Javascript Number format(thousandSep, decPoint)

Number.THOUSAND_STEP = ",";
Number.DECIMAL_POINT = ".";
Number.prototype.format = function(thousandSep, decPoint) {
 var decimal, str;
 thousandSep = thousandSep || Number.THOUSAND_STEP;
 decPoint = decPoint || Number.DECIMAL_POINT;
 decimal = this.toString().split(".");
 str = Math.abs(this)/*w  w w  .  j  ava2  s  .c o m*/
  .toFixed(0)
  .toString()
  .replace(/\B(?=(\d{3})+(?!\d))/g, thousandSep);
 if(decimal.length === 2) {
  str+= decPoint + decimal[1].toString();
 }
 if(this < 0) {
  str = "-" + str;
 }
 return str;
};