| name | kill-mutants |
| description | Raise the Stryker mutation score of taninsam — run mutation testing, read the surviving mutants, and add the unit tests that kill them. Use when asked about mutation testing, Stryker, mutation score, surviving mutants, or to harden the tests of a function beyond line coverage. |
Kill the surviving mutants
100% line coverage only proves the code ran. The mutation score proves the tests would notice if
it behaved differently. Maximising it is a goal of this library, so a survivor is a real defect in
the specification — never something to configure away.
Run it
yarn stryker
yarn stryker:debug
Full runs take a while (every src/**/*.ts except specs). To iterate on one function:
npx stryker run --mutate "src/take-last/*.ts"
Read the clear-text output: each survivor prints the file, the line, and the mutation applied
(- original, + mutant). dist/stryker-reports/report.html shows the same annotated in source.
coverageAnalysis: 'perTest' is on, so a mutant reported as survived really was covered by
tests that all still passed — as opposed to no coverage, which means the coverage report and the
mutation report disagree and the line is simply untested.
Diagnose
For each survivor, ask what input distinguishes the mutant from the original, then write the test
that uses it. The mutant is a question the documentation already answers; the tests just failed to
ask it.
Frequent survivors in this codebase:
| Mutation | The test that kills it |
|---|
< → <=, >= → > (take, max-by, boundary guards) | Assert exactly at the boundary: n, n - 1, n + 1, 0, and the equality case where both sides match |
0 === array.length → 1 === … / true / false | Test the empty collection and a one-element collection, both with explicit expected values |
| Early-return guard block removed | Provide the input that only that guard handles ([], undefined, single element) and assert the exact returned value |
Arithmetic flipped (+ → -, ** → /) | Use operands where both results differ — never 0, 1, or two equal numbers |
String literal → "" (error messages, hash prefixes like `b(${…})`) | toThrowErrorMatchingSnapshot() for errors; for hash, assert two values of different types do not collide |
Array literal [] → ["Stryker was here"] (accumulator seeds) | Assert the result of reducing an empty collection, not just a populated one |
Conditional expression forced to true/false | Cover both branches with distinct observable outputs, not with two inputs that happen to produce the same value |
Comparison of a sort/compare result (sort-by, ComparaisonResultChoice) | Sort data that needs the second and third key to break a tie, plus an already-sorted and a reverse-sorted input |
Logical operator swapped (|| → && in is-empty, is-nil) | One test per disjunct, each true in isolation, plus one input where all are false |
Strengthen, don't loosen
- Prefer
toEqual/toBe with the literal expected value over toBeTruthy/toBeFalsy:
expect(isEmpty(0)).toBe(false) kills mutants that toBeFalsy() lets through.
- A
toMatchSnapshot() whose .snap was generated from mutated or stubbed output pins the wrong
behaviour and survives everything. Read the .snap values against the @example blocks in the
TSDoc; regenerate the folder if they disagree.
- If a survivor is genuinely equivalent — a mutation that cannot change any observable output for
any admissible input — the code is saying something it does not need to say. Simplify the code
so the mutation point disappears, rather than adding a
// Stryker disable comment. Reach for a
disable comment only when the simplification would itself be a breaking change, and explain why
in the comment.
- New tests must keep coverage at 100% and the suite deterministic: no
Date.now(), no
Math.random(), no ordering assumption beyond what the function guarantees.
Finish
yarn vitest run --coverage
yarn stryker
yarn lint
Tests-only work is a test commit, and it releases nothing:
test(takeLast): cover boundary of n to kill surviving mutants
If killing a mutant required changing the function's behaviour, stop — that is a breaking change.
Read the api-stability skill first.
