具体案例分析node事件环

time 4s

const fs = require('fs');
Promise.resolve()
    .then(() => {
        console.log(1);
    });
process.nextTick(() => {
    console.log(2);
});
console.log('start');
fs.readFile('1.txt', 'utf-8', () => {
    setTimeout(() => {
        console.log(3);
    }, 0);
    process.nextTick(() => {
        console.log(4);
    });
    setImmediate(() => {
        console.log(5);
    });
    console.log(6);
});
console.log(7);
setTimeout(() => {
    console.log(8);
}, 0);
setImmediate(() => {
    console.log(9);
});
console.log('end');

time 2m59s

const fs = require('fs');
Promise.resolve()
// promise1.then() cb
.then(() => {
  console.log(1);
});
// nextTick1
process.nextTick(() => {
  console.log(2);
});
console.log('start');
// readFile1
fs.readFile('1.txt', 'utf-8', () => {
  // setTimeout2
  setTimeout(() => {
    console.log(3);
  }, 0);
  // nextTick2
  process.nextTick(() => {
    console.log(4);
  });
  // setImmediate2
  setImmediate(() => {
    console.log(5);
  });
  console.log(6);
});
console.log(7);
// setTimeout1 
setTimeout(() => {
  console.log(8);
}, 0);
// setImmediate1
setImmediate(() => {
  console.log(9);
});
console.log('end');

time 4m31s
放入主执行栈中的任务完成，清空，开始执行微任务，主执行栈切换到timers的时候需要清空微任务，从timers切换到check时也需要清空微任务，切换需要清空微任务
每次从主执行栈中切换到别的任务都需要清空为任务，每一个阶段切换都需要清空微任务
走Times、poll、Check之前，要先清空微任务，虽然nextTick1是在 promise1.then() cb后面添加的，但nextTick1比promise先运行

time 9m04s
setTimeout 、setImmediate执行，cb放入执行栈，setImmediate cb可能先放入执行栈，setTimeout cb需要4ms放入执行栈，谁先谁后不一定，io中运行setImmediate一定先运行

setTimeout cb等主执行栈中的任务，清空了，走readfile

time 10m
readfile cb进入主执行栈

打印6，栈中有了任务

继续清空微任务，因为io中的setImmediate cb先执行