---
title: "Client | Grafana k6 documentation"
description: "MQTT client for connecting to brokers and managing operations"
---

# Client

The `Client` class provides a high-level, event-driven interface for interacting with MQTT brokers. It supports both synchronous and asynchronous operations for connecting, subscribing, publishing, and unsubscribing.

## Constructor

JavaScript ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```javascript
new Client(options)
```

Creates a new MQTT client instance.

### Parameters

Expand table

| Parameter                     | Type     | Description                                                                                        |
|-------------------------------|----------|----------------------------------------------------------------------------------------------------|
| options                       | object   | Optional client configuration                                                                      |
| options.client\_id            | string   | Client identifier (must be unique per broker connection). If not provided, the broker assigns one. |
| options.username              | string   | Username for broker authentication                                                                 |
| options.password              | string   | Password for broker authentication                                                                 |
| options.credentials\_provider | function | Function returning `{username, password}` for dynamic authentication                               |
| options.will                  | object   | Last Will and Testament message configuration                                                      |
| options.will.topic            | string   | Topic for the will message                                                                         |
| options.will.payload          | string   | Payload for the will message                                                                       |
| options.will.qos              | number   | QoS level for the will message (0, 1, or 2)                                                        |
| options.will.retain           | boolean  | Whether the will message should be retained                                                        |
| options.tags                  | object   | Custom tags for metrics (key-value pairs)                                                          |

## Properties

Expand table

| Property  | Type    | Description                                                              |
|-----------|---------|--------------------------------------------------------------------------|
| connected | boolean | Read-only. Indicates if the client is currently connected to the broker. |

## QoS

Quality of Service enumeration for message delivery guarantees:

Expand table

| Value | Name            | Description                                                         |
|-------|-----------------|---------------------------------------------------------------------|
| 0     | QoS.AtMostOnce  | Fire and forget. Message delivered at most once, no acknowledgment. |
| 1     | QoS.AtLeastOnce | Message delivered at least once, with acknowledgment.               |
| 2     | QoS.ExactlyOnce | Message delivered exactly once, guaranteed and duplicate-free.      |

## Methods

Expand table

| Method                                                                                   | Description                                       |
|------------------------------------------------------------------------------------------|---------------------------------------------------|
| [connect()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/connect/)                    | Connect to an MQTT broker                         |
| [reconnect()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/reconnect/)                | Reconnect to the broker using previous parameters |
| [subscribe()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/subscribe/)                | Subscribe to one or more topics                   |
| [subscribeAsync()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/subscribe-async/)     | Subscribe to topics asynchronously                |
| [unsubscribe()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/unsubscribe/)            | Unsubscribe from topics                           |
| [unsubscribeAsync()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/unsubscribe-async/) | Unsubscribe from topics asynchronously            |
| [publish()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/publish/)                    | Publish a message to a topic                      |
| [publishAsync()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/publish-async/)         | Publish a message asynchronously                  |
| [on()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/on/)                              | Register event handlers                           |
| [end()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/end/)                            | Disconnect from the broker                        |
| [endAsync()](/docs/k6/latest/javascript-api/k6-x-mqtt/client/end-async/)                 | Disconnect from the broker asynchronously         |

## Example

### Basic Usage

JavaScript ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```javascript
import { Client } from "k6/x/mqtt";

export default function () {
  const client = new Client()

  client.on("connect", () => {
    console.log("Connected to MQTT broker")
    client.subscribe("greeting")

    client.publish("greeting", "Hello MQTT!")
  })

  client.on("message", (topic, message) => {
    const str = String.fromCharCode.apply(null, new Uint8Array(message))
    console.info("topic:", topic, "message:", str)
    client.end()
  })

  client.on("end", () => {
    console.log("Disconnected from MQTT broker")
  })

  client.connect(__ENV["MQTT_BROKER_ADDRESS"] || "mqtt://broker.emqx.io:1883")
}
```