理解Promise的执行顺序

1. 优先执行Promise实例化函数中的内容
2. then中的函数被加入微任务队列
3. 执行到resolve后，当前上下文执行完成后，会立刻执行下一个then中的内容

执行结果为序号顺序

new Promise(resolve => {
  resolve();
})
  .then(() => {
    new Promise(resolve => {
      console.log(100);
      resolve();
      console.log(101);
    })
      .then(() => {
        console.log(111);
      })
      .then(() => {
        console.log(222);
      });
  })
  .then(() => {
    new Promise(resolve => {
      resolve();
    })
      .then(() => {
        new Promise(resolve => {
          resolve();
        })
          .then(() => {
            console.log(444);
          })
          .then(() => {
            console.log(666);
          });
      })
      .then(() => {
        console.log(555);
      });
  })
  .then(() => {
    console.log(333);
  });