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

interface GenerateKeys {
  (startKey: number, endKey: number): number[],
}

const generateKeys: GenerateKeys = (startKey, endKey) => (
  Array(endKey! - startKey! + 1)
    .fill(0)
    .map((_, i) => startKey! + i)
)

export default generateKeys