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

|Name|Type|Default|Description| |:--:|:--:|:-----:|:----------| |debug|{Boolean}|false|Turn on debugging mode| |compact|{Boolean}|true|Generate compact code| |autoWrap|{Boolean}|false|Set to true to allow return statements outside of functions| |esModules|{Boolean}|false|Set to true to instrument ES2015 Modules| |coverageVariable|{String}|__coverage__|Name of global coverage variable| |preserveComments|{Boolean}|false|Preserve comments in output| |produceSourceMap|{Boolean}|false|Set to true to produce a source map for the instrumented code| |fixWebpackSourcePaths|{Boolean}|false|Set to true to prevent filepath error in coverage| |sourceMapUrlCallback|{Function}|null|A callback function that is called when a source map URL is found in the original code. This function is called with the source filename and the source map URL|

webpack.config.js

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