Shows how to calculate a note's position on a fretboard (e.g. guitar or bass) providing the note's key (e.g. 'e/4'), a string tuning and a maximum fret limit.

getStringAndFretFromKeyAndTuning.ts

type Tuning = string[];

// provide the standard 6-string guitar tuning (high to low)
const standardTuning: Tuning = ['E4', 'B3', 'G3', 'D3', 'A2', 'E2'];

// provide a limit for the fretboard length (24 frets => 0 to 24)
const maxFretLimit: number = 24;

interface FretBoardNote {
  string: number;
  fret: number;
}

interface StringBaseNote {
  baseNoteKey: string;
  baseNoteKeyScaleIndex: number;
  string: number;
}

class Scale {

  static getScale(): string[] {
    return [1, 2, 3, 4, 5, 6, 7].reduce((scale: string[], octave: number) => {
      return scale.concat(Scale.getOctaveScale(octave));
    }, ['A0', 'A#0', 'B0']).concat('C8');
  }

  private static getOctaveScale(octave: number): string[] {
    return ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B'].map((step: string) => `${step}${octave}`);
  }
}

export const getStringFretsByNoteTuning = (noteKey: string, tuning: Tuning, maxFrets: number = 24): FretBoardNote[] => {

  if (noteKey.indexOf('/') !== -1) {
    noteKey = noteKey.replace('/', '');
  }
  noteKey = noteKey.toUpperCase();

  const scale = Scale.getScale();

  const stringBaseNotes: StringBaseNote[] = tuning.map((baseNoteKey: string, index: number) => {
    return {baseNoteKey: baseNoteKey, baseNoteKeyScaleIndex: scale.indexOf(baseNoteKey), string: index + 1};
  });
  const noteKeyScaleIndex = scale.indexOf(noteKey);

  const filteredBase = stringBaseNotes
  // filter by string tuning base note (index)
    .filter((sbnd: StringBaseNote) => sbnd.baseNoteKeyScaleIndex <= noteKeyScaleIndex)
    // filter by max fret
    .filter((sbnd: StringBaseNote) => scale.slice(sbnd.baseNoteKeyScaleIndex).indexOf(noteKey) <= maxFrets);

  return filteredBase.map((fb: StringBaseNote) => {
    return {string: fb.string, fret: scale.slice(fb.baseNoteKeyScaleIndex).indexOf(noteKey)};
  });
};