Just a heads up: if you’re getting all kinds of errors with the how-to-npm, make sure to create a new directory like the tutorial says.
The freeCodeCamp page here https://www.freecodecamp.com/challenges/manage-packages-with-npm says to work in the ~/workspace directory. However, if you do this there are all kinds of dependencies installed by Cloud9 that break the tutorial and throws a bunch of 404 errors. If you create a new directory and cd into it, it runs very smoothly.