toFixed 不能四舍五入问题

Number.prototype.toFixed = function(s) {  
       var changenum = (parseInt(this * Math.pow(10, s) + 0.5) /         
       Math.pow(10, s)).toString();  
            index = changenum.indexOf(".");  
            if (index < 0 && s > 0) {  
                changenum = changenum + "."; 
                for (i = 0; i < s; i++) {  
                    changenum = changenum + "0";  
                }  
  
            } else {  
                index = changenum.length - index;  
                for (i = 0; i < (s - index) + 1; i++) {  
                    changenum = changenum + "0";  
                }  
  
        }  
  
        return changenum;  
  }