Can't send a message to Node.js websocket server from React client

I have following code as a server:

var http = require('http'),
    cors = require('cors'),
    connect = require('connect'),
    WebSocketServer = require('ws');

var DataController = function() {
    this.PORT = 8090;
    this.startWS();
    this.startServer();
}

DataController.prototype = {
    // code to start server...

    getMessage: function(_message) {
        var that = this,
        message = JSON.parse(_message),
        msgName = message.name;
        console.log('new message ' + msgName);
    switch(msgName){
        case 'start':
        // here is some code to perform actions
}

As you can see here I have getMessage function that fires some events and to invoke it I need to send a message start. That's easy enough, right? Surpisingly for me, it's not. That's how I try to perform this .send() method in my React component (my React app is running on different port of course):

import React, { Component } from 'react';

class App extends Component {

    componentDidMount() {
        const socket = new WebSocket('ws://localhost:8090');
        socket.onopen = function() {
            socket.send(JSON.stringify({message: "start"}));
        }
    }
    render() {
        return (
            <div className="container">App</div>
        )
    }
 }

 export default App;

What I try to accomplish here - I need to send just one word - start to actually start receieving data from server but I can't and do not understand why. See, there's a console.log() statement in my server code that logs a message start when it's sent. So it actually should log start but it logs undefined instead. What I tried so far: 1. Send just start 2. Define an object like this:

const data = {
    message: "start"
}

And then pass data to .send() function 3. Define just a variable like this: const data = "start"

And nothing above works!Everything returns undefined on server side. I'm just getting started with websockets and can't find a way to get it working. I refered mostly to Mozilla and it didn't work too. So what actually am I doing wrong in this case? Thanks in advance, I appreciate any help!