What typescript type do I use with useRef() hook when setting current manually?

Question

How can I use a React ref as a mutable instance, with Typescript? The current property appears to be typed as read-only.

I am using React + Typescript to develop a library that interacts with input fields that are NOT rendered by React. I want to capture a reference to the HTML element and then bind React events to it.

  const inputRef = useRef<HTMLInputElement>();   const { elementId, handler } = props;    // Bind change handler on mount/ unmount   useEffect(() => {     inputRef.current = document.getElementById(elementId);     if (inputRef.current === null) {       throw new Exception(`Input with ID attribute ${elementId} not found`);     }     handler(inputRef.current.value);      const callback = debounce((e) => {       eventHandler(e, handler);     }, 200);      inputRef.current.addEventListener('keypress', callback, true);      return () => {       inputRef.current.removeEventListener('keypress', callback, true);     };   }); 

It generates compiler errors: semantic error TS2540: Cannot assign to 'current' because it is a read-only property.

I also tried const inputRef = useRef<{ current: HTMLInputElement }>(); This lead to this compiler error:

Type 'HTMLElement | null' is not assignable to type '{ current: HTMLInputElement; } | undefined'.    Type 'null' is not assignable to type '{ current: HTMLInputElement; } | undefined'. 

Answer 1

Yeah, this is a quirk of how the typings are written:

function useRef<T>(initialValue: T): MutableRefObject<T>; function useRef<T>(initialValue: T|null): RefObject<T>; 

If the initial value includes null, but the specified type param doesn't, it'll be treated as an immutable RefObject.

When you do useRef<HTMLInputElement>(null), you're hitting that case, since T is specified as HTMLInputElement, and null is inferred as HTMLInputElement | null.

You can fix this by doing:

useRef<HTMLInputElement | null>(null) 

Then T is HTMLInputElement | null, which matches the type of the first argument, so you hit the first override and get a mutable ref instead.