koa里的compose函数实现

function compose(middleList) {
  return function fn(ctx, next) {
    let index = -1;
    return dispatch(0);
    function dispatch(i) {
      index = i;
      let middle = middleList[i];
      if (i === middleList.length) return next;
      if(!middle) return Promise.resolve();
      try {
        return Promise.resolve(middle(ctx, dispatch.bind(null, i + 1)));
      } catch (e) {
        return Promise.reject(e);
      }
    }
  };
}