Debugging and Fixing a Complex Jest Test Case in React

Introduction

Hi, So Recently, I was working on fixing a Jest test case for a React component. While I had some knowledge about testing, this task proved to be a steep learning curve. The issue involved a ChatMessagePrompt component with multiple contexts and dynamic behaviors. What made it more challenging was my lack of prior experience with testing such complex components using Jest.

In this blog, I'll share the challenges I faced, how I debugged the issue, and the eventual solution. If you're new to Jest or testing complex components, I hope this post will be helpful for you!

---

The Problem

The ChatMessagePrompt component provides users with a "scroll to bottom" prompt when there are unread messages. It relies on multiple React contexts (BotRefsContext, BotStatesContext, SettingsContext, and StylesContext) for its functionality.

When running the test file, one specific test consistently failed:

FAIL __tests__/components/ChatBotBody/ChatMessagePrompt.test.tsx
ChatMessagePrompt Component
    ✓ renders with the correct message prompt text
    ✕ applies visible class when conditions are met
    ✓ applies hidden class when conditions are not met

The error message indicated:

Expected the element to have class:
  rcb-message-prompt-container visible
Received:
  rcb-message-prompt-container hidden

---

Challenges Faced

Here are the main challenges I encountered:

1. Dynamic Context Mocks:

   • The component relied heavily on context values. Some tests required unreadCount and isScrolling values to change dynamically, which wasn't happening in my initial implementation.

2. State Contamination Between Tests:

   • unreadCount and isScrolling were global mock variables, leading to cross-test state contamination.

3. CSS Class Dependencies:

   • The visibility of the component depended on CSS classes, and Jest's handling of CSS can sometimes be tricky.

4. JSDOM Limitations:

   • The scrollToBottom function in the component used window.scrollTo, which isn't implemented by JSDOM.

5. Lack of Jest Expertise:

   • I was unfamiliar with advanced mocking techniques in Jest, such as dynamically returning mock values.

---

The Solution

After hours of debugging and research, here's how I solved the issue:

1. Dynamic Context Mocks

Instead of using global variables for mocking unreadCount and isScrolling, I moved to dynamic mocking within each test. This allowed me to configure the mock values independently for every test.

Updated Code:

jest.mock("../../../src/context/BotStatesContext", () => ({
    useBotStatesContext: jest.fn(),
}));

it("applies visible class when conditions are met", () => {
    (useBotStatesContext as jest.Mock).mockReturnValue({
        unreadCount: 2,
        isScrolling: true,
        setIsScrolling: jest.fn(),
    });

    render(<ChatMessagePrompt />);
    const messagePrompt = screen.getByText("Scroll to new messages");
    expect(messagePrompt.parentElement).toHaveClass("rcb-message-prompt-container visible");
});

---

2. Cleaning Up Between Tests

I ensured all timers and mock states were reset after each test to avoid cross-test contamination. Here's how I achieved that:

Updated Code:

afterEach(() => {
    jest.clearAllMocks();
    jest.runOnlyPendingTimers();
    jest.useRealTimers();
});

---

3. Mocking window.scrollTo

To handle the window.scrollTo issue, I mocked it globally in my test setup:

Updated Code:

beforeAll(() => {
    Object.defineProperty(window, 'scrollTo', { value: jest.fn(), writable: true });
});

---

4. Debugging CSS Class Issues

I verified that Jest's CSS mocking was correctly configured in the jest.config.js file. Here's the relevant snippet:

"moduleNameMapper": {
    "\.(css|less|scss|sass)$": "identity-obj-proxy"
}

This ensured that CSS classes like visible and hidden were correctly mocked and recognized in tests.

---

5. Using Proper Timer Management

The scrollToBottom function used animations (requestAnimationFrame) and timers. I ensured Jest's fake timers simulated this behavior accurately:

Updated Code:

jest.advanceTimersByTime(600); // Simulate the scrolling duration
expect(mockSetIsScrolling).toHaveBeenCalledWith(false);

---

Final Thoughts

This experience taught me a lot about Jest and testing React components:

• How to dynamically mock context values.
• The importance of isolating test states.
• Handling JSDOM limitations with mocks.
• Debugging CSS-related issues in Jest.

Here's the final takeaway: testing complex components isn't just about writing tests but also understanding how the component interacts with its dependencies.

If you're working on a similar problem, don't give up! Testing can be tricky, but it's also incredibly rewarding.

---

PR Generated

https://github.com/tjtanjin/react-chatbotify/pull/278

---

Acknowledgments

Although I solved this problem on my own, I want to thank the incredible testing and React communities for their resources and discussions that guided me along the way.

Thanks for reading!

— Madhur