karma-sourcemap-writer

Sourcemap writer for karma-webpack to work with istanbul.

Why

Testing webpack project with karma-webpack is cool, until you want to use istanbul to do code coverage.

The problem

The codes were transpiled by webpack loaders so the coverage is not accurate.

The Solution

Use karma-sourcemap-writer to write source map file into right place and let remap-istanbul remap Istanbul code coverage information to its original source positions.

Install

npm install --save-dev karma-sourcemap-writer

Getting started

1. Setup karma-webpack in the alternative way. Let's call this file tests.webpack.js.

const context = require.context('./test', true, /.spec\.js$/);
context.keys().forEach(context);

2. Append sourceMappingURL to tests.webpack.js.

const context = require.context('./test', true, /.spec\.js$/);
context.keys().forEach(context);
//# sourceMappingURL=tests.webpack.js.map

3. Set devtool to inline-source-map

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

4. Apply karma-sourcemap-writer and karma-coverage

const webpackConfig = require('./webpack/config.test.js');

module.exports = function set(config) {
  config.set({
    browsers: ['Chrome'],
    singleRun: true,
    frameworks: ['mocha'],
    files: [
      'tests.webpack.js'
    ],
    preprocessors: {
      'tests.webpack.js': [
        'webpack',
        'sourcemap',
        'sourcemap-writer', // important!
        'coverage'          // important!
      ]
    },
    reporters: [
      'mocha',
      'coverage'            // important!
    ],
    webpack: webpackConfig,
    webpackServer: {
      noInfo: true
    },    
    coverageReporter: { // important!
      type: 'json',
      subdir: '.',
      file: 'coverage-final.json'
    },
    plugins: [
      'karma-chrome-launcher',
      'karma-webpack',
      'karma-mocha',
      'karma-mocha-reporter',
      'karma-sourcemap-loader',
      'karma-sourcemap-writer', // important!
      'karma-coverage'          // important!
    ]
  });
};

5. Run karma. This will generate coverage/coverage-final.json and tests.webpack.js.map beside tests.webpack.js

karma start

6. Run remap-istanbul to generate mapped report

remap-istanbul -i coverage/coverage-final.json -o coverage/coverage-remapped.json -t json

7. Generate clean coverage report

At this stage this report contains lots of unrelated coverage information, we have to run a script to remove them. Thanks to @otbe for the solution.

Create a node script and execute it.

const istanbul = require('istanbul');
const collector = new istanbul.Collector();
const reporter = new istanbul.Reporter();

const remappedJson = require('./coverage/coverage-remapped.json');
const coverage = Object.keys(remappedJson).reduce((result, source) => {
  // only keep js files under src/
  if (source.match(/^src\/.*\.js$/)) {
    result[source] = remappedJson[source];
  }

  return result;
}, {});

collector.add(coverage);

reporter.add('html');
reporter.write(
  collector,
  true,
  () => console.log('open coverage/index.html to see the coverage report.')
);

Done.

Example project

React Starter Kit