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Stryker Typescript

A collection of plugins for native TypeScript support in Stryker, the JavaScript TypeScript Mutation testing framework.


First, install Stryker itself (you can follow the quickstart on the website)

Next, install this package:

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

Now open up your stryker.conf.js file and add the following components:

coverageAnalysis: 'off', // Coverage analysis with a transpiler is not supported a.t.m.
tsconfigFile: 'tsconfig.json', // Location of your tsconfig.json file
mutator: 'typescript', // Specify that you want to mutate typescript code
transpilers: [
    'typescript' // Specify that your typescript code needs to be transpiled before tests can be run. Not needed if you're using ts-node Just-in-time compilation.

Now give it a go:

$ stryker run

Peer dependencies

The @stryker-mutator/typescript package is collection a plugins 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.

These are marked as peerDependencies so you get a warning during installation when the correct versions are not installed.

Load the plugins

In order to use one of the @stryker-mutator/typescript's plugins it must be loaded into Stryker. The easiest way to achieve this, is not have a plugins section in your config file. That way, all node_modules starting with stryker- will be loaded.

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

3 Plugins

This package contains 3 plugins to support TypeScript

  1. TypescriptConfigEditor
  2. TypescriptMutator
  3. TypescriptTranspiler


The TypescriptConfigEditor is a handy plugin that reads your tsconfig.json file and loads it into stryker.conf.js. It will capture all your tsconfig settings to the tsconfig in stryker (this property is later used by the TypescriptMutator and the TypescriptTranspiler)

Enable the config editor by pointing the tsconfigFile property to your tsconfig location:

// stryker.conf.js
 tsconfigFile: 'tsconfig.json',

We always override some properties to enforce these rules (see issue 391 to find out why):

allowUnreachableCode: true
noUnusedLocals: false
noUnusedParameters: false


The TypescriptMutator is a plugin to mutate typescript code. It builds a Typescript Abstract Syntax Tree (AST) and mutates your code using different kind of mutators.

See test code to know which mutations are supported.

Configure the Typescript mutator in your stryker.conf.js file:

// stryker.conf.js
    mutator: 'typescript'


The TypescriptTranspiler is a plugin to transpile typescript source code before running tests. If you're using a bundler you might want to configure that instead.

Given your Typescript configuration (see TypescriptConfigEditor) it generates the javascript output. This is also used to transpile each mutant to javascript. Internally, it uses the same method as Typescript's watch mode (tsc -w), so it can transpile mutants fairly efficiently.

Configure the Typescript transpiler in your stryker.conf.js file:

// stryker.conf.js
    transpilers: [
        // You can specify more transpilers if needed
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