I'm trying to access a method from a class inside a assigned function to a websocket event, even if I call it with 'this' keyword, the function is unable to see it.

'use strict';
let clientSocket = undefined;

window.addEventListener("load", function() {
    class ClientSocket {

        id = undefined;

        constructor(id) {
            this.id = id;
        }

        printMsg = (obj) => {
            if (obj === null || obj === undefined)
                return;

            if (this.isDebug) {
                console.debug(obj)
            }
        }

        conecta() {
            const url = "ws://localhost:5000";
            this.printMsg('URL: ' + url)

            const socket = new WebSocket(url);

            //on open
            socket.onopen = function (event) {
                this.printMsg("SOCKET OPEN"); //CAN'T ACCESS THIS METHOD EVEN WITH 'THIS'
            }

            this.socket = socket;
        }
    }

    clientSocket = new ClientSocket(1);
});

On onopen socket event, the function can't access printMsg() method from ClientSocket.