MQTT Client Library Encyclopedia – Paho Javascript

Guest post by James Sutton

Paho Javascript
Language Javascript
License Eclipse Public Licence
API Style Asynchronous


The Eclipse Paho project provides a number of open-source clients of the MQTT and MQTT-SN messaging protocols. The Paho Javascript client is a browser based library that takes advantage of WebSockets to connect to an MQTT Broker. The Library was originally authored by Andrew Banks at IBM and was donated to Eclipse by IBM in 2013.

The library is considered to be very stable and is used in many MQTT based web applications. It’s also available to use via a GUI on the Eclipse Paho website as well as HiveMQ’s very own Websocket-Client. By using WebSockets, the Javascript library allows developers to take advantage of MQTT without having to worry about port 1883 being blocked, they can simply point it at a WebSocket enabled MQTT Broker on port 80 and they are ready to go.


MQTT 3.1 ok
MQTT 3.1.1 ok
LWT ok
Automatic Reconnect ok
QoS 0 ok
QoS 1 ok
QoS 2 ok
Authentication ok
Throttling nok



The Javascript Paho library is available as a download from the project page and is all that is required to use it in your project. Once downloaded, simply include it in your HTML file using the script tags:


Connecting to a broker is relatively simple with the Javascript client, a basic example is shown below.

Connect with MQTT 3.1 or MQTT 3.1.1

By default, the standard action is to connect using MQTT 3.1.1, however if that fails, you could fall back and try to connect as MQTT 3.1. This is managed from the connectOptions object.

3 – MQTT 3.1
4 – MQTT 3.1.1

Connect with LWT

In order to use the really handy Last-Will-and-Testament feature of MQTT, we can use the same options object as in previous examples.

Connect with Username / Password

The Javascript client can also authenticate using a Username and Password.


Once you are connected to the MQTT broker, you are able to publish a message. This is fairly straight forward and can be done as such:

Although destinationName is required, qos is optional. If not supplied, qos will default to 0

Publish a retained message

Publishing a retained message is very simple too, just set retained to true.


Subscribing to a topic can be done with this one liner:

When a message arrives, the onMessageArrived Callback will be called with a MQTT Message object. You can use it like this:

You can also set more advanced options when subscribing:


Unsubscribing is equally as simple as subscribing:

You can also set more advanced options when unsubscribing:


Disconnecting is done like this:

Using SSL / TLS

Connecting to your broker using TLS is also very straight forward. Make sure that you’ve set the port to your Brokers TLS / WebSocket port, then set the useSSL setting in the connectOptions:

Keep in mind that as the browser manages external connections, you may receive an error in the console if the Certificate is not trusted.

Example application

Below is a very simple JavaScript application that will subscribe to the topic /World, the publish the Message 'Hello' to it.

Author Information

James Sutton | IBM
I work for IBM in the Internet of Things Foundation team. I’m a contributer on the Eclipse Paho Project, mainly working on the Java, Android and Javascript Clients

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