How to install NPM modules on Glitch

Some days ago I was helping a friend hosting his project on Glitch. The question is, every time he wanted to add a module to his project, he would ask me to do so (not that it was a hassle for me). So, if you're here, it means you're asking the same question and, ladies and gentlemen, you found the right spot!

Alright, what you need to do is this:

• Go in your project's package.json
• Click the "Add Package" button up in the code
• Now input the name of the module you want to install. The options are two:
  • Glitch can find the module. Click on its name and reboot the project.
  • Glitch can not find the module. Unfortunately, Glitch doesn't update its package search very often. In this case, manually add an entry in the package.json.
• ???
• Profit!

If for some reason you can't do something reported here, ask in the comments and I'll reply ASAP.