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Tap Runner

Since v7.0

A plugin to use test files producing TAP-output in StrykerJS.


Test files producing TAP-output are usually produced by tests using the node-tap test runner, the build-in node test runner or Ava using --tap.


The tap runner doesn't use node-tap (or similar) to run your files. As such, you are responsible for compiling typescript files, or loading any other dependencies you might need.


Install @stryker-mutator/tap-runner locally within your project folder, like so:

npm i --save-dev @stryker-mutator/tap-runner


You can configure the tap test runner in the stryker.config.json (or stryker.config.js) file.

"testRunner": "tap",
"tap": {
"testFiles": ["test/**/*.@(js|ts)"],
"nodeArgs": ["--loader", "ts-node/esm"],
"forceBail": true

The tap runner will look for files that match the testFiles glob expressions and execute them using the nodeArgs as additional node arguments.

tap.testFiles [string[]]

Default: ["{**/@(test|tests|__test__|__tests__)/**,**/*.@(test|tests|spec)}.@(cjs|mjs|js|jsx|ts|tsx|mts|cts)"]

Specify glob expressions to your test files. By default, the tap runner will look for testy-looking files, like files in the test directory, as well as any files that end in *.spec.js or *.test.js. If you want to run a specific file, you should override this setting.

tap.nodeArgs [string[]]

7.1Add {{hookFile}} and {{testFile}} placeholder support

Default: ["-r", "{{hookFile}}", "{{testFile}}"]

Specify node arguments to be used when running the tests. You can use the following placeholders:

'{{hookFile}}'The actual location to the hook fileBy default it will be prepended ['-r', '{{hookFile}}', ...]
'{{testFile}}'The test file to runBy default it will appended last: [..., '{{testFile}}']

When the placeholder are not used, the defaults will be applied as described above.


ConfigGenerated actual node argumentsExplanation
[]["-r", "actual/hook.cjs", "test/foo.spec.js"]The default, works when using raw JavaScript test files as input
["--loader", "ts-node/esm"]["-r", "actual/hook.cjs", "--loader", "ts-node/esm", "test/foo.spec.js"]If you want to use ts-node to run your tests
["node_modules/ava/entrypoints/cli.mjs", "--tap", "--node-arguments='-r {{hookFile}}'"]["node_modules/ava/entrypoints/cli.mjs", "--tap", "--node-arguments='-r actual/hook.cjs'", "test/foo.spec.js"]If you are running test with Ava

The hook file is used to capture test metrics. It is important that it is loaded inside the test environment. Usually, this is done by using the -r node argument. However, some test runners (like Ava) run the test files in a separate process or worker_thread. For those use cases you should consult the documentation of the test runner in question to find a way to provide the hook file in the test environment.

tap.forceBail [boolean]

Default: true

This option is typically set to false when a test runner uses child processes to run the tests. When set to true, the tap runner will force the test runner to stop after the first failed test. This is useful when you want to speed up mutation testing, as it will prevent the test runner from running all tests, even when the first test already failed. But when the test runner uses child processes, it can cause the test runner to stop prematurely which results in Stryker failing to complete.

Tips and tricks

  • Configuring
    When you rely on your test runner to compile your typescript files, you should first try to run them yourself, for example by using node --loader ts-node/esm test/my-test-file.spec.ts. If that succeeds, you can proceed to add "nodeArgs": ["--loader", "ts-node/esm"] to your tap configuration.

  • Debugging
    You can run Stryker with --logLevel debug to see the actual node arguments that are used to run your tests.

  • Performance
    When you're normally using a JIT compiler like ts-node to run your tests, you can speed up mutation testing considerably by using a buildCommand to compile your files before running them.

    + "buildCommand": "tsc",
    "tap": {
    + "testFiles": ["dist/test/**/*.js"],
    - "nodeArgs": ["--loader", "ts-node/esm"],
    - "testFiles": ["test/**/*.ts"]


The tap runner is a simple test runner. As such, it has some limitations:

  • It doesn't support fine-grained test reporting. A test is always a test file.
  • Each test file is always run in a separate process. This can become slow when you have a lot of test files and a lot of mutants, especially when you're using a JIT compiler like ts-node.
  • Coverage is always recorded per test, which means that coverage is measured per test file. Static mutants are undetectable.