code: function readWebpackConfig( server ) {
  
    const devConfig = server.webpackConfig && server.webpackConfig.dev;
    const configFile = paths.resolveApp( devConfig ) || appWebpackDevConfig;
  
    try {
  
      assert(
        existsSync( configFile ),
        `File ${devConfig || WEBPACK_DEV_CONFIG} is not exsit.`
      );
  
      const wpConfig = getConfig( configFile );
  
      // plugins: function
      if ( is.Function( wpConfig.plugins )) {
        wpConfig.plugins = wpConfig.plugins( wpConfig );
      }
  
      // plugins: not array
      if ( !is.Array( wpConfig.plugins )) {
        wpConfig.plugins = [wpConfig.plugins];
      }
  
      // plugins: array in array
      wpConfig.plugins = flatten( wpConfig.plugins.map(( plugin ) => {
        return is.Function( plugin ) ? plugin( wpConfig ) : plugin;
      }));
  
      server.webpackDevConfig = wpConfig;
      watchFiles.push( configFile );
  
      return Right( server );
    } catch ( e ) {
      print(
        chalk.red( `Failed to read ${devConfig || WEBPACK_DEV_CONFIG}.` ),
        e.message
      );
      print();
      return Left( null );
    }
  }
docs: read config for webpack dev server