How to solve the "update was not wrapped in act()" warning in testing-library-react?

Updated answer:

Please refer to @mikaelrs comment below.

No need for the waitFor or waitForElement. You can just use findBy* selectors which return a promise that can be awaited. e.g await findByTestId('list');

Deprecated answer:

Use waitForElement is a correct way, from the docs:

Wait until the mocked get request promise resolves and the component calls setState and re-renders. waitForElement waits until the callback doesn't throw an error

Here is the working example for your case:

index.jsx:

import React, { useState, useEffect } from 'react';
import axios from 'axios';

export default function Hello() {
  const [posts, setPosts] = useState([]);

  useEffect(() => {
    const fetchData = async () => {
      const response = await axios.get('https://jsonplaceholder.typicode.com/posts');
      setPosts(response.data);
    };

    fetchData();
  }, []);

  return (
    <div>
      <ul data-testid="list">
        {posts.map((post) => (
          <li key={post.id}>{post.title}</li>
        ))}
      </ul>
    </div>
  );
}

index.test.jsx:

import React from 'react';
import { render, cleanup, waitForElement } from '@testing-library/react';
import axios from 'axios';
import Hello from '.';

jest.mock('axios');

afterEach(cleanup);

it('renders hello correctly', async () => {
  axios.get.mockResolvedValue({
    data: [
      { id: 1, title: 'post one' },
      { id: 2, title: 'post two' },
    ],
  });
  const { getByTestId, asFragment } = render(<Hello />);

  const listNode = await waitForElement(() => getByTestId('list'));
  expect(listNode.children).toHaveLength(2);
  expect(asFragment()).toMatchSnapshot();
});

Unit test results with 100% coverage:

 PASS  stackoverflow/60115885/index.test.jsx
  ✓ renders hello correctly (49ms)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |     100 |      100 |     100 |     100 |                   
 index.jsx |     100 |      100 |     100 |     100 |                   
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   1 passed, 1 total
Time:        4.98s

index.test.jsx.snapshot:

// Jest Snapshot v1

exports[`renders hello correctly 1`] = `
<DocumentFragment>
  <div>
    <ul
      data-testid="list"
    >
      <li>
        post one
      </li>
      <li>
        post two
      </li>
    </ul>
  </div>
</DocumentFragment>
`;

source code: https://github.com/mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/60115885

For me, the solution was to wait for waitForNextUpdate

it('useMyHook test', async() => {
      const {
        result,
        waitForNextUpdate
      } = renderHook(() =>
        useMyHook(),
      );
      await waitForNextUpdate()
      expect(result.current).toEqual([])
    }