setTimeout(() => {
  console.log('settimeout');
})
Promise.resolve().then(() => {
  console.log('promise1');
  Promise.resolve().then(() => {
    console.log('promise2');
    Promise.resolve().then(() => {
      console.log('promise3');
    })
  })
})
console.log('main');
// 输出：mian promise1 promise2 promise3 settimeout

// 第一个先进I/O
setTimeout(() => {
  Promise.resolve().then(() => {
    console.log('promise');
  })
})
Promise.resolve().then(() => {
  // 第二个进I/O
  setTimeout(() => {
    console.log('settimeout');
  })
})
console.log('main');
// 输出：mian promise settimeout

const fs = require('fs');
const path = require('path');
function someAsync(callback) {
  // 假设花费两毫秒
  fs.readFile(__dirname + '/' + __filename, callback);
}

const timeoutScheduled = Date.now();

let fileReadTime = 0;

setTimeout(() => {
  const delay = Date.now() - timeoutScheduled;
  console.log('setTimeout: ' + delay + 'ms have passed since i was scheduled');
  console.log('fileReadTime', fileReadTime - timeoutScheduled);
}, 10)

someAsync(function () {
  fileReadTime = Date.now();
  while (Date.now() - fileReadTime < 20) {

  }
})
// 输出：
//    setTimeout: 22ms have passed since i was scheduled
//    fileReadTime 2

// I/O -> poll -> timers
/* 
  1、setTimeout-callback、someAsync-callback进入I/O；
  2、当到达2ms，将someAsync-callback添加进poll，此时会阻塞poll；
  3、当到达22ms，会将setTimeout-callback添加进timers，再执行
  注意：因为poll的阻塞，导致setTimeout-callback原本应该在10ms后执行，结果却是22ms后执行
*/

setTimeout(() => {
  console.log('timeout');
})
setImmediate(() => {
  console.log('immed');
})
// 以上执行顺序不确定

fs.readFile('./test.js', () => {
  setTimeout(() => {
    console.log('timeout');
  })
  setImmediate(() => {
    console.log('immed');
  })
})
// 以上执行顺序：immed -> timeout

/*
  解释：
    1、因为在nodejs中setTimeout(callback, 0) === setTimeout(callback, 1)，
      而浏览器setTimeout(callback, 0) === setTimeout(callback, 4)；
      因为0ms是同步的；
    2、然后因为event loop启动也是需要时间的，所以有时候耗时1ms，有时候不需要1ms
*/

const fs = require('fs');

fs.readFile('./test.js', () => {
  setTimeout(() => {
    console.log('setTimeout');
  })
  setImmediate(() => {
    console.log('setImmediate');
    process.nextTick(() => {
      console.log('nextTick3');
    })
  })
  process.nextTick(() => {
    console.log('nextTick1');
  })
  process.nextTick(() => {
    console.log('nextTick2');
  })
})
// nextTick1 nextTick2 setImmediate nextTick3 setTimeout
// poll(fs.readFile) -> nextTick1、nextTick2 ->check(setImmediate) -> nextTick3 -> timers(setTimeout)