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Stryker can run Mutation Testing on all NodeJS projects. Either by using one of the test runner plugins or with the command test runner. It also supports a custom buildCommand. This command is useful to compile TypeScript or babel code or to bundle your code.

About transpiling

There are multiple scenarios of transpiling code when running your tests.

  • Ahead-of-time
    Use tsc or @babel/cli to compile your code before testing or use webpack or another bundler to create a bundle before running tests.
  • Just-in-time
    Use ts-node or @babel/register as a just-in-time compiler to compile your code on the fly.

Both scenarios are supported, however using just-in-time transpiling during mutation testing is not recommended because it means running the compiler a large number of times. Since Stryker uses mutation switching, compiling only once is preferred. Don't worry; this guide will help you configuring Stryker correctly.

If you manage your code's compilation through a tsconfig.json file, this guide ensures that your TypeScript code uses that configuration.

This guide does not cover using @stryker-mutator/typescript-checker. Please review the page here if you want to include the typescript checker.


Please follow this configuration guide. Place the configuration examples inside your stryker.config.json file.

Build command


"buildCommand": "tsc -b path/to/tsconfig.json"

You generally don't have to configure a buildCommand if you're using the Jest test runner.

Use the buildCommand to configure a command that Stryker can run in its sandbox, just after your code is mutated. If you're using a bundler, you will need to change this command by a command that creates a bundle, like "webpack --config webpack.test.config.js". You can also use a script you've defined in package.json, for example "npm run build".

Don't worry about your PATH environment variable; Stryker will make sure your local dependencies are available there before executing the build command inside the sandbox.

If you're using ts-node or @babel/register to just-in-time compile during unit testing, then it's a good idea to configure your build command-equivalent here. Some examples:

  • For ts-node: tsc -b path/to/tsconfig.json
  • For @babel/register: babel src --out-dir lib (using the @babel/cli)

Be sure to test them out yourself first.

Test runner

Next, configure the test runner you're using. If you're using a different test runner than described here, you can still use the default command test runner



"coverageAnalysis": "perTest",
"mochaOptions": {
"config": "path/to/your/.mocharc.js/file",
"spec": [ "dist/test/**/*.spec.js"]
"testRunner": "mocha"

Use the mochaOptions to configure the mocha test runner. If your project uses a mocha config file, you can specify it in mochaOptions.config; use other settings to override settings in the config file.

If you're using a buildCommand, be sure to configure the js output files in the mochaOptions.spec instead of the ts input files, otherwise mocha won't be able to find your test files.

If you choose to keep using your just-in-time compiler and accept the performance penalty, you can use mochaOptions.require to configure your ts-node or @babel/register transpiler. Also, you may want to override the ts-node configuration options via environment variables. You can do so using environment variables, for example:

"scripts": {
"test:mutation": "cross-env TS_NODE_PROJECT=path/to/your/tsconfig.json stryker run"

cross-env is a tool to help you set environment variables across platforms.



"coverageAnalysis": "perTest",
"jest": {
"projectType": "custom",
"configFile": "path/to/your/custom/jestConfig.js"
"tempDirName": "stryker-tmp",
"testRunner": "jest"

Jest has two ways to provide TypeScript compilation, either using babel (default) or using ts-jest. For both, you don't need to configure the buildCommand.



"coverageAnalysis": "perTest",
"jasmineConfigFile": "spec/support/jasmine.json",
"testRunner": "jasmine"


Run Stryker as per usual, either using a custom script in package.json or using npx.

npx stryker run
# OR, if your script is called "test:mutation"
npm run test:mutation


Please take a look at the troubleshooting page when you run into any problems setting up StrykerJS.