When you use a Class, and forget 'this', then this ReferenceError will come out.
ReferenceError: * is not defined at HTMLButtonElement
solution : Find the thing that is missing 'this' in front of it
클래스를 생성하면서 리팩토링할 때 곳곳에 this. 를 빠트려 해당 에러가 많이 발생했다.
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