Typescript, how to pass "Object is possibly null" error?

Question

I've got the "Object is possibly null" error many times and usually I use a safety "if statement" in case it returns null.

I've got the following function:

const ModalOverlay = (props: any[]) => {   const overlayEl = useRef(null);     useEffect(() => {     overlayEl.current.focus();     });     return <div {...props} ref={overlayEl} />;   } 

But overlayEl.current gets the error "Object is not defined". So I've tried:

if (!overlayEl) {     return null   } else {     useEffect(() => {     overlayEl.current.focus();     });     return <div {...props} ref={overlayEl} />;   } 

Which didn't work. I've tried also:

overlay && overlayEl.current.focus(); 

Any hints would be highly appreciated! Thanks

Answer 1

When you declare const overlayEl = useRef(null); Makes the type it comes out as is null because that's the best possible inference it can offer with that much information, give typescript more information and it will work as intended.

Try....

 const overlayEl = useRef<HTMLDivElement>(null); 

Alternatively some syntax sugar for if you don't care for when its undefined is to do something like this.

const overlayEl = useRef(document.createElement("div")) 

using the above syntax all common DOM methods just return defaults such as "0" i.e overlayEl.offsetWidth, getBoundingClientRect etc.

Usage:

if(overlayEl.current) {     // will be type HTMLDivElement NOT HTMLDivElement | null     const whattype = overlayEl.current;  } 

The way this works is typescripts static analysis is smart enough to figure out that if check "guards" against null, and therefore it will remove that as a possible type from the union of null | HTMLDivElement within those brackets.

Answer 2

const overlayEl = useRef() as MutableRefObject<HTMLDivElement>; 

It will cast overlayEl to an initiated MutableRefObject that is the returning value of useRef:

function useRef<T = undefined>(): MutableRefObject<T | undefined>; 

Yet in this case, the compiler will always think that overlayEl has a value.