code: function() {
          var background = '',
              style = this;
          if(style.getPropertyValue('background')) {
  			return;
  		}
          var positionText = this.removeProperty('background-position-x') + ' ' +
              this.removeProperty('background-position-y');
  
          style.setProperty('background-position', positionText.trim(), null);
  
          var toMergeAttrs = [
              'background-color', 'background-image', 'background-position',
              'background-repeat', 'background-attachment',
              'background-origin', 'background-clip'
          ];
          for (var i = 0, item; item = toMergeAttrs[i]; i++) {
              if (style.hasOwnProperty(item)) {
                  background += this.removeProperty(item) + ' ';
              }
          }
          if(background.trim()) {
              style.setProperty('background', background.trim(), null);
          }
      }
docs: 把 style 里面的 background 属性转换成简写形式, 用于减少代码