pd.js

var newFFTPitch = {

  calc: function (fft, samplerate, fftSize) {
    var fftLen = fft.length;
    var i;
    var max = fft[0];
    var curVal;
    for (i=1; i < fftLen; i += 1) {
      curVal = fft[i];
      if (curVal > max) max = curVal;
    }
    // now find the index of 1/3
    var leastIndex = -1;
    var minVal = max / 3;
    var leastVal = max;
    for (i = 0; i < fftLen; i += 1) {
      curVal = fft[i];
      if (curVal > minVal && curVal < leastVal) {
        leastVal = curVal;
        leastIndex = i;
      }
    }
    var prevIndex = leastIndex - 1;
    var nextIndex = leastIndex + 1;
    var prevVal = fft[prevIndex];
    var nextVal = fft[nextIndex];

    /*
    // https://ccrma.stanford.edu/~jos/parshl/Peak_Detection_Steps_3.html#sec:peakdet

      explanation uses alpha for prevVal, beta for leastVal, gamma for nextVal
      then gives 1/2 * ( (alpha - gamma)   /  (alpha - 2*beta + gamma) )
      for the parabole peak location, which is the position it would have in the array
      the frequency would be (sampleFreq*p)/ fftsize ?

     */
     var p = 1/2 * ( (prevVal-nextVal) / (prevVal - (2*leastVal) + nextVal));
     var freq = (samplerate * (p + leastIndex)) / fftSize;
     console.log('freq: ' + freq);
     return freq;
  }