promise的终极面试题，打印顺序你绝对想不到

new Promise(resolve => {
  resolve(2);
  new Promise(resolve => {
    resolve(1);
  }).then(res => {
    console.log(res);
    return 3;
  }).then(console.log);
}).then(res => {
    console.log(res);
    return 4;
}).then(console.log);