React Typescript – Argument of type is not assignable to parameter of type

const [user, setUser] = useState(null);

Since you havn’t given this a type, typescript has to try to infer it. It sees you passed in a null, so it assumes this state is (and always will be) null. Instead, you need to specify the type, as in:

interface UserData {
  username: string;
  password: string;
  prevState: null
}

//...
const [user, setUser] = useState<UserData | null>(null);