Awesome

Instrument JS files with istanbul-lib-instrument for subsequent code coverage reporting

npm i -D istanbul-instrumenter-loader

References

• karma-webpack
• karma-coverage-istanbul-reporter

Structure

├─ src
│ |– components
│ | |– bar
│ | │ |─ index.js
│ | |– foo/
│     |– index.js
|– test
| |– src
| | |– components
| | | |– foo
| | | | |– index.js

To create a code coverage report for all components (even for those for which you have no tests yet) you have to require all the 1) sources and 2) tests. Something like it's described in "alternative usage" of karma-webpack

test/index.js

// requires all tests in `project/test/src/components/**/index.js`
const tests = require.context('./src/components/', true, /index\.js$/);

tests.keys().forEach(tests);

// requires all components in `project/src/components/**/index.js`
const components = require.context('../src/components/', true, /index\.js$/);

components.keys().forEach(components);

ℹ️ This file will be the only entry point for karma

karma.conf.js

config.set({
  ...
  files: [
    'test/index.js'
  ],
  preprocessors: {
    'test/index.js': 'webpack'
  },
  webpack: {
    ...
    module: {
      rules: [
        // instrument only testing sources with Istanbul
        {
          test: /\.js$/,
          use: { loader: 'istanbul-instrumenter-loader' },
          include: path.resolve('src/components/')
        }
      ]
    }
    ...
  },
  reporters: [ 'progress', 'coverage-istanbul' ],
  coverageIstanbulReporter: {
    reports: [ 'text-summary' ],
    fixWebpackSourcePaths: true
  }
  ...
});

with Babel

You must run the instrumentation as a post step

webpack.config.js

{
  test: /\.js$|\.jsx$/,
  use: {
    loader: 'istanbul-instrumenter-loader',
    options: { esModules: true }
  },
  enforce: 'post',
  exclude: /node_modules|\.spec\.js$/,
}

The loader supports all options supported by istanbul-lib-instrument

webpack.config.js

{
  test: /\.js$/,
  use: {
    loader: 'istanbul-instrumenter-loader',
    options: {...options}
  }
}