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Announcing StrykerJS v6.0 Beta

ยท 8 min read
Nico Jansen
Stryker Team

We're excited to announce the beta of the next major release of the StrykerJS: StrykerJS 6.0.

From the 6th version onward, StrykerJS supports native ECMAScript modules, or ESM for short. To make this work, we changed some parts of StrykerJS, which includes small (yet technically breaking) changes. As a bonus, we've improved performance considerably in some use cases! We're curious to know what you think. Why not try the beta today?

Update your Stryker dependencies to get started using the beta.

npm install --save-dev @stryker-mutator/core@next
# OR
yarn add --dev @stryker-mutator/core@next

Don't forget to update Stryker plugins. For example, when using the mocha runner:

npm install --save-dev @stryker-mutator/mocha-runner@next
# OR
yarn add --dev @stryker-mutator/mocha-runner@next

If you're new to mutation testing, it's a way to measure your tests' effectiveness. Please take a look at our RoboCoasters ๐Ÿค–๐ŸŽข example and see how even 100% code coverage doesn't tell the whole story. Mutation testing is the only way to know if your tests are any good.

Now, let's dive into the changes! ๐ŸŠโ€โ™‚๏ธ

๐Ÿ“ฆ ESM support for NodeJS-based test runnersโ€‹

As stated in the introduction, StrykerJS v6 brings official support for ESM projects.

With ESM, you can use import and export statements. NodeJS has supported ESM since version 12. Browsers have gradually been adding support since 2017, and all major browsers support it by now. I recommend watching Gil Tayar's introduction to ESM when you want to brush up on the ESM basics.

The @stryker-mutator/karma-runner and @stryker-mutator/jest-runner already worked with ESM (for Jest, you will need to configure the --experimental-vm-modules flag), but our node-based runners did not. This changes with version 6.

You configure it like this:

As you can see no configuration change is needed. Updating StrykerJS is enough ๐Ÿ˜.

โฉ Hot reloadโ€‹

As you might know, StrykerJS uses mutation switching (sometimes called mutant schemata), meaning Stryker inserts all mutants into your source code yet only activates them one at a time.

It looks something like this:

function add(a, b) {
if (global.activeMutant === 0) {
// ๐Ÿ‘พ BlockStatement mutator
} else {
return global.activeMutant === 1
? a - b // ๐Ÿ‘ฝ ArithmeticExpression mutator
: a + b;

Next, Stryker creates worker processes. A worker process is responsible for activating a mutant, running tests, and reporting the result to the main process. To do that, it uses the test runner of your choice, for example, mocha. For NodeJS-based test runners, it would look something like this:

(old process)

  1. Activate the mutant
  2. Load all files, using require
  3. Run the tests with the test runner of choice.
  4. Unload all files, using delete require.cache['path/to/cjs/module'] to prepare for the next run.
  5. Report the result

As you can see, all files are loaded and unloaded for each mutant run. This way of reloading won't ever work with native ESM, as there is simply no cache we can delete the loaded files from ๐Ÿคทโ€โ™€๏ธ.

The reloading is also expensive. Think about it: if your project contains 1k files, with a total of 10k mutants, it could mean a total of ten million file IO actions. That's why mutation testing with StrykerJS would slow down considerably when the size of your project grows.

When you think about it, it is also unnecessary; why not simply keep all files loaded? Since they contain all the mutants anyway. This new process looks something like this:

(new process)

  1. (once) Load the code using import
  2. Activate the mutant
  3. Run the tests with the test runner of choice.
  4. Report the result

The result is a significant performance improvement ๐Ÿ‡. You can clearly see the difference when running StrykerJS on Stryker's core itself:

New *

* using --ignoreStatic

That's a whopping 70% performance improvement ๐Ÿคฏ.

We call this feature "hot reload," and StrykerJS supports it in @stryker-mutator/mocha-runner and @stryker-mutator/jasmine-runner. Please note that this feature is always on from version 6 onward, even for plain old CommonJS style projects! It does require a higher version of mocha or jasmine, respectively, see breaking changes. Support for @stryker-mutator/cucumber-runner for the release of version 8 of @cucumber/cucumber.

