as const

• We use as const to type assert. And this works great if :

function useInfiniteScroll(callback: () => void) {
  const [isFetching, setIsFetching] = useState(false);

  return [isFetching, setIsFetching];
}

Theoretically I could return

function useInfiniteScroll(callback: () => void)

function useInfiniteScroll(callback: () => void): [boolean, Dispatch<SetStateAction<boolean>>]

// or

const returnArr:[boolean, Dispatch<SetStateAction<boolean>>] = [isFetching, setIsFetching];

But those are a bit too verbose. So we could assert (aka just telling ts directly) that we know that this array won't change.

The return type of this function is a MUTABLE array and we want to tell TS that it's actually IMMUTABLE and therefore you can set the array's type as NON-widening. And Arrays and Objects are readonly so they never are able to change their order

Resource: https://blog.logrocket.com/const-assertions-are-the-killer-new-typescript-feature-b73451f35802/