code: function sanitize(config) {
    const configs = [].concat(config).map((conf) => {
      const result = merge({}, conf);
      delete result.progress;
      delete result.reporter;
      delete result.watchStdin;
  
      // TODO: remove the need for this
      for (const property of ['entry', 'output']) {
        const target = result[property];
        if (target && Object.keys(target).length === 0) {
          delete result[property];
        }
      }
  
      return result;
    });
  
    // if we always return an array, every compilation will be a MultiCompiler
    return configs.length > 1 ? configs : configs[0];
  }
docs: Removes invalid webpack config properties leftover from applying flags