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: 90 in 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.

mutation-testing

Mutation Testing Quality Gate

What It Does

Setup (one-time per project)

The Quality Gate Workflow

Survivor table and tips