Our web client imports the Paho-MQTT JavaScript client library at line (1):

<title>MQTT Web Socket Example</title>
<script src="./jquery.min.js"></script>
<script src="./paho-mqtt.js"></script>  <!-- (1) -->

paho-mqtt.js can be also found in the chapter04/mosquitto_www folder.

The official documentation page for the Paho-MQTT JavaScript library is available at https://www.eclipse.org/paho/clients/js, while its official GitHub page is found at https://github.com/eclipse/paho.mqtt.javascript.

Next, we encounter the global variables.