koa-compose解析

/**
 * Compose `middleware` returning
 * a fully valid middleware comprised
 * of all those which are passed.
 *
 * @param {Array} middleware
 * @return {Function}
 * @api public
 */

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 // 初始化index = -1, i = 0 , i > index
    return dispatch(0)
    function dispatch (i) {
    
    
      // i比index大1
      if (i <= index) return Promise.reject(new Error('next() called multiple times'))
      index = i // index = 0、1、2、3、4、5...middleware.length
      let fn = middleware[i] // fn = middleware[0]... fn=[middleware[middleware.length]] fn是一个async await异步函数
      if (i === middleware.length) fn = next // ? next是undefined 最后一次遍历,说明中间件函数执行完了，返回 Promise.resolve()
      if (!fn) return Promise.resolve()
      try {
    
    
      	// Promise.resolve(middleware[0](context, dispatch.bind(null, i + 1)))
      	// middleware[0](context, dispatch(i + 1)) 1、2、3、4、5...middleware.length
      	// promise队列
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (err) {
    
    
        return Promise.reject(err)
      }
    }
  }
}