Node.js app works locally but heroku says missing module

Question

I made a simple chat application using Node.JS and Socket.IO, everything works fine locally, but when I push it to heroku it gives me an application error, when I check the

Answer 1

In my case I had to add a few modules to dependencies, as they were only under devdependecies and were not built on production

Answer 2

Alright so after 2 hours I figured the problem out, multiple folders named "emitter" that contained the indexof module also had a gitignore file that made git ignore the module, no idea why that was even there, but deleting them fixed the problem