Add unit test code

[hendrixroa]

I've seen the popularity of this tool are increase, so I wondering if a good idea start adding new testing code that guarantee the code stay healthy in the next releases. I can make it in a separate PR. We can add unit test code in 2 phases.

• Phase 1: Add testing code for backend code (server.js)
• Phase 2: Add testing code for client (react)

what do you think?