getting an error when trying to use socket.io

Question

I am currently working with socket.io swift client. Running on Iphone SE. this is the swift code

 let socket = SocketIOClient(socketURL: URL(string: "http://example.com:4000")!, config: [.log(true), .forcePolling(true)]);
        socket.connect();
        socket.on("connect") {data, ack in
            print("socket is connected");
            socket.emit("getData", ["data": 3]);
        }

And on the server:

var express = require('express');
var app = express();
var http = require('http').Server(app);
var io = require('socket.io')(http);

io.on('connection', function(socket){
    console.log('a user connected');
    socket.on('disconnect', function(){
        console.log('user disconnected');
    });
    socket.on('getData', function(result){
        console.log(result);
    });
});


app.listen(4000, function () {
  console.log(' on at 4000!');
});

...And on the Xcode console, I get

2016-09-29 16:38:33.871895 proj[3070:1019256] LOG SocketEngine: Handshaking
2016-09-29 16:38:33.872301 proj[3070:1019256] LOG SocketEnginePolling: Doing polling request
2016-09-29 16:38:34.004312 proj[3070:1019256] LOG SocketEnginePolling: Got polling response
2016-09-29 16:38:34.004874 proj[3070:1019283] LOG SocketEngine: Got message: Cannot GET /socket.io/?transport=polling&b64=1
2016-09-29 16:38:34.005283 proj[3070:1019283] ERROR SocketIOClient: Got unknown error from server Cannot GET /socket.io/?transport=polling&b64=1

Which demonstrates a connection is made and the server is successfully found, but something else is wrong. Would appreciate any help.

Answer 1

(Sidenote: If you don't need support for old browsers (or any browsers for that matter, since your client is a native mobile app) then you may consider using WebSocket which is an open standard. Socket.io is usually used to have a WebSocket-like functionality on browsers that don't support WebSocket. WebSocket on the other hand is an open standard, has a wide support (not only in browsers) and it has a better performance. See this answer for more details.)

Now, since you are already using Socket.io then here is how you can diagnose the problem. I would try to connect from a browser, which is a main way to connect with Socket.io, and see if that works. If it doesn't then it would mean that there's a problem in your server code. If it does then it could mean that there's a problem in your client. That would be the first thing to check. Going from there you can narrow the problem and hopefully fix it.

If you want to have a starting point with some working code using Socket.io, both server-site (Node.js) and client-side (browser vanilla JavaScript), then you can see the examples that I wrote originally for this answer , that are available on GitHub and on npm :

Socket.IO Server

Socket.IO server example using Express.js:

var path = require('path');
var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);
app.get('/', (req, res) => {
  console.error('express connection');
  res.sendFile(path.join(__dirname, 'si.html'));
});
io.on('connection', s => {
  console.error('socket.io connection');
  for (var t = 0; t < 3; t++)
    setTimeout(() => s.emit('message', 'message from server'), 1000*t);
});
http.listen(3002, () => console.error('listening on http://localhost:3002/'));
console.error('socket.io example');

Source: https://github.com/rsp/node-websocket-vs-socket.io/blob/master/si.js

Socket.IO Client

Socket.IO client example using vanilla JavaScript:

var l = document.getElementById('l');
var log = function (m) {
    var i = document.createElement('li');
    i.innerText = new Date().toISOString()+' '+m;
    l.appendChild(i);
}
log('opening socket.io connection');
var s = io();
s.on('connect_error', function (m) { log("error"); });
s.on('connect', function (m) { log("socket.io connection open"); });
s.on('message', function (m) { log(m); });

Source: https://github.com/rsp/node-websocket-vs-socket.io/blob/master/si.html

You can compare the same code with WebSocket versions:

WebSocket Server

WebSocket server example using Express.js:

var path = require('path');
var app = require('express')();
var ws = require('express-ws')(app);
app.get('/', (req, res) => {
  console.error('express connection');
  res.sendFile(path.join(__dirname, 'ws.html'));
});
app.ws('/', (s, req) => {
  console.error('websocket connection');
  for (var t = 0; t < 3; t++)
    setTimeout(() => s.send('message from server', ()=>{}), 1000*t);
});
app.listen(3001, () => console.error('listening on http://localhost:3001/'));
console.error('websocket example');

Source: https://github.com/rsp/node-websocket-vs-socket.io/blob/master/ws.js

WebSocket Client

WebSocket client example using vanilla JavaScript:

var l = document.getElementById('l');
var log = function (m) {
    var i = document.createElement('li');
    i.innerText = new Date().toISOString()+' '+m;
    l.appendChild(i);
}
log('opening websocket connection');
var s = new WebSocket('ws://'+window.location.host+'/');
s.addEventListener('error', function (m) { log("error"); });
s.addEventListener('open', function (m) { log("websocket connection open"); });
s.addEventListener('message', function (m) { log(m.data); });

Source: https://github.com/rsp/node-websocket-vs-socket.io/blob/master/ws.html

I hope this can help you evaluate whether staying with Socket.io or going with WebSocket is the right decision for you, and will give you some working client-side code to test your backend. The code is released under the MIT license (open source, free software) so feel free to use it in your project.