mosquitto has been installed with a default configuration file.
You can make changes to the configuration by editing:

Python client bindings can be installed from the Python Package Index:
pip install mosquitto

Javascript client has been removed, see Eclipse Paho for an alternative.

To have launchd start mosquitto at login:
ln -sfv /usr/local/opt/mosquitto/*.plist ~/Library/LaunchAgents
Then to load mosquitto now:
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mosquitto.plist
Or, if you don’t want/need launchctl, you can just run:
mosquitto -c /usr/local/etc/mosquitto/mosquitto.conf
==> Summary
🍺  /usr/local/Cellar/mosquitto/1.3.1: 26 files, 628K