看到很多同学都有这个错误，好像都说是模块导出错误，具体怎么改还是不甚清楚，报错如下：

如下是我的代码：

//app.js
'use strict'

var Koa = require('koa')
var path = require('path')
var wechat = require('./wechat/g')
var util = require('./libs/util')
var wechat_file = path.join(__dirname,'./config/wechat.txt')
var config = {
    wechat:{
        appID:'wxfeac53b7ad50b5dc',
        appSecret:'d7912572793e386070d20a396790f951',
        token: 'freecodecamplxk',

        getAccessToken: function(){
        	return util.readFileAsync(wechat_file)
        },
        saveAccessToken: function(data){
        	data = JSON.stringify(data)
        	return util.writeFileAsync(wechat_file,data)
        }
    }
}

var app = new Koa()

app.use(wechat(config.wechat))

app.listen(1234)
console.log('Listening : 1234')

//wechat目录下的g.js
'use strict'

var sha1 = require('sha1')
var getRawBody = require('raw-body')
var Wechat = require('./wechat')
var util = require('./util')



module.exports = function(opts){
    var wechat = new Wechat(opts)

    return function *(next){

        // console.log('wechat1')
        // console.log(this.query)

        var that = this
        var token = opts.token
        var signature = this.query.signature
        var nonce = this.query.nonce
        var timestamp = this.query.timestamp
        var echostr = this.query.echostr

        var str = [token,timestamp,nonce].sort().join('')
        var sha = sha1(str)

        if(this.method === 'GET'){
            if(sha === signature){
                this.body = echostr + ''
                // console.log('123456');
            }
            else{
                this.body = 'wrong'
            }
        }
        else if(this.method === 'POST'){
            if(sha !== signature){
                this.body = 'wrong'
                return false
            }
            // console.log('123');
            var data = yield getRawBody(this.req,{
                length: this.length,
                limit: '1mb',
                encoding: this.charset
            })

            console.log('test')
            console.log(data.toString())
            // parseXMLAsync 是为了把 XML 解析为 JS 对象
            var content = yield util.parseXMLAsync(data)

            console.log(content)
            console.log('123')
            // formatMessage 是为了把 JS 对象解析为扁平的 JS 对象
            var message = util.formatMessage(content.xml)

            console.log(message)
            console.log('lxk');

            if(message.MsgType === 'event'){
                if(message.Event === 'subscribe'){
                    var now = new Date().getTime()

                    that.status = 200
                    that.type = 'application/xml'
                    that.body = '<xml>' + 
                    '<ToUserName><![CDATA[' + message.FromUserName + ']]></ToUserName>' + 
                    '<FromUserName><![CDATA[' + message.ToUserName +']]></FromUserName>' + 
                    '<CreateTime>' + now + '</CreateTime>' +
                    '<MsgType><![CDATA[text]]></MsgType>' + 
                    '<Content><![CDATA[Hi, Imooc同学！！]]></Content>' +
                    '</xml>'

                    return
                }
            }
        }
    }
}

//wechat目录下的util.js
'use strict'

var xml2js = require('xml2js')
var Promise = require('bluebird')

exports.parseXMLAsync = function(xml){
	return new Promise(function(resolve, reject){
		xml2js.parseString(xml, {trim: true}, function(err,content){
			if (err) reject(err)
			else resolve(content)
		})
	})
}

function formatMessage(result){
	var message = {}

	if(typeof result === 'object'){
		var keys = Object.keys(result)

		for(var i = 0; i < keys.length; i++){
			var item = result[keys[i]]
			var key = key[i]

			if(!(item instanceof Array) || item.length === 0){
				continue
			}

			if(item.length ===1){
				var val = item[0]

				if(typeof val === 'object'){
					message[key] = formatMessage(val)
				}
				else{
					message[key] = (val || '').trim()
				}
			}
			else{
				message[key] = []

				for(var j = 0, k = item.length; j < k; j++){
					message[key].push(formatMessage(item[j]))
				}
			}
		}
	}
	return message
}

exports.formatMessage = function(xml){
	console.log('456');
	return new Promise(function(resolve, reject){
		xml2js.parseString(xml, {trim: true}, function(err,content){
			if (err) reject(err)
			else resolve(content)
		})
	})
}

// exports.formatMessage = formatMessage