属性扩展

// 属性扩展

        // @param isCover 是否覆盖已有属性  默认true

        this.extend = function(destination, source, isCover) {

            var isUndefined = this.isUndefined;

            (isUndefined(isCover)) && ( isCover = true);

            for (var name in source) {

                if (isCover || isUndefined(destination[name])) {

                    destination[name] = source[name];

                }

            }

            return destination;

        };