Question 1

What operating systems does MQTT-JS support?

Accepted Answer

MQTT-JS currently ships as a native desktop app for Linux (.deb for Debian/Ubuntu, .rpm for Fedora/RHEL). Windows and macOS builds are planned for a future release.

Question 2

Is MQTT-JS free?

Accepted Answer

Yes. MQTT-JS is completely free to download and use. There are no accounts, subscriptions, or cloud services involved — it runs entirely on your machine.

Question 3

How do I connect to a broker with TLS?

Accepted Answer

When adding a connection, select the "mqtts" protocol and provide the paths to your CA certificate, client certificate, and private key files. Self-signed certificates and mutual TLS are both supported. You can also disable certificate verification for development brokers.

Question 4

What is protobuf decoding and how do I use it?

Accepted Answer

Protobuf (Protocol Buffers) is a binary serialisation format used in many IoT and microservice systems. MQTT-JS lets you load your .proto schema files and map MQTT topic patterns to message types. Incoming binary payloads on matching topics are automatically decoded and shown as human-readable JSON in the message detail view.

Question 5

Can I filter messages by topic?

Accepted Answer

Yes. The filter bar supports substring matching and MQTT wildcards (+ for a single level, # for multiple levels). You can apply multiple filters simultaneously — only messages matching all active filters are shown. Filters can be saved and reused across sessions.

Question 6

How many messages can MQTT-JS hold in memory?

Accepted Answer

The message list is a ring buffer that holds the last 10,000 messages per session. Older messages are automatically discarded as new ones arrive. You can pause the stream at any time to freeze the view without losing incoming messages.

Question 7

Is my broker configuration stored securely?

Accepted Answer

Connection settings (including hostnames, usernames, and passwords) are persisted locally using the OS user data directory via electron-store. Data never leaves your machine — there is no cloud sync or telemetry.

Question 8

Can I monitor multiple brokers at the same time?

Accepted Answer

Yes. You can add as many connections as you like. Each broker connects independently and all messages flow into the same unified message list. Use the topic filter to narrow down messages from a specific broker or topic namespace.

Observe your brokers.
Understand
your data.

Everything you need to inspect MQTT traffic

Multiple Brokers

TLS / MQTTS

Protobuf Decoding

Topic Filtering

Persistent Config

Real-time Stream

Frequently asked questions

Download MQTT-JS

Observe your brokers.Understandyour data.