karma webpack --grep

grep.js

module.exports = function (config) {
  // karma parses cmd line args, therefore
  // when executing `karma start --grep some/path`
  // config will have `grep` set to 'some/path'
  if (config.grep) {
    var pattern = config.grep
      // we then remove the .js suffix, this is useful
      // because if you're typing `karma start --grep some/test_fi` and tab
      // the terminal autocompletes to `karma start --grep some/test_file.js`
      // however, we want to remove this to effectively make use of the
      // ContextReplacementPlugin below
      .replace(/\.js$/, '')
      // do the samewith the Test suffix if any
      .replace(/Test$/, '')
    // this needs to match exactly the string you used in your
    // testMain.js file in require.ensure() call
    var testContext = './tests'
    // add a new plugin to your webpack config
    webpackConfig.plugins = (webpackConfig.plugins || []).concat([
      // now we're gonna use the little known webpack plugin called
      // ContextReplacementPlugin to replace the './tests' context,
      // with a different regex instead of /Test$/ or /_test$/ or
      // whatever you have it configured as.
      new webpack.ContextReplacementPlugin(new RegExp(testContext), function (result) {
        // need to do this, since webpack calls this function multiple times
        // for some reason with various different objects. By checking the
        // result.request, we can find the object we're looking for
        if (result.request === testContext) {
          // finally create an entirely new pattern for the dynamic require.ensure,
          // e.g. with this new pattern executing
          // $ karma start --grep tests/module/foo
          // will only build those files that match: /.*tests/module/foo.*Test/
          result.regExp = new RegExp('.*' + pattern + '.*Test$')
        }
      })
    ])
  }
  
  // the usual karma config setup
  config.set({ /* ... */ })
}