some changes

src/main.ts

@@ -1,14 +1,18 @@
 import * as log from './log'
-import * as MqttClient from './mqttclient'
+import * as mqtt from './mqttclient'
 
 class Dispatcher {
-    private _mqttClient: MqttClient.MqttClient
+    private _mqttClient: mqtt.MqttClient
 
     constructor() {
-        this._mqttClient = new MqttClient.MqttClient()
+        this._mqttClient = new mqtt.MqttClient()
 
-        this._mqttClient.register('IoT/test', null)
-        this._mqttClient.register('IoT/Device/#', null)
+        this._mqttClient.register('IoT/test', (message: any) : any => {
+            log.info("Callback for IoT/test")
+            log.info(`message is ${message}`)
+            return `<<${message}>>`
+        })
+        this._mqttClient.register('IoT/Device/#', mqtt.passThrough)
     }
 
     exec() : void {

src/mqttclient.ts

@@ -7,7 +7,11 @@ type TopicHandlerCallback = (message: any) => any
 
 interface TopicHandler {
     topic: string,
-    callback: TopicHandlerCallback|null
+    callback: TopicHandlerCallback
+}
+
+export function passThrough(message: any) {
+    return message
 }
 
 export class MqttClient {
@@ -20,7 +24,7 @@ export class MqttClient {
         this._topicHandlers = []
     }
 
-    register(topic: string, callback: TopicHandlerCallback|null) : void {
+    register(topic: string, callback: TopicHandlerCallback) : void {
         this._topicHandlers.push({topic, callback})
         log.info(`handler registered for topic ${topic}`)
     }
@@ -39,9 +43,7 @@ export class MqttClient {
             for (let topicHandler of this._topicHandlers) {
                 if (this.topicMatch(topicHandler.topic, topic)) {
                     log.info(`received topic ${topic} matches registered topic ${topicHandler.topic}`)
-                    if (topicHandler.callback != null) {
-                        topicHandler.callback(payload)
-                    }
+                    topicHandler.callback(payload)
                 }
             }
         })