Why We Write Unit Tests

• 2023/07/08

Why We Write Unit Tests

Do you write unit tests? I think they are essential for any long-lived product—web services, batch jobs, anything multiple people touch (less so for disposable scripts). Experienced engineers may take testing for granted, but I wanted to revisit why unit tests matter.

1. Higher product quality

Unit tests check whether expected outputs come back for given inputs (happy paths) and whether unexpected inputs are handled (error paths). For an if statement, cover both true and false. For multiple branches, decide whether to pursue C0/C1/C2 coverage, etc. Coverage tools (JUnit/JaCoCo, Python’s coverage) tell you how much code the tests exercise and serve as CI gates.

2. Resilience to change

As systems grow, change impact becomes harder to predict. Running unit tests after modifications helps surface regressions—assuming tests are written properly. Even baseline coverage-focused tests are useful; add edge cases when possible. Without tests, touching the system feels risky, and teammates avoid it.

3. Encouraging reuse

Teams often copy-paste similar logic because helper functions lack tests, the culture does not value abstraction, or both. Controllers might have tests, but what about file upload helpers or validation utilities they call? When helpers lack tests, people avoid reusing them. Write tests for shared functions, centralize logic, and reuse becomes more attractive.

Closing

Unit tests shine during incidents and large refactors. Even for personal projects, they help you remember intent and catch bugs. Tests matter.