实现 compose 函数, 类似于 koa 的中间件洋葱模型

// 题目需求
let middleware = [];
middleware.push((next) => {
  console.log(1);
  next();
  console.log(1.1);
});
middleware.push((next) => {
  console.log(2);
  next();
  console.log(2.1);
});
middleware.push((next) => {
  console.log(3);
  next();
  console.log(3.1);
});

let fn = compose(middleware);
fn();

/*
1
2
3
3.1
2.1
1.1
*/

//实现compose函数
function compose(middlewares) {}

/**
 * Compose `middleware` returning
 * a fully valid middleware comprised
 * of all those which are passed.
 *
 * @param {Array} middleware
 * @return {Function}
 * @api public
 */
function compose(middleware) {
  // 校验传入的参数是数组，校验数组中每一项是函数
  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) {
      // 一个函数中多次调用报错
      // await next()
      // await next()
      if (i <= index)
        return Promise.reject(new Error("next() called multiple times"));
      index = i;
      // 取出数组里的 fn1, fn2, fn3...
      let fn = middleware[i];
      // 最后 相等，next 为 undefined
      if (i === middleware.length) fn = next;
      // 直接返回 Promise.resolve()
      if (!fn) return Promise.resolve();
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err);
      }
    }
  };
}