Promise 对象一章关于 finally 的实现的问题

[bsc2xp]

文中对于 finally 的实现给出了示例：

Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};

这里为什么要使用 P.resolve(callback())?写成下面不行吗？

return this.then(
value => {try {callback();} catch(err){}; return value},
reason => {try {callback();} catch(err){}; throw reason }
);

[Ruan YiFeng]

此处不答疑