TypeError：无法读取undefined的属性'emit' [英] TypeError: Cannot read property 'emit' of undefined

问题描述

问题在于，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。

The problem is that Whenever I try to fire "this.io.emit" event a TypeError occurs. It gives only while I write this statement "this.io.emit" inside "socket.on" block otherwise, if I write it outside this block it generates no error.

这是调用其他库的主要server.js文件：

This is the main server.js file to call the other libraries:

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。

I also tried to access "this.users.The length" inside socket.on the statement and it generated the same TypeError: cannot read property length. I have no idea why it is occurring. Please help me to resolve this problem.

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>

推荐答案

的上下文是您代码中的问题。要传递当前上下文，请使用 bind 或箭头函数。在javascript中，这个的值是由你如何调用函数定义的，在你的情况下是 socket 对象。

The context of this is an issue in your code. To pass the current context use bind or the Arrow Function. In javascript, the value of this is defined by how you called the function, which in your case is socket object.

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

P.S。：编辑，现在这段代码工作正常。我想建议下面描述的帖子。

P.S.: Edited, now this code works fine. I would like to suggest the below post described about it.

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）？

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