前端 Promise 问题?

Question

请大佬帮忙分析一下，以下代码执行顺序(正确的顺序是：a 1 2 3 b 4 c 5)：

Promise.resolve()
  .then(() => {
    console.log('a');
    return new Promise(res => res());
  })
  .then(() => {
    console.log('b');
  })
  .then(() => {
    console.log('c');
  });
Promise.resolve()
  .then(() => {
    console.log(1);
  })
  .then(() => {
    console.log(2);
  })
  .then(() => {
    console.log(3);
  })
  .then(() => {
    console.log(4);
  })
  .then(() => {
    console.log(5);
  });

Answer 1

参考
https://segmentfault.com/q/1010000039387146
https://www.zhihu.com/question/453677175

记住return Promise.resolve()、 return new Promise(resolve => { reolve() }) 与 return new Promise(resolve => { reolve() }).then(res => {return res})这三个返回值会卡两次