MQTT Client Library Encyclopedia – MQTT.js

Guest post by Matteo Collina

API-StyleAsynchronous, Callbacks, Streams, Offline
License MIT


The MQTT.js provides a full-featured Javascript library for the MQTT protocol. It is fully isomorphic, which means it can run in the browser and in node.js (>= 0.8), including Intel Edison and Raspberry PIs. In fact, it is bundled in base image of the Intel Edison.
The library was originally written by Adam Rudd in May 2011, and it has been upgraded to all versions of node.js since then. In 2014, Matteo Collina took over as a maintainer, and since version 1.0.0 it supports MQTT over Websockets, both in Node.js and in the Browser.
This library is production ready and used by some fortune 500 companies. It can support long running clients with spotty connectivity. It has an optional on-disk storage for local offline messaging.
MQTT.js became Open Open Source last January, and it had 55 contributors in its history. There is a team of 4 that maintains the library.


MQTT 3.1ok
MQTT 3.1.1ok
Automatic Reconnectok
QoS 0ok
QoS 1ok
QoS 2ok



You need node.js installed to use MQTT.js, then:

If you want to use the embedded CLI tools, you need to


Connects to the broker specified by the given url and options and returns a Client.

The URL can be on the following protocols: 'mqtt', 'mqtts', 'tcp', 'tls', 'ws', 'wss'. The URL can also be an object as returned by URL.parse(), in that case the two objects are merged, i.e. you can pass a single object with both the URL and the connect options.

You can also specify a servers options with content: [{ host: 'localhost', port: 1883 }, ... ], in that case that array is iterated at every connect.

Connect with MQTT 3.1 or MQTT 3.1.1

The library connects with MQTT 3.1.1 by default. To connect with MQTT 3.1

Connect with LWT

LWT can be set in the connect function, like the following:

Connect with Username / Password

Username and password can be set in the connect function, like the following:


Publish a message to a topic

  • topic is the topic to publish to, String
  • message is the message to publish, Buffer or String
  • options are the options to publish with, including:
    • qos QoS level, Number, default 0
    • retain retain flag, Boolean, default false
  • callback callback fired when the QoS handling completes, or at the next tick if QoS 0.


Subscribe to a topic or topics

  • topic is a String topic to subscribe to or an Array of topics to subscribe to. It can also be an object, the object key is the topic name and the value is the QoS, like {'test1': 0, 'test2': 1}.
  • options is the options to subscribe with, including:
    • qos qos subscription level, default 0
  • callbackfunction(err, granted) callback fired on suback where:
    • err is a subscription error
    • granted is an array of {topic, qos} where:
      • topic is a subscribed to topic
      • qos is the granted qos level on it


Unsubscribe from a topic or topics

topic is a String topic or an array of topics to unsubscribe from
callback fired on unsuback


Using SSL / TLS

The user should call mqtt.connect(), here is a full example:

Author Information

Matteo Collina
Matteo is a code pirate and mad scientist. He spends most of his days programming in node.js, but in the past he worked with Ruby, Java and Objective-C. He recently got a Ph.D. with a thesis titled “Application Platforms for the Internet of Things”. Now he is a Software Architect at nearForm, where he consults for the top brands in world. Matteo is also the author of the Node.js MQTT Broker, Mosca and of the LevelGraph database. Matteo spoke at several international conferences: NodeSummit,, LXJS, Distill by Engine Yard, and JsDay to name a few. He is also co-author of the book “Javascript: Best Practices” edited by FAG, Milan. In the summer he loves sailing the Sirocco.


  1. Saili Ghavat says:

    I am trying to connect to an mqtt broker but I get this error when I try top use require.js
    Uncaught Error: Module name “mqtt” has not been loaded yet for context: _. Use require([])

    1. Mosquitto says:

      Have you imported de mqtt module?: –> var mqtt = require(‘mqtt’);

  2. bedinsky says:

    I’m using MQTT.js 2.2.1 with websocket
    I’ve set
    var mqtt = require(‘mqtt’);
    var client = mqtt.connect(‘ws://’,{
    protocolId: ‘MQIsdp’,
    protocolVersion: 3

    I cannot connect to hiveMQ, client stays in a connection loop
    client is working with mqtt://

    please tell me how I can connect to HiveMQ using websocket
    it seems to me that MQTT.js is setting protocolId to mqttv3.1
    and broker does not recognize such protocol


    1. Hi there,

      please follow this instruction, to correctly configure your HiveMQ for Websockets.
      Note that there is a known issue for paho.js and websockets.

      Hope that helps.

      Florian, from the HiveMQ Team.

  3. bedinsky says:

    Thank you !
    I successfully connected from an ionic2 app using
    mqtt.connect(‘ws://’, {port: 8000,path: ‘/mqtt’ });


    1. Frank says:

      Thank you bedinsky, this format worked for me!

  4. Vikas Mujumdar says:

    I am building an AngularJS application to connect to Mosquitto. I use the Paho client successfully to connect to the broker via websockets. No problems there. I wanted a client that could connect to the broker via the basic MQTT (port 1883) since Paho does not support that. I am trying with mqtt.js but it always forces the URL to ws://hostname even though my code has mqtt://hostname. I am running my application on Chrome.

    I am including the library from the CDN:
    I am not adding the “require” statement since it is a client side script
    Code has: var client = mqtt.connect(‘mqtt://’);

    Error in console:
    WebSocket connection to ‘ws://’ failed: Error during WebSocket handshake: net::ERR_CONNECTION_RESET

    What am I doing wrong? Or is my understanding incorrect that I can choose to connect via mqtt or ws?

    1. Hallo Vikas,

      this is actually a browser / security issue. A tcp connection that is not wrapped in websockets can not be opened in JavaScript.

      This is not a restriction of the language itself but of it’s use inside the sandbox within the browser. Just imagine that a script somewhere on the internet gets loaded into the browser and could from inside the browser access every computer reachable by the browser with arbitrary protocols. You could easily misuse this to send spam through a companies internal mail server or attack/misuse other internal and external resources.

      Hope that helps.

      Best regards,
      Florian, from The HiveMQ Team.

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