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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.conf.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.


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



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

Troubleshooting FAQ#

Build command fails#

If you're running into issues with your buildCommand, try to execute it by hand to make sure it works. For example, if your buildCommand is "tsc -b path/to/tsconfig.json", try to run npx tsc -b path/to/tsconfig.json yourself, did it have the desired effect?

If that works, the next step would be to try the command yourself inside the Stryker sandbox directory. Please cd into the .stryker-tmp/sandbox-xxxxx directory and try the command there. For example:

cd .stryker-tmp/sandbox-12345678
node ../../node_modules/typescript/bin/tsc -b path/to/tsconfig.json

Note: since the sandbox dir doesn't have you node_modules, you will need to specify the full path to the TypeScript compiler.

Initial test run fails#

You might run into issues like this:

One or more tests resulted in an error [...] Cannot assign to 'stryNS_9fa48' because it is not a variable [...]

The initial test run might fail when you're using ts-jest or ts-node. The reason for this is that Stryker will mutate your code and, by doing so, introduce type errors into your code. Stryker tries to ignore these errors by adding // @ts-nocheck in your source files. However, this is only done for TypeScript-like files inside your lib, src, and test directories by default. If you have your code somewhere else, you will need to override disableTypeChecks yourself:

"disableTypeChecks": "app/**/*.{js,ts,jsx,tsx,html,vue}"

No tests executed - jest runner#

You might run into issues like this when using the @stryker-mutator/jest-runner:

No tests found, exiting with code 1 Run with --passWithNoTests to exit with code 0

You will need to override the tempDirName to a directory without a . in front of it.

"tempDirName": "stryker-tmp"

See #1691 for more info.