Javascript Number Type Format with comma

function formatCommas(numString) {
    var re = /(-?\d+)(\d{3})/;
    while (re.test(numString)) {
        numString = numString.replace(re, "$1,$2");
    }/*  w w  w. jav a  2s  .  co m*/
    return numString;
}

function formatNumber (num, decplaces) {
    // convert in case it arrives as a string value
    num = parseFloat(num);
    // make sure it passes conversion
    if (!isNaN(num)) {
        // multiply value by 10 to the decplaces power;
        // round the result to the nearest integer;
        // convert the result to a string
        var str = "" + Math.round (eval(num) * Math.pow(10,decplaces));
        // exponent means value is too big or small for this routine
        if (str.indexOf("e") != -1) {
            return "Out of Range";
        }
        // if needed for small values, pad zeros
        // to the left of the number
        while (str.length <= decplaces) {
            str = "0" + str;
        }
        // calculate decimal point position
        var decpoint = str.length - decplaces;
        return formatCommas(str.substring(0,decpoint)) + "." + str.substring(decpoint,str.length);

    } else {
        return "NaN";
    }
}


function stripCommas(numString) {
    var re = /,/g;
    return numString.replace(re,"");
}
console.log(formatNumber(3123123.123,3));