Building a Saas application with ReactJS (NextJS) and NodeJS -- 2

I'm looking to build a Saas application that leverages ReactJS (NextJS) and NodeJS. To build the application, I'll be using ReactJS (NextJS), NodeJS, and MongoDB. Unit tests are a necessary part of making sure our software works correctly, so I'm planning to write unit tests for the application as well. Additionally, I plan to scale the application depending on user demand.