mutation-testing-stryker-gotchas

A survivor no test can kill is likely an equivalent mutant. First prove equivalence, then prefer adopting the simpler program by deleting the dead code path. Annotate with a disable directive only when no honest refactor exists, and put the equivalence proof in the reason.

Stryker reads the directive from a node's leading comments and keys next-line off that node's line. A comment dangling above a closing line like `}, []);` leads no node and is silently ignored. Expand the call so the annotated element starts on its own line.

ArrayDeclaration rewrites an empty deps array to a one-element constant, but React compares deps with Object.is so the effect fires identically, making the mutant equivalent. Hoisting the literal would remove it, but react-hooks/exhaustive-deps rejects non-literal deps, so annotate with the equivalence proof.

Shard membership is packed by file size, so editing a file moves it between shards while incremental caches retain old results. Merging on first occurrence lets stale results outrank the current owner. Rebuild the packing with the shards' algorithm and keep each file only from its current owner.

Do not use a disable directive for ordinary survivors that a real assertion would kill — write the assertion instead. Also avoid it before checking whether the simpler program is available, and never lower the break threshold or narrow mutate scope to get past a survivor.