stryker-tsgo-checker

Experimental StrykerJS checker plugin that type-checks mutants with tsgo, the native TypeScript compiler: 3.6× faster (54s→15s) on one 238-mutant / 5-file project; your numbers will vary.

Install

npm i -D stryker-tsgo-checker @stryker-mutator/typescript-checker

Add the checker to your Stryker config:

{
  "checkers": ["tsgo"],
  "appendPlugins": ["stryker-tsgo-checker"],
  "tsconfigFile": "tsconfig.json"
}

Use appendPlugins, not plugins. plugins overwrites Stryker's defaults and can break your test runner.

The parity tool uses the official typescript checker as the tsc baseline, so @stryker-mutator/typescript-checker must be installed alongside this package.

⚠️ Experimental — verify parity before trusting your score

This package uses @typescript/native-preview, a pre-GA TypeScript compiler preview with APIs under unstable/* paths. Mutation-score correctness depends on tsgo classifying compile-error mutants the same way tsc does for your codebase.

A mismatch can silently skew the score:

• tsc reports CompileError, tsgo does not: the mutant may run and survive even though it should be excluded.
• tsgo reports CompileError, tsc does not: a valid mutant may be excluded, inflating the score.

Run the parity gate before trusting tsgo results:

npx stryker-tsgo-parity

Only trust the tsgo score if the command exits 0. The package pins @typescript/native-preview exactly because the preview API and diagnostics can change between date-stamped builds. Re-run parity on every bump.

The current pin was parity-checked on one real project: 5 files, 238 mutants, 54 matching CompileError mutants.

How it works

stryker-tsgo-checker keeps one resident tsgo API instance from @typescript/native-preview/unstable/sync. It gives tsgo a virtual file system that reads from disk but overlays mutant contents in memory. For each single-mutant group, the checker writes the mutated file into the overlay, calls updateSnapshot({ fileChanges }), reads structured semantic diagnostics, maps TypeScript error diagnostics to Stryker's CompileError, then restores the original file in the overlay.

That means no subprocess per mutant and no stderr parsing.

Limitations

• 0.x targets one tsconfig project. It does not implement --build or TypeScript project-reference orchestration.
• The project must typecheck cleanly under tsgo before mutation checking starts. Baseline tsgo errors abort the checker.
• The parity tool supports JSON Stryker configs in 0.x. If your config is .js or .mjs, convert it to JSON for the parity run.

License

CC0-1.0. Public domain. Copy it, fork it, upstream it, and remove attribution if that helps Stryker absorb the idea.