.nextTick(() => { // add to current micro-task & run
        resolve(5)
      })
    }))
    .then((v) => {console.log(v)})
    .then(() => new Promise(resolve => { // add to current micro-task & run
      setImmediate(() => { // add to macro-task, current micro-task clear
        resolve(7) // added after 6, FIFO
      })
    }))
    .then((v) => {console.log(v)})
    .then(() => new Promise(resolve => {
      setTimeout(() => {
        resolve(9) // run after 8, FIFO
      }, 0)
    }))
    .then((v) => {console.log(v)})

  process.nextTick(() => {
    console.log(2) // done main script, run micro-task
  })

  console.log(1) // main script
})()

// queue:
//   macro-task(task):
//     main script > setTimeout > setInterval > setImmediate > I/O > UI rendering
//   micro-task(jobs):
//     process.nextTick > Promise > Object.observe > MutationObserver

// run program in order:
//   script -> micro-task -> macro-task -> micro-task -> macro-task ... -> end

// FYI: https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/
//      http://stackoverflow.com/questions/25915634/difference-between-microtask-and-macrotask-within-an-event-loop-context