Socket.IO双火

Question

I am using Socket.IO to send data from my client to my node.js server. When I receive the data from the client I am simply logging it. However the data logs two times where I only expect it to log once?

Here is the client side code

btn.addEventListener("click", (e) => {
    socket.emit("comparison", "hello");
 });

Here is the server-side code

app.get("/", (req, res) => {
  io.on("connection", (socket) => {
   socket.on("comparison", (data) => {
    console.log(data);  
    })
 })

It logs 'hello hello'

Any idea as to what this is?

UPDATE

I still haven't fixed the problem although I've noticed when I take the socket code outside of the express route code there is only one log

Answer 1

You shouldn't put io.on('connection') inside app.get('/'). What you are doing here is registering an event listener on every get request. The reason why you are seeing two statements is directly related to the number of requests handled inside your app.get().

Take a look at the example provided in the socket.io site https://socket.io/docs/#Using-with-Express

I have added a simillar boilerplate of the code here,

const express        = require('express');
const bodyParser     = require('body-parser');
const app            = express();
const port           = 8000;

var server = require('http').createServer(app);
var io = require('socket.io')(server);

app.use(express.static(__dirname + '/app'));
app.use(bodyParser.urlencoded({ extended: true }));

server.listen(port,() => {
    console.log("server listening on"+port);
});

app.get('/', function(req, res){
  //do something
});


io.on('connection',function(socket){

  socket.on('comparison', function () {

  });

});