简述 koa 的中间件原理，手写 koa-compose 代码

[MrSeaWave]

koa中间件的原理：参考

原理：中间件之间通过 next 函数联系，当一个中间件调用 next() 后，会将控制权交给下一个中间件，直到下一个中间件不再执行 next() 时沿路返回，依次将控制权交给上一个中间件。

function koaCompose(middlewares) {
  return function (context, next) {
    function dispatch(i) {
      let fn = middlewares[i];
      if (i === middlewares.length) fn = next;
      if (!fn) return Promise.resolve();
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (e) {
        return Promise.reject(e);
      }
    }

    return dispatch(0);
  };
}

[MrSeaWave]

源码

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]
      if (i === middleware.length) fn = next
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)))
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}