react-musical-keyboard/src/services/groupKeysIntoOctaves.ts

export default (dummyKeys: number[]): Record<number, number[]> => (
  dummyKeys
    .map(k => [k, Math.floor(k / 12)])
    .reduce<Record<number, number[]>>(
      (theOctaves, [key, keyOctave, ]) => ({
        ...theOctaves,
        [keyOctave]: (
          Array.isArray(theOctaves[keyOctave])
            ? [...theOctaves[keyOctave], key]
            : [key]
        )
      }),
      {}
    )
)