useFetch.js

import { useEffect, useState } from 'react';

export default function useFetch(apiUrl) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    const getData = async (url) => {
      setLoading(true);
      try {
        const response = await fetch(url);
        const data = await response.json();
        setData(data);
        
        return data;
      } catch(e) {
        setError(e);

        return e;
      } finally {
        setLoading(false);
        return;
      }
    }

    getData(apiUrl);
  }, [apiUrl]);


  return {data, loading, error};
};

useFetch.test.js

import { renderHook } from '@testing-library/react-hooks'
import useFetch from './useFetch';

const DATA_MOCK = {
  "userId": 1,
  "id": 1,
  "title": "delectus aut autem",
  "completed": false
};

describe('useFetch', () => {
  // beforeAll(() => {
  //   global.fetch = jest.fn().mockReturnValue(Promise.resolve({
  //     json: () => Promise.resolve({
  //       "userId": 1,
  //       "id": 1,
  //       "title": "delectus aut autem",
  //       "completed": false
  //     })
  //   }))
  // });

  // afterAll(() => {    
  //   global.fetch.mockClear();
  // });

  test('should return data', async () => {

    global.fetch = jest.fn().mockReturnValue(Promise.resolve({
      json: () => Promise.resolve({
        "userId": 1,
        "id": 1,
        "title": "delectus aut autem",
        "completed": false
      })
    }))

    const { result, waitForNextUpdate } = renderHook(() => useFetch('https://jsonplaceholder.typicode.com/todos/1'));
    await waitForNextUpdate();

    console.log(result.current);
    // console.log
    // {
    //   data: null,
    //   loading: false,
    //   error: TypeError: Cannot read property 'json' of undefined
    //       at getData (/random-components-practice/src/hooks/useFetch/useFetch.js:13:37)
    // }
    // expect(result.current.data).toBeTruthy();  // problem
    expect(result.current.data).toMatchObject(DATA_MOCK);
    expect(result.current.loading).toBeFalsy();
    expect(result.current.error).toBeNull();
});
});