function walk(holder, key) {
          var k, v, value = holder[key];
          if (value && typeof value === 'object') {
              for (k in value) {
                  if (Object.prototype.hasOwnProperty.call(value, k)) {
                      v = walk(value, k);
                      if (v !== undefined) {
                          value[k] = v;
                      } else {
                          delete value[k];
                      }
                  }
              }
          }
          return reviver.call(holder, key, value);
      }
parse the json

@memberof vjs.JSON
@return {Object|Array} The parsed JSON