simple financial rate function in javascript

Question

I'm looking for a simple javascript financial RATE function, and I found this one. But it seems too difficult to understand. I want to simplify this function, and I need your help. If anyone has a simplest function, please answer. (It's a excel RATE function equivalent.)

var rate = function(nper, pmt, pv, fv, type, guess) {
  if (guess == null) guess = 0.01;
  if (fv == null) fv = 0;
  if (type == null) type = 0;

  var FINANCIAL_MAX_ITERATIONS = 128;//Bet accuracy with 128
  var FINANCIAL_PRECISION = 0.0000001;//1.0e-8

  var y, y0, y1, x0, x1 = 0, f = 0, i = 0;
  var rate = guess;
  if (Math.abs(rate) < FINANCIAL_PRECISION) {
     y = pv * (1 + nper * rate) + pmt * (1 + rate * type) * nper + fv;
  } else {
     f = Math.exp(nper * Math.log(1 + rate));
     y = pv * f + pmt * (1 / rate + type) * (f - 1) + fv;
  }
  y0 = pv + pmt * nper + fv;
  y1 = pv * f + pmt * (1 / rate + type) * (f - 1) + fv;

  // find root by Newton secant method
  i = x0 = 0.0;
  x1 = rate;
  while ((Math.abs(y0 - y1) > FINANCIAL_PRECISION) && (i < FINANCIAL_MAX_ITERATIONS)) {
     rate = (y1 * x0 - y0 * x1) / (y1 - y0);
     x0 = x1;
     x1 = rate;

     if (Math.abs(rate) < FINANCIAL_PRECISION) {
        y = pv * (1 + nper * rate) + pmt * (1 + rate * type) * nper + fv;
     } else {
        f = Math.exp(nper * Math.log(1 + rate));
        y = pv * f + pmt * (1 / rate + type) * (f - 1) + fv;
     }

     y0 = y1;
     y1 = y;
     ++i;
  }
  return rate;}

Thanks!

Answer 1

The math is too complicated for me to understand, but this might be easier for you to read. Some of the variables have been renamed to make more sense, and it's formatted to be easier on your eyes

function rate(paymentsPerYear, paymentAmount, presentValue, futureValue, dueEndOrBeginning, interest)
{
    //If interest, futureValue, dueEndorBeginning was not set, set now
    if (interest == null)
        interest = 0.01;

    if (futureValue == null)
        futureValue = 0;

    if (dueEndOrBeginning == null)
        dueEndOrBeginning = 0;

    var FINANCIAL_MAX_ITERATIONS = 128;//Bet accuracy with 128
    var FINANCIAL_PRECISION = 0.0000001;//1.0e-8

    var y, y0, y1, x0, x1 = 0, f = 0, i = 0;
    var rate = interest;
    if (Math.abs(rate) < FINANCIAL_PRECISION)
    {
        y = presentValue * (1 + paymentsPerYear * rate) + paymentAmount * (1 + rate * dueEndOrBeginning) * paymentsPerYear + futureValue;
    }
    else
    {
        f = Math.exp(paymentsPerYear * Math.log(1 + rate));
        y = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue;
    }
    y0 = presentValue + paymentAmount * paymentsPerYear + futureValue;
    y1 = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue;

    // find root by Newton secant method
    i = x0 = 0.0;
    x1 = rate;
    while ((Math.abs(y0 - y1) > FINANCIAL_PRECISION)
        && (i < FINANCIAL_MAX_ITERATIONS))
    {
        rate = (y1 * x0 - y0 * x1) / (y1 - y0);
        x0 = x1;
        x1 = rate;

        if (Math.abs(rate) < FINANCIAL_PRECISION)
        {
            y = presentValue * (1 + paymentsPerYear * rate) + paymentAmount * (1 + rate * dueEndOrBeginning) * paymentsPerYear + futureValue;
        }
        else
        {
            f = Math.exp(paymentsPerYear * Math.log(1 + rate));
            y = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue;
        }

        y0 = y1;
        y1 = y;
        ++i;
    }
    return rate;
}