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TypeScript Checker

A TypeScript type checker plugin for Stryker, the JavaScript TypeScript Mutation testing framework. This plugin enables type checking on mutants, so you won't have to waste time on mutants which result in a type error.


👽 Type check each mutant. Invalid mutants will be marked as CompileError in your Stryker report.
🧒 Easy to setup, only your tsconfig.json file is needed.
🔢 Type check is done in-memory, no side effects on disk.
🎁 Support for both single typescript projects as well as projects with project references (--build mode).


First, install Stryker itself (you can follow the getting started)

Next, install this package:

npm install --save-dev @stryker-mutator/typescript-checker


You can configure the typescript checker in the stryker.config.json (or stryker.config.js) file.

"checkers": ["typescript"],
"tsconfigFile": "tsconfig.json",
"typescriptChecker": {
"prioritizePerformanceOverAccuracy": true

tsconfigFile [string]

Default: 'tsconfig.json'

The path to your tsconfig. Project references are supported, --build mode will be enabled automatically when references are found in your tsconfig.json file.

Note: the following compiler options are always overridden by @stryker-mutator/typescript-checker to avoid false positives. See issue 391 for more information on this

"compilerOptions": {
"allowUnreachableCode": true,
"noUnusedLocals": false,
"noUnusedParameters": false

typescriptChecker.prioritizePerformanceOverAccuracy [boolean]

Default: true

Sets the performance strategy for the typescript-checker. Defaults to true which the fastest strategy with the consequence of losing some accuracy. The accuracy that is lost comes down to having mutants with a status other than CompileError while they should have this status. This result in a report that may not be 100% accurate. Setting this option to false results in an accurate report but may take (way) longer.

Peer dependencies

The @stryker-mutator/typescript-checker package for stryker to enable typescript support. As such, you should make sure you have the correct versions of its dependencies installed:

  • typescript
  • @stryker-mutator/core

For the current versions, see the peerDependencies section in the package.json.

Load the plugin

In this plugin the @stryker-mutator/typescript-checker' must be loaded into Stryker. The easiest way to achieve this, is not have a plugins section in your config file. That way, all plugins starting with "@stryker-mutator/" will be loaded.

If you do decide to choose specific modules, don't forget to add "@stryker-mutator/typescript-checker" to the list of plugins to load.