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npm npm i -D karma-rspack

yarn yarn add -D karma-rspack

karma.conf.js

module.exports = (config) => {
  config.set({
    // ... normal karma configuration

    // make sure to include rspack as a framework
    frameworks: ['mocha', 'rspack'],
    
    plugins: [
      'karma-rspack',
      'karma-mocha',
    ],

    files: [
      // all files ending in ".test.js"
      // !!! use watched: false as we use rspacks watch
      { pattern: 'test/**/*.test.js', watched: false }
    ],

    preprocessors: {
      // add rspack as preprocessor
      'test/**/*.test.js': [ 'rspack' ]
    },

    rspack: {
      // karma watches the test entry points
      // Do NOT specify the entry option
      // rspack watches dependencies

      // rspack configuration
    },
  });
}

Default rspack configuration

This configuration will be merged with what gets provided via karma's config.rspack.

const defaultRspackOptions = {
  mode: 'development',
  output: {
    filename: '[name].js',
    path: path.join(os.tmpdir(), '_karma_rspack_') + Math.floor(Math.random() * 1000000),
  },
  stats: {
    modules: false,
    colors: true,
  },
  watch: false,
  optimization: {
    runtimeChunk: 'single',
    splitChunks: {
      chunks: 'all',
      minSize: 0,
      cacheGroups: {
        commons: {
          name: 'commons',
          chunks: 'initial',
          minChunks: 1,
        },
      },
    },
  },
  plugins: [],
};

How it works

This project is a framework and preprocessor for Karma that combines test files and dependencies into 2 shared bundles and 1 chunk per test file. It relies on rspack to generate the bundles/chunks and to keep it updated during autoWatch=true.

The first preproccessor triggers the build of all the bundles/chunks and all following files just return the output of this one build process.

Rspack typescript support

By default karma-rspack forces *.js files so if you test *.ts files and use rspack to build typescript to javascript it works out of the box.

If you have a different need you can override by setting rspack.transformPath

// this is the by default applied transformPath
rspack: {
  transformPath: (filepath) => {
      // force *.js files by default
      const info = path.parse(filepath);
      return `${path.join(info.dir, info.name)}.js`;
    },
},

Source Maps

You can use the karma-sourcemap-loader to get the source maps generated for your test bundle.

npm i -D karma-sourcemap-loader

And then add it to your preprocessors.

karma.conf.js

preprocessors: {
  'test/test_index.js': [ 'rspack', 'sourcemap' ]
}

And tell rspack to generate sourcemaps.

rspack.config.js

rspack: {
  // ...
  devtool: 'inline-source-map'
}