编辑代码

function compose (middleware) {
  if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
  for (const fn of middleware) {
    if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
  }

  /**
   * @param {Object} context
   * @return {Promise}
   * @api public
   */

  return function (context, next) {
    // last called middleware #
    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]
      console.log('i: ' + i, next);
      if (i === middleware.length) fn = next
      console.log(fn);
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}

var middleware = []
async function a(ctx, next) {
    console.log(1);
    await next();
    console.log(2);
}

async function b(ctx, next) {
    console.log(3);
    await next();
    console.log(4);
}

async function c(ctx, next) {
    console.log(5);
    await next();
    console.log(6);
}

middleware.push(a,b,c);

var aaa = compose(middleware);

aaa({});