var changeTwoDecimal_f= function (floatvar){ 
    var f_x = parseFloat(floatvar); 
    if (isNaN(f_x)){ 
        return '0.00'; 
    } 
    var f_x = Math.round(f_x*100)/100; 
    var s_x = f_x.toString(); 
    var pos_decimal = s_x.indexOf('.'); 
    if (pos_decimal < 0){ 
        pos_decimal = s_x.length; 
        s_x += '.'; 
    } 
    while (s_x.length <= pos_decimal + 2){ 
        s_x += '0'; 
    } 
    return s_x; 
} 

Js provides a rounding function:

js  Round the function  toFixed (), the parameter inside   I'm just keeping the number of decimal places.  

<script language="javascript">  
document.write("<h1>JS Keep the two decimal examples </h1><br>");   
   var a=2.1512131231231321;   
   document.write(" Original value: "+a+"<br>");   
   document.write(" Two decimal places :"+a.toFixed(2)+"<br> Four digit decimal point "+a.toFixed(4));   
</script>