Node.js 网络框架koa compose中间件使用解析

// sendHandle.js
const sendHandle = () => {
    // 处理请求成功方法
    const render = ctx => {
        return (data, msg = '请求成功') => {
            ctx.set('Content-Type', 'application/json');
            ctx.body = {
                code: '000001',
                data,
                msg
            }
        }
    }
    // 处理请求失败方法
    const renderError = ctx => {
        return (code, msg = '请求失败') => {
            ctx.set('Content-Type', 'application/json');
            ctx.body = {
                code,
                data: null,
                msg
            }
        }
    }
    return async (ctx, next) => {
        ctx.send = render(ctx);
        ctx.sendError = renderError(ctx);
        await next();
    }
}
module.exports = sendHandle;

'use strict'
/**
 * Expose compositor.
 */
module.exports = compose
/**
 * 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
    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)
      }
    }
  }
}