code: function (offset) {
                  if (undefined === offset) {
                      this.offset = 0;
                  }
  
                  if (offset < 0 || offset > this.length) {
                      error.raise(10002, this.length, offset);
                  }
  
                  this._offset = this.offset;
                  this.offset = offset;
  
                  return this;
              }
docs: 跳转到指定偏移
  
  @param {number=} offset 偏移
  @return {this}