Javascript Function to calculate present value of an investment.

// Function to calculate present value of an investment..

// Parameters are rate, total number of periods, payment made each period, future value and type (when payments are due)

function pv(rate, per, nper, pmt, fv) {

    nper = parseFloat(nper);

    pmt = parseFloat(pmt);

    fv = parseFloat(fv);

    rate = eval((rate) / (per * 100));

    if (nper == 0) {
        //(pmt == 0) ||
        alert("Why do you want to test me with zeros?");

        return (0);

    }

    if (rate == 0) // Interest rate is 0
    {

        pv_value = -(fv + (pmt * nper));

    }

    else {

        x = Math.pow(1 + rate, -nper);

        y = Math.pow(1 + rate, nper);

        pv_value = -(x * (fv * rate - pmt + y * pmt)) / rate;

    }

    pv_value = conv_number(pv_value, 2);

    return (pv_value);

}

function conv_number(expr, decplaces) { // This function is from David Goodman's Javascript Bible.

    var str = "" + Math.round(eval(expr) * Math.pow(10, decplaces));

    while (str.length <= decplaces) {

        str = "0" + str;

    }

    var decpoint = str.length - decplaces;

    return (str.substring(0, decpoint) + "." + str.substring(decpoint, str.length));

}
//end of pv