문제 (오류 발생상황)

ref로 할당된 signRef를 클래스 컴포넌트에서 정의하려고 하면, 아래와 같은 오류 발생

해결

Definite Assignment Assertion

The definite assignment assertion is a feature that allows a ! to be placed after instance property and variable declarations to relay to TypeScript that a variable is indeed assigned for all intents and purposes, even if TypeScript’s analyses cannot detect so.

'!'를 instance 변수명에 추가하니 해결!

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html