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

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'.

79116

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最佳答案

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.

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I came to this question by searching how to type useRef with Typescript when used with setTimeout or setInterval. The accepted answer helped me solve that.

You can declare your timeout/interval like this

const myTimeout = useRef<ReturnType<typeof setTimeout> | null>(null)

And to clear it and set it again, you do it as usual:

const handleChange = () => {
if (myTimeout.current) {
clearTimeout(myTimeout.current)
}
myTimeout.current = setTimeout(() => {
doSomething()
}, 500)
}

The typing will work both if you're running in Node or in a Browser.

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as key.

You can use it like this for input component.

const inputRef = useRef() as MutableRefObject<HTMLInputElement>;