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1.3.0 • Public • Published


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Black magic on top of MQTT.js

Provides MQTT topic listeners with wildcards!

Also, Promises.

Table of Contents


This is my MQTT.js wrapper. There are many like it, but this one is mine.

MQTT.js is an excellent MQTT client for Node.js & the browser. It is somewhat of a low-level module, however.

There are two (2) main issues which users of MQTT.js will quickly encounter:

  1. You can't just set a listener function for a subscribed topic; you have to stuff logic into a listener on the message event, and figure out what to do with the topic.
  2. Received messages are all Buffer objects, and you can only publish a Buffer or string.
  3. (BONUS ISSUE) It doesn't use Promises, which some people prefer over callbacks.

mqttletoad solves the above problems. These are problems I have, and likely many others have as well.

Listeners for Specific Topics

When subscribing to an MQTT topic, you can use wildcards (+ and #). If you do, then you have to match an incoming message's topic. Say we subscribed to:


with a listener function quux(). And an we receive a message with topic:


How do we know to execute quux()?

We need something like a router (think express) to be able to match the topic to the proper listener.

EventEmitter2 does exactly this by supporting wildcards in event names. It's flexible, so you don't need to use Express-style routes (foo/:param/baz), which is what several other libraries tackling the same problem have done.

What's better is that EventEmitters are standardized. They are easy to consume. Think RxJs's Observable.fromEvent(). This should help those using a "reactive" programming model.

Encoding and Decoding

MQTT makes no prescriptions about what a message looks like. It's just a blob.

But as a developer, you know your data. Maybe that message is JSON, maybe it's base64-encoded, or maybe it's just a string. You are unlikely to be surprised about what you get.

Out-of-the-box, mqttletoad provides several common decoders for received messages, and encoders for publishing messages.

These are:

  • json - Convert to/from a JSON representation of an object
  • text - Convert to/from a UTF-8 encoded string
  • binary - Convert to a Buffer (all received messages are Buffers, so no decoding necessary)
  • base64 - Convert to/from a base64 (string) representation of just about anything

To use these, you can specify a default encoder and/or decoder when connecting:

const toad = require('mqttletoad');
(async function () {
  const client = await toad.connect('wss://test.mosquitto.org', {
    encoder: 'json',
    decoder: 'json'
  await client.subscribe('foo/bar', message => {
    console.log(message.baz); // quux
  // see listener above
  await client.publish('foo/bar', {baz: 'quux'});  

Or you can do this on a per-subscription/publish basis:

const toad = require('mqttletoad');
(async function () {
  // "text" is the default encoder/decoder
  const client = await toad.connect('wss://test.mosquitto.org', {
    encoder: 'text',
    decoder: 'text'
  await client.subscribe('foo/bar', message => {
    console.log(message.baz); // quux
  }, {decoder: 'json'});
  // see listener above
  await client.publish('foo/bar', {baz: 'quux'}, {encoder: 'json'});   

You can also provide your own either way:

const toad = require('mqttletoad');
(async function() {
  const client = await toad.connect('wss://test.mosquitto.org', {
    decoder: parseFloat 
  await client.subscribe('foo/bar', message => {
    console.log(message); // 123.4
  await client.subscribe('foo/bar', message => {
    console.log(message); // '123.4'
  }, {decoder: 'text'});
  // see listeners above
  await client.publish('foo/bar', 123.4); // default encoder is "text"  

Promise Support

async-mqtt does the same thing here--more or less.

The following functions are promisified:

  • MqttClient#publish
  • MqttClient#subscribe
  • MqttClient#unsubscribe
  • MqttClient#end

IPC Support

mqttletoad supports connecting to an MQTT broker running on a named pipe.


Node.js v7.0.0 or greater required.

$ npm install mqttletoad


This is a fancypants wrapper around MQTT.js, so most everything there applies here, except the differences noted above.

const toad = require('mqttletoad');
const myfunc = async () => {
  const client = await toad.connect('wss://test.mosquitto.org');
  client.on('disconnect', () => {
    console.warn('client disconnected');
  .on('offline', () => {
    console.warn('client offline; reconnecting...');
  // uses default "text" decoder
  const suback = await client.subscribe('winken/+/nod', (str, packet) => {
    console.log(`topic: "${packet.topic}", message: "${str}"`);
  }, {qos: 1});
  console.log(`subscribed to ${suback.topic} w/ QoS ${suback.qos}`);
  // uses default "text" encoder
  await client.publish('winken/blinken/nod', 'foo');
  const someOtherListener = (message, packet) => {
    // does stuff with MESSAGE
  // a custom decoder
  await client.subscribe('winken/+/nod', someOtherListener, {
    decoder: value => String(value).toUpperCase()
  // remove only this particular listener for this topic;
  // no actual unsubscription occurs because this isn't the only listener
  // on the topic.
  await client.unsubscribe('winken/+/nod', someOtherListener);
  // remove ALL listeners from this topic and unsubscribe
  await client.unsubscribe('winken/+/nod');
  // disconnect
  await client.end();
  // IPC support (mqtt only; not ws)
  const client = await toad.connect({path: '/path/to/my/named/pipe'});
  • Use client.subscribe(topic, [opts], listener) to register a listener for the topic.
    • opts are the standard options MqttClient#subscribe() supports, including decoder
    • While MqttClient#subscribe() supports an Array of topics, our topic is singular, and must be a string.
    • Standard MQTT topic wildcards are supported, and listeners are executed first in order of specificity; i.e. foo/bar will take precedence over foo/+ and foo/+ will take precedence over foo/#.
  • Use client.unsubscribe(topic, listener) to remove the listener for the topic.
    • This will not necessarily unsubscribe from the topic (at the broker level), because there may be other listeners, but it will remove the listener.
    • If listener is omitted, all listeners are removed, which forces unsubscription.
  • Use client.end(force=false) to disconnect
  • Use client.publish(topic, message, [opts]) with standard MqttClient#publish() options, including encoder
  • Use connect(url, [opts]) to connect; url is a string, or you could just pass an opts object. Includes encoder and decoder options, which set the default encoder and decoder, respectively. The default is text in both cases.


  • Something something Rollup?




Is this module useful for you? Please let me know. PRs accepted!


Apache-2.0 © 2017 Christopher Hiller


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