TypeError:无法读取undefined的属性'emit'

Question

问题是,每当我尝试触发"this.io.emit"事件时,就会发生TypeError.它仅在我在"socket.on"块内写入此语句"this.io.emit"时给出,否则,如果我在该块之外写入它则不会产生错误.

这是调用其他库的主要server.js文件:

const express = require('express'),
http = require('http'),
socketio = require('socket.io');

class App{

constructor()
{
    this.port =  process.env.PORT || 81;
    this.host = `localhost`;        
    this.app = express();
    this.http = http.Server(this.app);
    this.socket = socketio(this.http);
}
appConfig(){
    new config(this.app);
}
appRoutes(){
    new routes(this.app,this.socket).routesDefault();
}
appDefault(){
    this.appConfig();
    this.appRoutes();
    this.http.listen(this.port,this.host,()=> {
        console.log(`Listening`);
    });
}}

我的服务器端代码是:

'use strict';
class Routes {

constructor(app,socket) {
    this.app = app;
    this.io = socket;
    this.users=[];
}

routesTemplate()
{
    this.app.get('/',function(req,res){
        res.render('index');
    });
}

socketEvents()
{
    this.io.on('connection',(socket) => {
        socket.on('send message',function(data)
        {
            this.io.emit('new message',data);//here the error lies.
        });
    }); 
}
routesDefault()
{
    this.routesTemplate();
    this.socketEvents();
}}  
module.exports = Routes;

我还尝试在socket.on语句中访问"this.users.The length"并生成相同的TypeError:无法读取属性长度.我不知道它为什么会发生.请帮我解决这个问题.

客户端:

        <script>
        $(function($){
            var socket = io.connect();
            var $messageForm = $('#send-message');
            var $messageBox = $('#message');
            var $chat = $('#chat');

            $messageForm.submit(function(e){
                e.preventDefault();
                socket.emit('send message',$messageBox.val());
                $messageBox.val("");
            });
            socket.on('new message',function(data){
                $chat.append(data+ "<br/>");
            });
        });
    </script>

Answer 1

上下文this是您的代码中的问题.传递当前上下文使用bind或Arrow函数.在javascript中,值this是由您调用函数的方式定义的,在您的情况下,该函数是socket对象.

socketEvents()
{
    this.io.on('connection',(socket) => {
        socket.on('send message',function(data)
        {
            this.io.emit('new message',data);//here the error lies.
        }bind(this));
    }); 
}

PS:编辑,现在这段代码工作正常.我想建议下面描述的帖子.

ES6箭头函数和与Function.prototype.bind绑定的函数之间有什么区别(如果有的话)？