All these answers above are great. And the way to go on a full featured application.
But if you want to prototype fast, want flexibility as the collection still changes as well as to minimize your early code base, there is a much simpler way that is not much discussed.
You can basically forego mongoimport by now. I could have saved 3 hours if it was mentioned here on this question. So let me share for others:
Mongodb has a GUI called Mongo Compass has both csv and json import features out of the box in a matter of clicks. It is an official part of the Mongo ecosytem. At the time of writing it is free and it works very well for my use case. https://www.mongodb.com/products/compass