Javascript Number currencyDisplay()

Number.prototype.currencyDisplay = function() {
  var num = this.valueOf().toString()
    var value = Number(num);
    var res = num.split(".");
    if(num.indexOf('.') === -1) {
        value = value.toFixed(2);/*  w w  w.  j  a  v  a 2  s . com*/
        num = value.toString();
    } else if (res[1].length < 3) {
        value = value.toFixed(2);
        num = value.toString();
    }
    return num
};