in mempory webpack testing

webpack helpers.js

export function run(config, assert) {
  return new Promise((resolve, reject) => {
    let compiler = webpack(config)
    let fs = compiler.outputFileSystem = new MemoryFileSystem();

    compiler.run(function (err, stats) {
      if (err) return reject(err)
      let errors = stats.compilation.errors || [];
      if (assert) expect(errors.length).to.equal(0, errors.join('\n'))

      resolve({
        fs,
        output: getOutputFiles(fs, config.output.path),
        stats,
        errors,
        warnings: stats.compilation.warnings || []
      })
    })
  })
}

function getOutputFiles(fs, dir) {
  let files = []
  dir = path.normalize(dir)
  getFiles(dir)

  return files.map(f => path.normalize(f).replace(dir + path.sep, ''))

  function getFiles(dir) {
    fs.readdirSync(dir).forEach(filename => {
      let file = path.join(dir, filename)
      if (fs.statSync(file).isDirectory())
        getFiles(file)
      else
        files.push(file)
    })
  }
}