Conversion of midi note numbers to a linear scale so they can be drawn more easily. The testFrequency method show how to use it

NoteMath.js

// get frequency from piano key number t
export function keyToFrequency(n, base = 440, keys = 12) {
  return base * Math.pow(2, n / keys);
}

// get inverse frequency from piano key number t
export function keyToInverseFrequency(n, base = 440, keys = 12) {
  return base * -Math.pow(2, -n / keys);
}

export function toBinaryString(dec) {
  return (dec >>> 0).toString(2);
}

export function padLeft(input, amount) {
  return amount - input.length > 0 ? Array(amount + 1 - input.length).join("0") + input : input;
}

export function testFrequency() {
  // freq = 440 * Math.pow(2, n/12)

  // get frequency from piano key number t
  keyToFrequency = function(t, c = 440, d = 12) {
    return c * Math.pow(2, t / d);
  };

  // get inverse frequency from piano key number t
  keyToInverseFrequency = function(t, c = 440, d = 12) {
    return c * -Math.pow(2, -t / d);
  };

  // add both together to get a linear curve
  console.log(440 + keyToInverseFrequency(0)); // = 0

  const freq = {
    49: 440,
    50: 466.164,
    51: 493.883,
    52: 523.251,
    53: 554.365,
    54: 587.33,
    55: 622.254,
    56: 659.255,
    57: 698.456,
    58: 739.989,
    59: 783.991,
    60: 830.609,
    61: 880
  };

  // console.log(440 * Math.pow(2, 0 / 12)); // = 440
  const A4noteNumber = 49;

  console.log(keyToFrequency(0), freq[A4noteNumber + 0]);
  console.log(keyToFrequency(1), freq[A4noteNumber + 1]);
  console.log(keyToFrequency(2), freq[A4noteNumber + 2]);
  console.log(keyToFrequency(3), freq[A4noteNumber + 3]);
  console.log(keyToFrequency(4), freq[A4noteNumber + 4]);
  console.log(keyToFrequency(5), freq[A4noteNumber + 5]);
  console.log(keyToFrequency(6), freq[A4noteNumber + 6]);
  console.log(keyToFrequency(7), freq[A4noteNumber + 7]);
  console.log(keyToFrequency(8), freq[A4noteNumber + 8]);
  console.log(keyToFrequency(9), freq[A4noteNumber + 9]);
  console.log(keyToFrequency(10), freq[A4noteNumber + 10]);
  console.log(keyToFrequency(11), freq[A4noteNumber + 11]);
  console.log(keyToFrequency(12), freq[A4noteNumber + 12]);

  // to linear
  console.log(keyToInverseFrequency(0) + freq[A4noteNumber + 0]);
  console.log(keyToInverseFrequency(1) + freq[A4noteNumber + 1]);
  console.log(keyToInverseFrequency(2) + freq[A4noteNumber + 2]);
  console.log(keyToInverseFrequency(3) + freq[A4noteNumber + 3]);
  console.log(keyToInverseFrequency(4) + freq[A4noteNumber + 4]);
  console.log(keyToInverseFrequency(5) + freq[A4noteNumber + 5]);
  console.log(keyToInverseFrequency(6) + freq[A4noteNumber + 6]);
  console.log(keyToInverseFrequency(7) + freq[A4noteNumber + 7]);
  console.log(keyToInverseFrequency(8) + freq[A4noteNumber + 8]);
  console.log(keyToInverseFrequency(9) + freq[A4noteNumber + 9]);
  console.log(keyToInverseFrequency(10) + freq[A4noteNumber + 10]);
  console.log(keyToInverseFrequency(11) + freq[A4noteNumber + 11]);
  console.log(keyToInverseFrequency(12) + freq[A4noteNumber + 12]);

  const note = { frequency: 880, noteNumber: 61 };
  const value = note.frequency + keyToInverseFrequency(note.noteNumber - A4noteNumber);
  console.log(">>", value); // 660!

  console.log(padLeft("", 3));
  console.log(padLeft("1", 3));
  console.log(padLeft("11", 3));
  console.log(padLeft("111", 3));
  console.log(padLeft("1111", 3));
}