platform events lightning web component variable scope shenanigans

geofenceevents.js

import { LightningElement, track } from 'lwc';
import { subscribe, unsubscribe, onError, setDebugFlag, isEmpEnabled } from 'lightning/empApi';

export default class Geofenceevent extends LightningElement {
    @track channelName = '/event/GeoFenceTrip__e';
    @track isSubscribeDisabled = false;
    @track isUnsubscribeDisabled = !this.isSubscribeDisabled;

    subscription = {};

    @track latestMessageReceived = "dummy for now."; // Fryan

    handleChannelName(event) {
        this.channelName = event.target.value;
    }

    handleTest() {
        console.log(this.latestMessageReceived); 
        // Dummy for now - no matter if we get the events. It never gets set.
    }

    handleSubscribe() {
        const messageCallback = function(response) {
            console.log(this.latestMessageReceived); // Undefined
            this.latestMessageReceived = JSON.stringify(response);
            
            console.log('FRYAN last message: ', this.latestMessageReceived); 
            // Works so much be getting set locally to this "scope"?
        };

        subscribe(this.channelName, -1, messageCallback).then(response => {
            console.log('Successfully subscribed to : ', JSON.stringify(response.channel));
            this.subscription = response;
            this.toggleSubscribeButton(true);
        });
    }

    handleUnsubscribe() {
        this.toggleSubscribeButton(false);
        unsubscribe(this.subscription, response => {
            console.log('unsubscribe() response: ', JSON.stringify(response));
        });
    }

    toggleSubscribeButton(enableSubscribe) {
        this.isSubscribeDisabled = enableSubscribe;
        this.isUnsubscribeDisabled = !enableSubscribe;
    }

    registerErrorListener() {
        // Invoke onError empApi method
        onError(error => {
            console.log('Received error from server: ', JSON.stringify(error));
            // Error contains the server-side error
        });
    }
}