useState.ts

// inferred as number
const [value, setValue] = useState(0);

// explicitly setting the types
const [value, setValue] = useState<number | undefined>(undefined);
const [value, setValue] = useState<Array<number>>([]);

interface MyObject {
  foo: string;
  bar?: number;
}
const [value, setValue] = useState<MyObject>({ foo: 'hello' });