Unit tests are colocated with the code they test. Good examples of test files can be found in the /apps/app/src/utils/__tests__ directory, which contains unit tests for the files in the /apps/app/src/utils/ folder.
Additional examples of unit test files exist throughout the frontend codebase. They have a .test.{ts,tsx} extension and are sometimes located in __tests__ directories.
Add as many unit tests as possible, especially for pure functions and complex business logic that can be tested independently without relying on extensive mocking and external dependencies.
Prioritize testing different properties and scenarios to catch hard-to-miss edge cases instead of only following the happy path with a few examples.