https://kyounghwan01.github.io/blog/TS/object-null/#%E1%84%90%E1%85%A1%E1%84%8B%E1%85%B5%E1%86%B8-%E1%84%83%E1%85%A1%E1%86%AB%E1%84%8B%E1%85%A5%E1%86%AB-%E1%84%89%E1%85%A1%E1%84%8B%E1%85%AD%E1%86%BC-type-assertion
