music_theory.js

class Note {
  constructor(noteValue, pitch){
    this.pitch = pitch;
    this.noteValue = noteValue;
  }
}

class Bar {
  constructor(...notes) {
    this.notes = notes;
  }
}

const theScore = [
  new Bar(new Note(4, 3), new Note(4, 3), new Note(4, 1), new Note(4, 3)),
  new Bar(new Note(1, 2)),
  new Bar(new Note(4, 5))
];

class TimeSignature {
  constructor(a, b) {
    this.a = a; // "beats per bar"
    this.b = b; // "note value per beat"
  }
}

// noteValue * (beat / noteValue) * (minute / beat) = duration of noteValue

function noteDuration(note, timeSignature, bpm){
  const realNoteValue = 1 / note.noteValue;
  const beatPerRealNoteValue = timeSignature.b; // flips twice
  return realNoteValue * beatPerRealNoteValue * (1 / bpm) * 60;
}

function barNoteDurations(bar, timeSignature, bpm){
  return bar.notes.map(note => noteDuration(note, timeSignature, bpm));
}

function barDuration(bar, timeSignature, bpm) {
  return barNoteDurations(bar, timeSignature, bpm).reduce((a, b) => a + b, 0);
}

function scoreBarDurations(score, timeSignature, bpm){
  return score.map(bar => barDuration(bar, timeSignature, bpm), timeSignature);
}

// ======================================================

console.log(theScore);
// compute duration in seconds of each bar in the score, given a time signature and BPM
console.log(scoreBarDurations(theScore, new TimeSignature(4,4), 120));
// gives [ 2, 2, 0.5 ], which looks right

what am i doing wrong? if nothing, what's the point of a in TimeSignature?