我一直收到这个错误消息：/socket.io/socket.io.js 404 (Not Found) Uncaught ReferenceError: io is not defined
我的代码是：
var express = require('express'), http = require('http');
var app = express();
var server = http.createServer(app);
var io = require('socket.io').listen(server);
server.listen(3000);
和
<script src="/socket.io/socket.io.js"></script>
问题是什么？
欢迎任何帮助！
把 socket.io.js 复制到公共文件夹（例如 resources/js/socket.io.js）并不是正确的做法。
如果 Socket.io 服务器正确监听了你的 HTTP 服务器，它会自动通过 http://localhost:<端口号>/socket.io/socket.io.js 提供客户端文件，你不需要手动查找或复制到公开可访问的文件夹中，如 resources/js/socket.io.js 并进行手动服务。
代码示例
Express 3.x -
Express 3 要求你先实例化一个 http.Server，然后才能附加上 socket.io
var express = require('express')
, http = require('http');
//make sure you keep this order
var app = express();
var server = http.createServer(app);
var io = require('socket.io').listen(server);
//...
server.listen(8000);
编程愉快 :)
app.listen(3000)。然而这并没有起作用。当我将其改为server.listen(3000)时，它就开始正常工作了。 - edufinnnpm install socket.io
寻找socket.io客户端
find ./ | grep client | grep socket.io.js
结果：
./node_modules/socket.io/node_modules/socket.io-client/dist/socket.io.js
将socket.io.js复制到您的资源文件夹中：
cp ./node_modules/socket.io/node_modules/socket.io-client/dist/socket.io.js /home/proyects/example/resources/js/
<script type="text/javascript" src="resources/js/socket.io.js"></script>
看起来这个问题可能从未得到答复（尽管对于提问者来说可能为时已晚），但我会回答它以供未来遇到此类问题的人使用解决方案。
你需要执行 npm install socket.io --save 而不是 npm install socket.io，这样socket.io模块就会安装在你的web开发文件夹中（在基础位置/索引.html或index.php所在位置运行此命令）。这将使socket.io安装在命令运行的区域，而不是全局，并且还会自动更正/更新您的package.json文件，以便node.js知道它的存在。
然后将源路径从'/socket.io/socket.io.js'改为'http://' + location.hostname + ':3000/socket.io/socket.io.js'。
您可能会想知道/socket.io/socket.io.js文件是从何处来的，因为我们既没有添加它，也没有在文件系统中存在。这是io.listen在服务器上完成的魔法的一部分。它在服务器上创建一个处理程序，用于提供socket.io.js脚本文件。
来自书籍《Socket.IO实时Web应用开发》，第56页
res.sendFile(__dirname + '/index.html');。请确保您的 index.html 文件在正确的目录中。你只需按照 https://socket.io/get-started/chat/ 的步骤操作，一切都会正常工作。
var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);
http.listen(3000, function(){
console.log('listening on *:3000');
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.0.1/socket.io.min.js" integrity="sha512-eVL5Lb9al9FzgR63gDs1MxcDS2wFu3loYAgjIH0+Hg38tCS8Ag62dwKyH+wzDb+QauDpEZjXbMn11blw8cbTJQ==" crossorigin="anonymous"></script>
调试步骤
npm install socket.io --save in static files (index.html) for example, you may have installed it globally and when you look at the debugger, the file path is empty.
Change your script file and instantiate the socket explicitly adding your localhost that you have set up in your server file
<script src="http://localhost:5000/socket.io/socket.io.js"></script>
<script>
const socket = io.connect("localhost:5000");
$(() =>
Double check that the data is flowing by opening a new browser tab and pasting http://localhost:5000/socket.io/socket.io.js you should see the socket.io.js data
Double check that your server has been set-up correctly and if you get a CORs error npm install cors then add it to the server.js (or index.js whatever you have chosen to name your server file)
const cors = require("cors");
const http = require("http").Server(app);
const io = require("socket.io")(http);
Then use the Express middleware app.use() method to instantiate cors. Place the middleware this above your connection to your static root file
app.use(cors());
app.use(express.static(__dirname));
As a final check make sure your server is connected with the http.listen() method where you are assigning your port, the first arg is your port number, for example I have used 5000 here.
const server = http.listen(5000, () => {
console.log("your-app listening on port", server.address().port);
});
As your io.on() method is working, and your sockets data is connected client-side, add your io.emit() method with the callback logic you need and in the front-end JavaScript files use the socket.on() method again with the call back logic you require. Check that the data is flowing.
我还编辑了上面的一条评论，因为它对我最有用 - 但是我需要采取一些额外的步骤来使客户端-服务器连接正常工作。
app.use(express.static(path.join(__dirname, '/')))
/ 并发送 index.html 文件。app.get('/', function (req, res) {
res.sendFile(__dirname + '/index.html')
})
index.html中，您有一个脚本标签(<script src="/socket.io/socket.io.js"></script>)请求了socket.io-client的资源，但在index.js中没有路由它(可以在控制台网络中找到url为http://localhost:3000/socket.io/socket.io.js)。<script src="/socket.io/socket.io.js"></script> 在客户端上可用。const path = require('path');
const express = require('express');
const socketio = require('socket.io');
const port = 3001 || process.env.PORT;
const app = express();
const server = http.createServer(app);
const io = socketio(server);
//MiddleWares
app.use(express.json());
app.use(
express.urlencoded({
extended: false,
})
);
app.use(express.static(__dirname + '/public'));
app.get('/', (req, res) => {
res.sendFile('index.html');
});
io.on('connect', (socket) => {
console.log('New user joined');
}
server.listen(port, () => {
console.log(`App has been started at port ${port}`);
});
在执行以下命令后运行您的服务器文件：
node index.js
然后打开localhost:${port}，将端口替换为index.js文件中给定的端口并运行它。
这解决了我的问题。希望它也能解决你的问题。
http://localhost:3000/socket.io/socket.io.js，试试看。 - robertklep