Scope of "this"

transport.js

/* Doesn't work */
class Transport extends EventEmitter.EventEmitter {
    constructor() {
        super();
        this.connected = false;
        this.ws = new WebSocket("ws://localhost:8081");
        this.ws.onmessage = fromServer;
        this.ws.onopen = function(event) {
            this.connected = true;
        }
        this.ws.onclose = function(event) { 
            this.connected = false;
            console.log("Connection closed");
            // TODO: reconnect?
        }
        this.ws.onerror = function(event) {
            console.log("Error in websocket");
        }
    }
  
    fromServer(data) {
      var edit = JSON.parse(data.data);
      edit.delta = new Delta(edit.delta);
      this.emit('edit', edit); // "this" refers to websocket 'ws'
    }

    push(edit) {
        this.ws.send(JSON.stringify(edit));
    }
}

/** This works **/
import EventEmitter from 'events';
import Delta from 'quill-delta';


class Transport extends EventEmitter.EventEmitter {
    constructor() {
        super();
        this.ws = new WebSocket("ws://localhost:8081");
        //////////////////////////////////////////////////////////////////////////
        // workaround
        this.ws.onmessage = (data) => {
            var edit = JSON.parse(data.data);
            edit.delta = new Delta(edit.delta);
            this.emit('edit', edit);
        }
        /////////////////////////////////////////////////////////////////////////
        this.ws.onopen = function(event) {
            this.connected = true;
        }
        this.ws.onclose = function(event) { 
            this.connected = false;
            console.log("Connection closed");
            // TODO: reconnect?
        }
        this.ws.onerror = function(event) {
            console.log("Error in websocket");
        }
    }

    push(edit) {
        this.ws.send(JSON.stringify(edit));
    }
}