# Connect to Deployment using Node.js via MQTT.js SDK

This article mainly introduces how to use [MQTT](https://www.emqx.com/en/mqtt) in the Node.js project to realize the functions of connecting, subscribing, unsubscribing, sending and receiving messages between the client and the MQTT broker.

## Prerequisites

Before connecting your application to the EMQX Cloud using the Node.js client, ensure you have completed the following prerequisites:

- Deploy MQTT Broker
- Check Node.js version

### Deploy MQTT Broker

<!-- @include: ./deploy_mqtt_broker.md -->

### Check Node.js Version

This project uses Node.js v16.19.1 for development and testing. You can confirm the version of Node.js with the following command.

```shell
node --version

v16.19.1
```

## Install Dependencies

[MQTT.js](https://github.com/mqttjs/MQTT.js) is a fully open-source client-side library for the MQTT protocol, written in JavaScript and available for Node.js and browsers. For more information and usage of `MQTT.js`, please refer to the [MQTT.js GitHub](https://github.com/mqttjs/MQTT.js#table-of-contents).

MQTT.js can be installed via NPM or Yarn, or can be imported through CDN or relative path. This example will install MQTT.js through Yarn command.

- Using NPM or Yarn：

  Install MQTT.js

  ```shell
  # NPM
  npm install mqtt
  # or Yarn
  yarn add mqtt
  ```

## Connect over TCP port

You can set a client ID, username, and password with the following code. The client ID should be unique.

```js
const clientId = 'emqx_nodejs_' + Math.random().toString(16).substring(2, 8)
const username = 'emqx_test'
const password = 'emqx_test'
```

Establish a connection between the client and MQTT Broker using the following code.

```js
const client = mqtt.connect('mqtt://broker.emqx.io:1883', {
  clientId,
  username,
  password,
  // ...other options
})
```

## Connect over TCP Secure port

If TLS/SSL encryption is enabled, the connection [parameter options](https://github.com/mqttjs/MQTT.js#mqttclientstreambuilder-options) are consistent with establishing a connection through the TCP port. You only need to pay attention to changing the protocol to `mqtts` and matching the correct port number.

Establish a connection between the client and MQTT Broker using the following code.

```js
const client = mqtt.connect('mqtts://broker.emqx.io:8883', {
  clientId,
  username,
  password,
  // ...other options
})
```

## Connect over WebSocket Port

MQTT WebSocket uniformly uses `/path` as the connection path, which needs to be specified when connecting, while EMQX Broker uses `/mqtt` as the path.

Therefore, when using WebSocket connection, in addition to modifying the port number and switching the protocol to `ws`, you also need to add the `/mqtt` path.

Establish a connection between the client and MQTT Broker using the following code.

```js
const client = mqtt.connect('ws://broker.emqx.io:8083/mqtt', {
  clientId,
  username,
  password,
  // ...other options
})
```

## Connect over WebSocket Secure Port

If TLS/SSL encryption is enabled, the connection [parameter option](https://github.com/mqttjs/MQTT.js#mqttclientstreambuilder-options) is consistent with establishing a connection through the WebSocket port. You only need to pay attention to changing the protocol to 'wss' and matching the correct port number

Establish a connection between the client and MQTT Broker using the following code.

```js
const client = mqtt.connect('wss://broker.emqx.io:8084/mqtt', {
  clientId,
  username,
  password,
  // ...other options
})
```

## Subscribe and Publish

### Subscribe to Topics

Specify a topic and the corresponding [QoS level](https://www.emqx.com/zh/blog/introduction-to-mqtt-qos) to be subscribed.

```javascript
const topic = '/nodejs/mqtt'
const qos = 0

client.subscribe(topic, { qos }, (error) => {
  if (error) {
    console.log('subscribe error:', error)
    return
  }
  console.log(`Subscribe to topic '${topic}'`)
})
```

### Unsubscribe to Topics

You can unsubscribe using the following code, specifying the topic and corresponding QoS level to be unsubscribed.

```javascript
const topic = '/nodejs/mqtt'
const qos = 0
client.unsubscribe(topic, { qos }, (error) => {
  if (error) {
    console.log('unsubscribe error:', error)
    return
  }
  console.log(`unsubscribed topic: ${topic}`)
})
```

### Publish Messages

When publishing a message, the MQTT broker must be provided with information about the target topic and message content.

```javascript
const topic = '/nodejs/mqtt'
const payload = 'nodejs mqtt test'
const qos = 0

client.publish(topic, payload, { qos }, (error) => {
  if (error) {
    console.error(error)
  }
})
```

### Receive Messages

The following code listens for message events and prints the received message and topic to the console when a message is received.

```javascript
client.on('message', (topic, payload) => {
  console.log('Received Message:', topic, payload.toString())
})
```

### Disconnect from MQTT Broker

To disconnect the client from the broker, use the following code:

```javascript
if (client.connected) {
  try {
    client.end(false, () => {
      console.log('disconnected successfully')
    })
  } catch (error) {
    console.log('disconnect error:', error)
  }
}
```

The above section only shows some key code snippets, for the full project code, please refer to [MQTT-Client-Node.js](https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-Node.js). You can download and try it out yourself.

## Test the Connection

We add a line of startup script to the script field in the package.json file.

```json
"scripts": {
  "start": "node index.js"
}
```

Then we can simply use `npm start` to run the project.

```shell
npm start
```

After running, we can see the output information of the console as follows:

![NodeJS MQTT Start](https://assets.emqx.com/images/9897e6cd56163dfe7139cf6d84361e63.png)

We see that the client has successfully connected to the [MQTT broker](https://www.emqx.io) and subscribed to the topic, received and published messages successfully. At this point, we will use [MQTT 5.0 Client Tool - MQTTX](https://mqttx.app) as another client for the message publishing and receiving test.

![MQTT 5.0 Client Tool - MQTTX](https://assets.emqx.com/images/5c841598f78eed0b186572165832f861.png)

We can see that the message sent by MQTTX is printed in the console.

![MQTT messages](https://assets.emqx.com/images/02d8a35312ca1309f18a628dacca8910.png)

## More

In conclusion, we have implemented creating MQTT connections in a Node.js project and simulated scenarios of subscribing, publishing messages, unsubscribing, and disconnecting between clients and MQTT servers. You can download the complete example source code on the [MQTT-Client-Node.js page](https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-Node.js), and we also welcome you to explore more demo examples in other languages on the [MQTT Client example page](https://github.com/emqx/MQTT-Client-Examples).