Unfortunately, we're not supporting hot reload for the @stryker-mutator/jest-runner or @stryker-mutator/karma-runner yet; please up-vote #3455 or #3454 respectively if you want to see it happen ๐Ÿ’™.

๐Ÿ—ฟ Ignore staticโ€‹

Yes, indeed, hot reload sounds incredible! But what about static mutants?

A static mutant is a mutant that is executed once on file load rather than during test runtime.

Take this small example:

// greet.js
const hi = '๐Ÿ‘‹';

export function greet(name) {
return `${hi} ${name}`;

// greet.spec.js
import { greet } from './greet.js';

it('should greet me', () => {
expect(greet('me')).toBe('๐Ÿ‘‹ me');

When you run StrykerJS on this code, it will create a mutant for the hi constant:

-const hi = '๐Ÿ‘‹';
+const hi = '';

When you consider mutant schemata, the actual code produced by StrykerJS looks more like this:

const hi = global.activeMutant === '1' ? '' : '๐Ÿ‘‹';

When you now factor in hot reload, you see that the mutant is only active during the runtime of the tests instead of at load time. This would mean that the mutant survives no matter how good your tests are. We can't have that ๐Ÿ˜ช!

That's why StrykerJS will detect static mutants and still follow the old process to test them. However, instead of reloading the files (remember, this won't work for ESM files), it will create a shiny, new worker process for every static mutant run (for NodeJS-based test runners).

Creating new worker processes to test static mutants makes them much more expensive in StrykerJS v6. Also, they are generally less interesting for you to spend your time on since they include constant strings, numbers, etc. You might even want to ignore them entirely! This is where --ignoreStatic comes in.

With --ignoreStatic static mutants are ... well ignored. You can still see them in your HTML report, but they won't count towards your mutation score.

Ignored static mutant
An ignored static mutant

Your mileage may vary, but for running StrykerJS on Stryker's core itself, we saw a whopping 50% performance improvement by ignoring 6% of our mutants (the static mutants) ๐Ÿคฏ. For more details on static mutants, see the new documentation on on them.

Please try this feature out and provide feedback. We're also thinking of ways to point this feature out to users of Stryker, or even enable it by default. Feel free to voice your opinion in the GitHub issue about it.

๐Ÿ“ƒ ESM-based configโ€‹

From v6 onward, StrykerJS also supports your config as ESM. Either by adding { "type": "module" } to your package.json file, or by renaming stryker.conf.js to stryker.conf.mjs.

// stryker.conf.mjs
/** @type {import('@stryker-mutator/api/core').PartialStrykerOptions} */
const config = {
reporters: ['html', 'clear-text', 'progress'],
testRunner: 'mocha',
coverageAnalysis: 'perTest',
export default config;

We've updated the stryker init command to emit this new config file format when choosing for "JavaScript" config format.

Note: for completeness sake, stryker.conf.cjs is also supported.

๐Ÿ’ฅ Breaking changesโ€‹

This is the list of breaking changes.

  • #3450 checker-api: The check method of checker plugins now receives a group of mutants and should provide a CheckResult per mutant id.
  • #3438 html reporter: Configuration option htmlReporter.baseDir is deprecated and will be removed in a later version. Please use htmlReporter.fileName instead.
  • #3432 esm config: Exporting a function (using module.exports = function(config) {}) from your stryker.conf.js file is no longer supported. This was already deprecated but now will give an error.
  • #3409 esm: StrykerJS is now a pure ESM. Please read this.
  • #3409 esm: Node 12.20 is now the min version.
  • #3409 esm: Karma v6.3 is now the min supported karma version for @stryker-mutator/karma-runner, since that version added support for async config loading
  • #3396 esm: The @stryker-mutator/jasmine-runner now requires [email protected] or higher.
  • #3393 esm: The @stryker-mutator/mocha-runner now requires [email protected] or higher.
  • #3369 reload test environment: Test runner plugins must provide TestRunnerCapabilities by implementing the capabilities method.

๐Ÿ”ฎ What's nextโ€‹

We'll be focussing on getting StrykerJS 6.0 released in about a month or so. We would appreciate your feedback. Feel free to report any issues you find or voice your opinion in the comment section below ๐Ÿ‘‡ or on our slack channel.