提取了部分koa-compose代码，并作了修改，如下：

class koa {
  constructor() {
    this.middleware = [];
  }

  listen() {
    const cb = this.compose(this.middleware);
    const x = cb();
    console.log(x);
  }

  use(fn) {
    this.middleware.push(fn);
  }

  compose(middleware) {
    return function () {
      let index = -1;     
      return dispatch(0);

      function dispatch(i) {
        if (i <= index) return Promise.reject(new Error('next() called multiple times'));
        index = i;
        let fn = middleware[i];
        if (!fn) return Promise.resolve();
        try {
          return Promise.resolve(fn(function next() {
            return dispatch(i + 1);
          }));
        } catch (error) {
          return Promise.reject(error);
        }
      }
    };
  }
}

const ko = new koa();

ko.use(async next => {
  console.log(1);
  await next();
  console.log(5);
});

ko.listen();

output:

1
Promise { pending }
5

问题： 1、 为什么输出是1 promise { pending } 5 , 而不是1 5 promise 2、

          return Promise.resolve(fn(function next() {
            return dispatch(i + 1);
          }));

能不能改为

          return fn(function next() {
            return dispatch(i + 1);
          });

根据MDN ： The Promise.resolve(value) method returns a Promise object that is resolved with the given value. if the value was a promise, that object becomes the result of the call to Promise.resolve; 那么既然这个fn是async函数，且async函数返回的是promise.resovle()，那岂不是可以去掉外面这一层Promise.resolve？

大佬们，need help help ~~~