Implementing findBy Queries for Asynchronous Operations in Jest

This challenge focuses on testing asynchronous functionalities using Jest's findBy queries. findBy queries are essential for gracefully handling operations that take time to complete, such as network requests or animations, and asserting their eventual state. Mastering this will improve the robustness of your tests for modern, dynamic user interfaces.

Problem Description

Your task is to implement a set of Jest tests for a simple asynchronous component that fetches and displays user data. You will primarily use Jest's findBy queries to assert the presence and state of elements after an asynchronous operation has completed.

What needs to be achieved: You need to write Jest tests for a hypothetical UserCard component that fetches user data asynchronously when it mounts. The component should initially display a "Loading..." message and then, upon successful data fetch, display the user's name and email.

Key requirements:

1. Initial State: Assert that the UserCard component initially displays a "Loading..." message.
2. Asynchronous Update: Assert that after a simulated delay (representing data fetching), the "Loading..." message disappears, and the user's name and email are displayed.
3. Error Handling (Optional but Recommended): If time permits, implement a test case to handle a simulated data fetch error, asserting that an error message is displayed instead of user data.

Expected behavior:

• When the component mounts, a "Loading..." text should be visible.
• After the simulated asynchronous operation completes successfully, the "Loading..." text should no longer be visible, and the fetched user's name and email should be present.

Edge cases to consider:

• What happens if the asynchronous operation takes a long time? findBy queries are designed to handle this by default.
• What if the data fetch fails? The UI should reflect an error state.

Examples

Example 1: Testing the initial loading state.

Input: A `UserCard` component that, upon mounting, immediately displays "Loading...".
Output: A Jest test that asserts the presence of an element containing "Loading...".

Explanation: This test verifies that the component correctly enters its initial loading state.

Example 2: Testing the successful data fetch and display.

Input: A `UserCard` component that, after a simulated 2-second delay, fetches user data `{ name: 'John Doe', email: 'john.doe@example.com' }` and displays it.
Output: A Jest test using `findByText` or `findByRole` to assert the presence of "John Doe" and "john.doe@example.com" after the delay, and the absence of "Loading...".

Explanation: This test confirms that the asynchronous data fetching works as expected and the UI updates correctly with the fetched data.

Constraints

• Your solution should be written in TypeScript.
• You will be simulating asynchronous operations using setTimeout within your hypothetical component or its mock.
• Assume you have access to a testing environment like @testing-library/react and Jest.
• Focus on using findBy queries for your assertions.

Notes

• findBy queries in @testing-library/dom (and by extension, its framework-specific versions like @testing-library/react) are asynchronous. They return Promises that resolve when an element is found within the default timeout period or reject if the element is not found within that period.
• You can control the timeout for findBy queries if needed, but the default is usually sufficient.
• Consider mocking the asynchronous data fetching mechanism to control the delay and the success/failure of the operation during your tests. This is a common practice in unit testing asynchronous code.
• Think about what primitive queries (getBy, queryBy) might be useful for asserting the absence of an element after the asynchronous operation completes.