js对浮点数进行四舍五入，按需要保留小数位数

function formatNumber(pnumber,decimals){
 if (isNaN(pnumber) || pnumber=='') { return 0};
 
 var snum = new String(pnumber);
 var sec = snum.split('.');
 var whole = parseFloat(sec[0]);
 var result = '';
 
 if (sec.length > 1){
  var dec = new String(sec[1]);
  dec = String(parseFloat(sec[1])/Math.pow(10,(dec.length - decimals)));
  dec = String(whole + Math.round(parseFloat(dec))/Math.pow(10,decimals));
  var dot = dec.indexOf('.');
  if (dot == -1){
   dec += '.';
   dot = dec.indexOf('.');
  }
  while(dec.length <= dot + decimals) { dec += '0'; }
  result = dec;
 } else{
  var dot;
  var dec = new String(whole);
  dec += '.';
  dot = dec.indexOf('.');   
  while(dec.length <= dot + decimals) { dec += '0'; }
  result = dec;
 } 
 return result;
}