For this application, we are going to use mLab (https://mlab.com/) to host our database as a service. But if you would like to install MongoDB for your local development, you can do so from here: https://docs.mongodb.com/manual/installation/. Once MongoDB is successfully installed, you can run the following command to verify the installation:

$ mongo --version
MongoDB shell version v3.4.10
git version: 078f28920cb24de0dd479b5ea6c66c644f6326e9
OpenSSL version: OpenSSL 1.0.2m 2 Nov 2017
allocator: system
modules: none
build environment:
    distarch: x86_64
    target_arch: x86_64

We are going to use MongoDB 3.4 for our application.