Generally speaking, it’s an unwritten rule that a IRC/Chat bot should initiate a conversation, it should only reply and when it replies it replies in the same medium. (So PRIVMSG reply with PRIVMSG whisper reply with whisper)
You are using TMI.js? Your example paste above looks like the TMI.js library
New followers are not “easy” to detect, you’d need your bot to make constant API requests (once per minute and then spam the chat as new followers clear thru the API cache), or use a Web Server to receive follower webhooks in real time which then tells the bot to send a message to chat.
As I noted you can hook onto the USERNOTICE in the docs I already linked.
But if you are using TMI.js you’d need to refer to the TMI.js docs for how their library presents the events for usage.