This application is buggy because the authors didn't develop it using TDD. Can you please help debugging by creating some unit tests?
Use the tools you consider the application needs to fit the best code quality or working guarantees.
Tip: E2E testing is a plus.
In the project directory, you can run:
Runs the app in the development mode.
Open http://localhost:3000 to view it in the browser.
The page will reload if you make edits.
You will also see any lint errors in the console.
Launches the test runner in the interactive watch mode.
See the section about running tests for more information.
Document your code assuming that the next engineer looking at your code is a serial killer who knows where you live.
Use JSDoc 3 docummentation style. http://usejsdoc.org/