Mutation Testing Quality Gate
Use Stryker Mutator to verify that tests actually catch breakage — not just that they pass. Often used after TDD (e.g. in Ralph’s quality gate).
What It Does
Stryker modifies your source code (mutates it) and checks if your tests catch the change. If a mutation survives (tests still pass), there's a gap in your test coverage.
Examples of mutations:
> 0→>= 0(boundary condition)a + b→a - b(arithmetic operator)if (condition)→if (true)(removed conditional)return value→return ""(string literal){ ...body }→{}(removed block)
If your tests don't catch these, they're not testing real behavior.
Setup (one-time per project)
See references/setup.md: install Stryker packages, add stryker.config.mjs, npm scripts (test:mutate, test:mutate:incremental), and .gitignore entries.
The Quality Gate Workflow
Run after all unit tests pass:
- Run Stryker:
npm run test:mutate:incremental(first run 10–60 min; incremental 2–5 min). - Read survivors: Clear-text output shows file, line, and what was changed.
- Kill survivors on files you touched: understand the mutation → write a targeted test that would fail with the mutation → re-run incremental.
- Repeat until mutation score ≥ 95% on touched files. Use
thresholds.break: 90in config to fail the build if you regress.
Survivor table and tips
| Mutation | What It Means | How to Kill |
|---|---|---|
> 0 → >= 0 | No test with zero value | Add test with amount = 0, assert rejection |
a + b → a - b | No test verifying arithmetic result | Assert exact computed values |
if (x) → if (true) | Condition not tested as false | Add test where condition is false |
return value → return "" | Return value not asserted | Assert the specific return value |
{ ...body } → {} | Block removal not detected | Assert side effects of the block |
a.filter(fn) → a | Filter not tested | Add test with mixed data, assert filtered result |
a === b → a !== b | Only testing the happy path | Test both matching and non-matching cases |
- Run incremental, not full: After the first baseline, use
npm run test:mutate:incremental. Much faster. - Focus on your changes: Kill survivors only in code you touched.
- NoCoverage / CompileError: NoCoverage = add tests later; CompileError = no action.
- Don't chase 100%: 95% on covered code is excellent.