Is writing Unit Tests worth it for already exciting functionality?

Writing unit tests for already existing functionality in legacy code can be a challenging but highly beneficial practice. Here are some key points to consider:

Benefits of Writing Unit Tests for Existing Functionality

1. Regression Testing:

   • Unit tests help ensure that changes or additions to the code do not break existing functionality. This is particularly important in legacy systems where the codebase is large and complex[5][12][13].

2. Documentation:

   • Unit tests serve as executable documentation, providing clear examples of how the code is supposed to behave. This can be invaluable for new developers onboarding onto the project or for future maintenance[5][12][18].

3. Facilitates Refactoring:

   • Having a suite of unit tests gives developers the confidence to refactor and improve the code without the fear of introducing new bugs. This is crucial for maintaining and improving the quality of legacy code[1][3][12].

4. Early Detection of Defects:

   • Unit tests can catch bugs early in the development process, which are cheaper and easier to fix compared to bugs found later in production[6][13].

5. Improved Code Quality:

   • Writing unit tests often forces developers to write more modular and decoupled code, which is easier to maintain and extend[6][18].

Challenges and Considerations

1. Initial Effort and Time:

   • Retrofitting unit tests into an existing codebase can be time-consuming and may require significant effort, especially if the code was not designed with testability in mind[1][2][4].

2. Hard-Wired Dependencies:

   • Legacy code often has tightly coupled components and hard-wired dependencies, making it difficult to isolate units for testing. This might necessitate some refactoring to make the code more testable[2][7][17].

3. Characterization Tests:

   • Instead of traditional unit tests, characterization tests can be used to capture the current behavior of the system. These tests help ensure that any changes do not alter the existing behavior, even if the code is not fully understood[1][9].

4. Strategic Testing:

   • It is often impractical to achieve 100% test coverage in legacy systems. Focus on critical areas of the code that are prone to changes or have a history of bugs. This strategic approach can provide the most value with the least effort[4][10][19].

Best Practices

1. Incremental Approach:

   • Start by writing tests for new code and for areas of the codebase that are being modified. Gradually increase test coverage over time[1][4][12].

2. Use of Tools:

   • Utilize tools like ApprovalTests or other automated testing frameworks to simpli...