How do I transpose music chords using JavaScript?

Question

I was wondering how would one create a javascript function for transposing music chords.

Since I don\'t expect everyone to be a musician here, I\'ll try to explain how i

Answer 1

All of these solutions are lacking the fact that after transposing a note, it needs to be converted to a sharp or flat depending on the key or chord.

So the API must be:

transpose(note, semitones, useSharps)

Here is my implementation. It also handles multiple # and b modifiers.

function transposeNote(note, semitones, useSharps) {
  // parse root followed by modifiers (# and b)
  const rx = /^([a-gA-G])([#b]*)$/;
  const m = rx.exec(note);
  if (!m) {
    return null;
  }
  // convert note from 0 to 11 based off of A
  let root;
  switch (m[1].toUpperCase()) {
    case "A":
      root = 0;
      break;
    case "B":
      root = 2;
      break;
    case "C":
      root = 3;
      break;
    case "D":
      root = 5;
      break;
    case "E":
      root = 7;
      break;
    case "F":
      root = 8;
      break;
    case "G":
      root = 10;
      break;
  }
  // modify root
  let mods = m[2];
  if (mods) {
    for (var i = 0; i < mods.length; i++) {
      if (mods.charAt(i) === "#") {
        root++;
      } else {
        root--;
      }
    }
  }
  // transpose note
  root = (root + semitones) % 12;

  if (root < 0) {
    root += 12
  }

  // convert back to a note
  const sharps = [
    "A",
    "A#",
    "B",
    "C",
    "C#",
    "D",
    "D#",
    "E",
    "F",
    "F#",
    "G",
    "G#"
  ];
  const flats = [
    "A",
    "Bb",
    "B",
    "C",
    "Db",
    "D",
    "Eb",
    "E",
    "F",
    "Gb",
    "G",
    "Ab"
  ];
  const transposedNote = useSharps ? sharps[root] : flats[root];
  return transposedNote;
}

function transposeChord(chord, semitones, useSharps) {
  const rx = /^([a-gA-G][#b]*)\s*([^\/]*)(\/[a-gA-G][#b]*)?$/;
  const m = rx.exec(chord);
  if (!m) {
    return null;
  }
  const root = transposeNote(m[1], semitones, useSharps);
  const quality = m[2] || "";
  let bass = m[3] || "";
  if (bass.length > 0) {
    bass = "/" + transposeNote(bass.substring(1), semitones, useSharps);
  }
  return root + quality + bass;
}

Use it like this

console.log(transposeChord("Cmin7/Eb", 3, false));