You can still verify it if you have the right secret key and algorithm.

Implementation with jsonwebtoken module

client

const {token} = sessionStorage;
const socket = io.connect('http://localhost:3000', {
  query: {token}
});

Server

const io = require('socket.io')();
const jwt = require('jsonwebtoken');

io.use(function(socket, next){
  if (socket.handshake.query && socket.handshake.query.token){
    jwt.verify(socket.handshake.query.token, 'SECRET_KEY', function(err, decoded) {
      if (err) return next(new Error('Authentication error'));
      socket.decoded = decoded;
      next();
    });
  }
  else {
    next(new Error('Authentication error'));
  }    
})
.on('connection', function(socket) {
    // Connection now authenticated to receive further events

    socket.on('message', function(message) {
        io.emit('message', message);
    });
});

Implementation with socketio-jwt module

This module makes the authentication much easier in both client and server side. Just check out their examples.

client

const {token} = sessionStorage;
const socket = io.connect('http://localhost:3000');
socket.on('connect', function (socket) {
  socket
    .on('authenticated', function () {
      //do other things
    })
    .emit('authenticate', {token}); //send the jwt
});

Server

const io = require('socket.io')();
const socketioJwt = require('socketio-jwt');

io.sockets
  .on('connection', socketioJwt.authorize({
    secret: 'SECRET_KEY',
    timeout: 15000 // 15 seconds to send the authentication message
  })).on('authenticated', function(socket) {
    //this socket is authenticated, we are good to handle more events from it.
    console.log(`Hello! ${socket.decoded_token.name}`);
  });