js substring从右边获取指定长度字符串(示例代码)

如下所示：

 /*    Get the rightmost substring, of the specified length, 
   from a String object. 
*/  
String.prototype.right = function (length_)  
{  
        var _from = this.length - length_;  
        if (_from < 0) _from = 0;  
        return this.substring(this.length - length_, this.length);  
};