드롭다운 바깥 클릭시 닫기 https://thinkforthink.tistory.com/286
깊은복사 얕은 복사 https://helloinyong.tistory.com/267 https://velog.io/@th0566/Javascript-%EC%96%95%EC%9D%80-%EB%B3%B5%EC%82%AC-%EA%B9%8A%EC%9D%80-%EB%B3%B5%EC%82%AC