Issues with testing React functional components using useEffect when dependencies change dynamically
This might be a silly question, but Quick question that's been bugging me - Hey everyone, I'm running into an issue that's driving me crazy... I'm facing a challenge while testing a React functional component that utilizes the `useEffect` hook, specifically when the dependencies change dynamically. My component fetches data from an API based on user input, but when I attempt to test the behavior of the `useEffect` hook with changing dependencies, it's not triggering the effect as expected. Here's a simplified version of my component: ```javascript import React, { useEffect, useState } from 'react'; const DataFetcher = ({ userId }) => { const [data, setData] = useState(null); useEffect(() => { const fetchData = async () => { const response = await fetch(`https://api.example.com/data/${userId}`); const result = await response.json(); setData(result); }; fetchData(); }, [userId]); return <div>{data ? JSON.stringify(data) : 'Loading...'}</div>; }; export default DataFetcher; ``` In my test file, I'm using React Testing Library along with Jest. Here’s the test case I wrote: ```javascript import { render, screen } from '@testing-library/react'; import DataFetcher from './DataFetcher'; test('fetches and displays data based on userId', async () => { render(<DataFetcher userId="1" />); expect(screen.getByText(/Loading.../i)).toBeInTheDocument(); // Mocking the fetch function globally global.fetch = jest.fn(() => Promise.resolve({ json: () => Promise.resolve({ name: 'John Doe' }), }) ); // How to wait for the effect to trigger and expect the result? // Using waitFor but it hangs indefinitely. await waitFor(() => { expect(screen.getByText(/John Doe/i)).toBeInTheDocument(); }); }); ``` The problem is that when I run this test, it hangs indefinitely at `waitFor`, and I never see the expected text in the document. I have verified that the `fetch` function is correctly mocked, but it seems like the `useEffect` never completes the fetch operation. I've tried several approaches, including using `act()` to wrap my rendering and state updates, but the issue persists. I suspect it might be related to how the mock is set up or possibly the way the component handles the asynchronous fetch operation. How can I properly test this component to ensure the `useEffect` hook behaves as expected when the `userId` prop changes dynamically? Any insights or best practices would be greatly appreciated! For context: I'm using Javascript on Linux. I'm working on a CLI tool that needs to handle this. Is there a better approach? What's the best practice here?