You can easily deploy into a heroku instance, you only need to set two env vars:
TOKENIt's your Slack Slash Command token, you can find this token when create/edit a Slash Command.
The idea is very simple you can execute a JS command inline into the a Slack Channel and get the result after send the command, for instance:
/js [1,2,3].reduce((a, b) => a +b)
The result will be:
User eduardonunesp snippet:[1,2,3].reduce((a, b) => a +b)result:6