https://joshua1988.github.io/ts/guide/enums.html#%EC%BB%B4%ED%8C%8C%EC%9D%BC-%EC%8B%9C%EC%A0%90%EC%97%90%EC%84%9C%EC%9D%98-%EC%9D%B4%EB%84%98-%ED%8A%B9%EC%A7%95
