function returnFloat(value){
 var value=Math.round(parseFloat(value)*100)/100;
 var xsd=value.toString().split(".");
 if(xsd.length==1){
 value=value.toString()+".00";
 return value;
 }
 if(xsd.length>1){
 if(xsd[1].length<2){
  value=value.toString()+"0";
 }
 return value;
 }
}
var num=3.1;
console.log(returnFloat(num));

if(xsd.length>1){
 if(xsd[1].length<2){
  value=value.toString()+"0";
 }
 return value;
 }

 var a = 9.39393; 
 alert(a.toFixed(2));

 function roundFun(numberRound,roundDigit) //四舍五入，保留位数为roundDigit  
 { 
 if (numberRound>=0) 
 { 
 var tempNumber = parseInt((numberRound * Math.pow(10,roundDigit)+0.5))/Math.pow(10,roundDigit); 
 return tempNumber; 
 } 
 else  
 { 
 numberRound1=-numberRound 
 var tempNumber = parseInt((numberRound1 * Math.pow(10,roundDigit)+0.5))/Math.pow(10,roundDigit); 
 return -tempNumber; 
 } 
   } 

<script> 
 tmp = "1234567.57232" 
 result = tmp.substr(0,tmp.indexOf(".")+3); 
 alert(result); 
 </script> 

<script type="text/javascript"> 
  //保留两位小数 
  //功能：将浮点数四舍五入，取小数点后2位 
  function toDecimal(x) { 
   var f = parseFloat(x); 
   if (isNaN(f)) { 
    return; 
   } 
   f = Math.round(x*100)/100; 
   return f; 
  } 
 
 
  //制保留2位小数，如：2，会在2后面补上00.即2.00 
  function toDecimal2(x) { 
   var f = parseFloat(x); 
   if (isNaN(f)) { 
    return false; 
   } 
   var f = Math.round(x*100)/100; 
   var s = f.toString(); 
   var rs = s.indexOf('.'); 
   if (rs < 0) { 
    rs = s.length; 
    s += '.'; 
   } 
   while (s.length <= rs + 2) { 
    s += '0'; 
   } 
   return s; 
  } 
   
  function fomatFloat(src,pos){  
    return Math.round(src*Math.pow(10, pos))/Math.pow(10, pos);  
  } 
  //四舍五入 
  alert("保留2位小数：" + toDecimal(3.14159267)); 
  alert("强制保留2位小数：" + toDecimal2(3.14159267)); 
  alert("保留2位小数：" + toDecimal(3.14559267)); 
  alert("强制保留2位小数：" + toDecimal2(3.15159267)); 
  alert("保留2位小数：" + fomatFloat(3.14559267, 2)); 
  alert("保留1位小数：" + fomatFloat(3.15159267, 1)); 
   
  //五舍六入 
  alert("保留2位小数：" + 1000.003.toFixed(2)); 
  alert("保留1位小数：" + 1000.08.toFixed(1)); 
  alert("保留1位小数：" + 1000.04.toFixed(1)); 
  alert("保留1位小数：" + 1000.05.toFixed(1)); 
   
  //科学计数 
  alert(3.1415.toExponential(2)); 
  alert(3.1455.toExponential(2)); 
  alert(3.1445.toExponential(2)); 
  alert(3.1465.toExponential(2)); 
  alert(3.1665.toExponential(1)); 
  //精确到n位，不含n位 
  alert("精确到小数点第2位" + 3.1415.toPrecision(2)); 
  alert("精确到小数点第3位" + 3.1465.toPrecision(3)); 
  alert("精确到小数点第2位" + 3.1415.toPrecision(2)); 
  alert("精确到小数点第2位" + 3.1455.toPrecision(2)); 
  alert("精确到小数点第5位" + 3.141592679287.toPrecision(5)); 
 </script>