React 19 support

[franknoel]

After upgrading to React 19, the useCountUp hook started to throw a TypeScript warning.

Type 'RefObject<HTMLElement | null>' is not assignable to type 'string | RefObject<HTMLElement>'.
  Type 'RefObject<HTMLElement | null>' is not assignable to type 'RefObject<HTMLElement>'.
    Type 'HTMLElement | null' is not assignable to type 'HTMLElement'.
      Type 'null' is not assignable to type 'HTMLElement'.ts(2322)

Adding support for React 19 and generating new types should fix the issue.

[mmarkelov]

can you please provide more details? ideally repro, cause i don't see any TS related errors
https://codesandbox.io/p/sandbox/zjfntd?file=%2Fsrc%2FApp.tsx%3A6%2C1

[franknoel]

Hi @mmarkelov , thank you for responding.
Unfortunately, I don't have access to your sandbox.

The issue is caused by UseCountUpProps ref property, set as the React 18 RefObject type.
With React 19, RefObject requires a union of the HTML Element type and null.

react-countup/src/useCountUp.ts

Line 14 in 771f6a3

ref: string | React.RefObject<HTMLElement>;