Day 11 of 60
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Unit, integration, contract
Unit testing
The cheapest defects to fix are the ones the engineer who wrote them catches before lunch. Unit tests are the discipline of catching them at the keyboard, not at the dashboard.
ProblemLogic regressions and incorrect implementations.
How it works
Small, fast, isolated tests of pure functions and small modules. Run on every save and every PR. Foundation of the test pyramid.
What it catches
Logic errors in pure code. Catches ~30–40% of defects when paired with type checking. Diminishing returns past 70% line coverage.
Tools
Vitest · OSS Jest · OSS pytest · OSS JUnit / xUnit / NUnit · OSS
Verdict by project size
Small
Must
Medium
Must
Large
Must
Extra-large
Must
Cost
| Project size | Setup | Maint / mo | Tool / mo | CI / run |
|---|---|---|---|---|
| Small <10k LOC | 4h | 4h | $0 | +1m |
| Medium 10–100k LOC | 2d | 20h | $0 | +3m |
| Large 100k–1M LOC | 5d | 80h | $0 | +8m |
| Extra-large >1M LOC | 15d | 300h | $0 | +20m |
Setup = engineer-days to first useful run ·
Maint = engineer-hours / month at steady state ·
Tool = out-of-pocket $ / month ·
CI = minutes added (or saved) per pipeline run
Lifecycle & ownership
When in lifecycle
Code Test
Per pull request · Runs in CI on every PR; gates merge.
Who owns it
Developer
Authoring + the inner loop
Collaborates with: QA / Test Engineer
Reference implementations
-
Vitest examples
Modern fast unit-test patterns across common JavaScript and TypeScript setups.
-
pytest examples
Official examples for fixtures, parametrization, and test organization.
-
JUnit 5 samples
Reference JVM unit-test projects using modern JUnit patterns.
Quick check
Where do unit tests show diminishing returns?
One question. Pick the best answer. Your streak is saved locally on this device.
Save the lesson
Download SVG ↓
Screenshot for a 1:1, drop it in Slack, or download the SVG.