There is generic get method defined in axios/index.d.ts

get<T = never, R = AxiosResponse<T>>(url: string, config?: AxiosRequestConfig<T>): Promise<R>;

Example

interface User {
    id: number;
    firstName: string;
}


axios.get<User[]>('http://localhost:8080/admin/users')
        .then(response => {
            console.log(response.data);
            setUserList( response.data );
        });

I think you are passing list the wrong way to child component.

const [users, setUserList] = useState<User[]>([]);

<UserList items={users} />

interface UserListProps {
    items: User[];
};

const UserList: React.FC<UserListProps> = ({items}) => {
    return (
        <Fragment>
            <ul>
            {items.map(user => (
                <li key={user.id}>
                    <span>{user.firstName}</span>
                </li>
            ))}
            </ul>
        </Fragment>
    );
};

The reason is in this line:

const [data, setData] = useState(null)

Because there is no type parameter provided explicitly for useState, TypeScript infers the type of data based on the type of initial state. In this case, the initial state is null and TypeScript treats this type as the only possible state.

You know that the state will either be null or something else — but TypeScript doesn't. Let's use type parameters to tell it what's really going on.

const [data, setData] = useState<AxiosResponse | null | void>(null);

This gets rid of the error, but looks strange — why is void there? The reason for that is that you catch before you then — and since catch emits a side effect (returns void, in other words), the type propagated to catch is either void or AxiosResponse. Let's fix that by replacing then and catch.

The final solution:

import axios, { AxiosResponse } from "axios";
import { useEffect, useState } from "react";

const useAxiosFetch = (url: string, timeout: number) => {
  const [data, setData] = useState<AxiosResponse | null>(null);
  const [error, setError] = useState(false);
  const [errorMessage, setErrorMessage] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    let unmounted = false;
    const source = axios.CancelToken.source();
    axios
      .get(url, {
        cancelToken: source.token,
        timeout,
      })
      .then(a => {
        if (!unmounted) {
          setData(a);
          setLoading(false);
        }
      })
      .catch(function(e) {
        if (!unmounted) {
          setError(true);
          setErrorMessage(e.message);
          setLoading(false);
          if (axios.isCancel(e)) {
            console.log(`request cancelled:${e.message}`);
          } else {
            console.log("another error happened:" + e.message);
          }
        }
      });

    return function() {
      unmounted = true;
      source.cancel("Cancelling in cleanup");
    };
  }, []);

  return { data, loading, error, errorMessage };
};

export default useAxiosFetch;