Logarithmic fader (envelope) in javascript

fader.js

/**
 * Fades values in and out logarithmically.
 * @param {number} sampleRate - Number of samples per duration unit
 * @param {number} duration - Duration of an envelope
 * @param {number} unity - Unity approximation
 */
function Fader(sampleRate, duration, unity = 0.999) {
  this._sampleDuration = duration / sampleRate;

  let T = -duration / Math.log(1 - unity);
  this._exp = Math.exp(this._sampleDuration / T);

  this._numSamples = duration / this._sampleDuration;
  this._isActive = false;
}

Fader.fadeIn = 0;
Fader.fadeOut = 1;

/**
 * Starts an envelope with initial and target values.
 * Note that initial value isn't included in the envelope.
 */
Fader.prototype.start = function(initialValue, targetValue) {
  this._currentUnitySample = 0.0;
  this._currentSampleIndex = 0;
  this._fadeType = targetValue <= initialValue ? Fader.fadeOut : Fader.fadeIn;
  this._isActive = true;

  switch (this._fadeType) {
    case Fader.fadeIn:
      this._minValue = initialValue;
      this._maxValue = targetValue;
      this._getCurrentExponentSample = function() {
        return this._currentUnitySample / this._exp + 1 - 1 / this._exp;
      };

      break;
    case Fader.fadeOut:
      this._minValue = targetValue;
      this._maxValue = initialValue;

      this._getCurrentExponentSample = function() {
        return this._currentUnitySample / this._exp;
      };

      break;
  }

  this._currentUnitySample =
    (initialValue - this._minValue) / (this._maxValue - this._minValue);
};

/**
 * Returns the next unity sample (in the range of [0.0, 1.0] for this envelope.
 */
Fader.prototype.getNextUnitySample = function() {
  let unitySample;
  if (this._currentSampleIndex < this._numSamples - 1) {
    unitySample = this._getCurrentExponentSample();
    this._currentSampleIndex++;
  } else {
    switch (this._fadeType) {
      case Fader.fadeIn:
        unitySample = 1;
        break;
      case Fader.fadeOut:
        unitySample = 0;
        break;
    }

    this._isActive = false;
  }

  this._currentUnitySample = unitySample;

  return unitySample;
};

/**
 * Returns the next sample for this envelope.
 */
Fader.prototype.getNextSample = function() {
  let unitySample = this.getNextUnitySample();
  return (this._maxValue - this._minValue) * unitySample + this._minValue;
};

/**
 * Returns the total number of samples for this envelope.
 */
Fader.prototype.getNumSamples = function() {
  return this._numSamples;
};

/**
 * Returns the duration of a single sample
 */
Fader.prototype.getSampleDuration = function() {
  return this._sampleDuration;
};

/**
 * Returns a boolean, indicating whether the target value is reached
 */
Fader.prototype.isActive = function() {
  return this._isActive;
};

/* Usage */

let sampleRate = 10;
let duration = 0.5;

let fader = new Fader(sampleRate, duration);

/* Fade in */

fader.start(0.1, 0.5);

console.log("Fade in samples:");

for (let i = 0; i < fader.getNumSamples(); i++) {
  console.log(fader.getNextSample());
}

/* Fade out */

fader.start(0.5, 0.3);

console.log("Fade out samples:");

for (let i = 0; i < fader.getNumSamples(); i++) {
  console.log(fader.getNextSample());
}