const a = arg||'default' 주로 이렇게 초기화용도로 쓰는데 0, ''도 정상값일 경우는 ??를 쓰면 됨
Nullish coalescing operator
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing
Logical or operator
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR