js四舍五入封存指定小数位

//四舍五入保留二位小数
function round(num,dec){ 
    var sNum = num + '';
    var idx = sNum.indexOf(".");    
    if(idx<0)   return num; 
    var n = sNum.length - idx -1; 
    if(dec < n){ 
        var e = Math.pow(10,dec);
        return Math.round(num * e) / e; 
    }else{ 
        return num; 
    } 

}