Software Design - Interaction between two classes

First off, I am sorry if there is a trivial answer to this question but I am unable to find one.

The concept that I have in question is the following. I have a client app and a server app that exchange data. I am trying to separate the logic that handles a server update in the client in the following way.

A "main" class that handles application logic and works with parsed and sanitized data and a "worker" class that parses the received data and calls the appropriate method of the "main" class. I guess this is in some way imitating events. Here is an example structure to make things more clear.

var Main = function(){
    this.init();
};

Main.prototype = {

    init: function(){
        //Do init logic

    },

    connect: function(){
        var client = new Client();
        client.onStateUpdate(this.executeStateUpdate);
        client.connect('localhost', 3001);
    },

    executeStateUpdate: function(data){
        // Problem - the "this" operator is now Client and not Main
    }

};


var Client = function(){
};

Client.prototype = {
    connect: function(host, port){
        this.client = new WebSocket("ws://"+host+":"+port);

        this.client.onmessage = function(e){
            var data = JSON.parse(e);
            //handle received data
            stateUpdate(data);
        };
    },

    stateUpdate: function(data){
        //sanitize data to application objects
        this.stateUpdateCallback(sanitizedData);
    },

    onStateUpdate: function(callback){
        this.stateUpdateCallback = callback;
    }
}

The biggest problem here is that calling "main" methods from the "worker" makes them point to the wrong object(e.g "Client" and not "Main"). I have thought about passing the "Main" class object to the "Client" object instead of callbacks, but this way I have to hardcode all the callback methods that would be called on each event that arises from the server. I am open to any suggestions on how to solve this matter and sticking to this architecture of the code is not mandatory for me.