transpose chords

transpose.js

/**
 * transpose.js
 * @author SHIN Suzuki
 * 
 **/
 
/**
 * function transpose
 * 
 * @param chordStr (string): documents containing chords
 * @param degree (number): e.g. 1: F -> F#, -3 F -> D
 * @param toFlat (boolean): if true, all chords are expressed as flat.
 *                          e.g.  F# -> Gb
 **/
function transpose(chordStr, degree, toFlat) {
  if (!degree) degree = 0;
  // validation
  if (typeof degree != "number") throw "put number to 2nd argument";
  degree = parseInt(degree);

  var chordRegex = /[A-G][#b]?(maj|aug|dim|sus|[Mm0-9b#△])*(\/[A-G][#b]?)?$/g;
  var toneRegex = /[A-G][#b]?/g;
  var tonesArray =  ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"];
  var flatToSharp =  {"Db":"C#", "Eb":"D#", "Gb":"F#", "Ab":"G#", "Bb":"A#"};
  var sharpToFlat =  (function() {
    var obj = {};
    Object.keys(flatToSharp).forEach(function(flat) {
      obj[flatToSharp[flat]] = flat;
    });
    return obj;
  })();

  return chordStr.split(/([ \t\n]+)/).map(function(word) {
    if (!chordRegex.test(word)) return word;

    // here, chords come, do transpose
    return word.replace(toneRegex,function(tone) {
      // flat to sharp
      var sharpTone = flatToSharp[tone];
      if (sharpTone) tone = sharpTone;

      // transpose 
      var key = tonesArray.indexOf(tone);
      var newKey = (key + degree + 12) % 12;
      var newTone = tonesArray[newKey];

      // flatize
      if (toFlat) {
        var flatTone = sharpToFlat[newTone];
        if (flatTone) newTone = flatTone;
      }

      return newTone;
    });
  }).join("");
}