text: 
  // 提升性能
  module.exports = {
      extend: function(target, source, flag) {
          for(var key in source) {
              if(source.hasOwnProperty(key))
                  flag ?
                      (target[key] = source[key]) :
                      (target[key] === void 0 && (target[key] = source[key]));
          }
          return target;
      }
  }