DOM Tree Testing: A Deep Dive into Snapshot Testing with Jest or Vitest

Let's take a look at snapshot testing and see how to use this concept in our visual regression testing strategy.

Introduction

As software applications become increasingly complex, maintaining their integrity during rapid iterations is paramount. Testing strategies play a crucial role in ensuring that software functions as expected, and DOM tree testing — particularly snapshot testing — has gained significant traction. Tools like Jest and Vitest offer snapshot testing features that enable developers to capture the structure of a rendered component and compare it over time. This blog will dive into the benefits of DOM tree testing, when it should be used, and scenarios where it might be avoided.

What is Snapshot Testing?

Snapshot testing involves capturing the rendered output of a component or piece of UI at a specific point in time and saving it as a snapshot file. These snapshots serve as a source of truth for future comparisons. When changes occur in the component, the new rendered output is compared against the saved snapshot. If there are differences, the test will either fail or prompt the user to update the snapshot.

Tools like Jest and Vitest simplify the process, offering mechanisms to automate snapshot creation and validation.

How Snapshot Testing Works

Here's a quick overview of how snapshot testing typically works:

1. Rendering: A component is rendered by the testing framework (Jest or Vitest).
2. Snapshot Creation: The framework captures the rendered output and saves it as a snapshot file (usually in a __snapshots__ folder).
3. Comparison: On subsequent test runs, the current rendered output is compared with the saved snapshot.
4. Failure or Pass: If the rendered output matches the snapshot, the test passes. If they differ, the test fails, and the developer can either update the snapshot (if the change was intentional) or investigate the source of the discrepancy.

Benefits of DOM Tree (Snapshot) Testing

1. Automated UI Regression Detection:

• Snapshot testing provides a simple way to capture the UI structure at a given point in time. If the component changes unexpectedly, the snapshot will fail, indicating a regression in the UI. Thus, ensuring consistent user interfaces.

  Use Case: Let’s say a button changes from normal to disabled state unintentionally. Snapshot testing would capture this change and alert developers of a discrepancy, prompting them to either correct the unintended change or update the snapshot.

2. Fast Feedback:

• Snapshot tests are relatively lightweight and quick to execute, providing fast feedback for developers. Since snapshots are usually simple text files, it’s easy to add them to version control systems and compare changes over time.

  Use Case: In fast-paced development environments, where multiple features or components are built simultaneously, snapshot testing provides quick checks to ensure that visual changes do not inadvertently break existing functionality.

3. Minimal Setup:

• Tools like Jest and Vitest have integrated snapshot functionality, requiring minimal configuration. Developers can write a test, run it, and have a snapshot file generated automatically. This simplicity makes snapshot testing an attractive addition to the testing arsenal, even for teams with little experience in testing.

4. Integration with Modern Testing Tools:

• Modern testing tools (Jest and Vitest) make it easy to integrate snapshot testing into CI/CD pipelines, ensuring that no unintentional visual changes are deployed without oversight.

5. Component Documentation:

• Snapshot tests also serve as a form of documentation. The snapshot files clearly indicate what the DOM structure should look like for various component states. This can be helpful for teams to understand the expected structure of UI elements without needing to dive into implementation details.

When to Use DOM-Tree Snapshot Testing

While snapshot testing is a powerful tool, it isn’t the right fit for every scenario. Below are some cases where snapshot testing shines:

1. Static or Predictable UI Components:

• For components that render static content, like forms, buttons, and layout containers, snapshot testing provides a reliable means of ensuring their structure remains unchanged across different iterations.

  Example: If you’re testing a stateless React component that renders a form with predefined fields, snapshot testing is perfect for ensuring that the form structure remains intact.

2. Testing Component Layouts and Styles:

• When verifying that layout and structural changes to UI components don’t occur unintentionally, snapshot testing can be a powerful mechanism.

  Example: In a design-heavy application, snapshot tests can catch when padding, margins, or other visual layout changes affect the overall design unexpectedly.

3. Component Library Development:

• Teams working on reusable UI libraries can benefit from snapshot testing to ensure that core components do not accidentally change their appearance due to internal refactors.

  Example: A company developing a shared component library for use across multiple projects can use snapshot testing to ensure that updates to one project don’t break shared components for others.

4. Quick Refactoring Feedback:

• Snapshot testing can be helpful during refactoring to ensure that no unintended changes have been introduced to the rendered output.

When Snapshot Testing Should Be Avoided

Snapshot testing, while useful, has some limitations. Here are scenarios where it should be approached with caution:

1. Overuse Leading to Fragile Tests:

• Snapshot testing can be a double-edged sword. Overuse or reliance on snapshot tests can lead to a large number of snapshot files, which may become difficult to manage. Developers might end up blindly updating snapshots rather than investigating changes, defeating the purpose of testing.

  Avoid: If your tests frequently require snapshot updates, or if you’re dealing with components that change frequently, it might be better to avoid snapshot testing or use alternative testing strategies like functional testing or visual regression testing.

2. Dynamic Content or Frequent UI Changes:

• If your components render dynamic content, such as live data fetched from an API or user-generated content, snapshot testing can be fragile. Dynamic content often leads to constant snapshot changes, causing tests to fail unnecessarily.

  Example: For components that render timestamps, dynamic user data, or constantly updating feeds, snapshot testing might not provide meaningful results.

3. Complex Components with Internal State:

• Snapshot testing works best with simple or predictable components. For more complex components that rely on internal state, interaction testing using tools like React Testing Library might provide more value than a simple snapshot of the DOM structure.

  Avoid: If your component has complex logic, with different states based on user interaction, consider using more targeted unit tests or integration tests to cover the various states.

4. Over-Reliance on Snapshots:

• If your tests rely solely on snapshots, you may miss important behaviors, logic, and user interactions that should be tested through more meaningful assertions and user-event simulations.

5. False Sense of Security:

• Passing snapshot tests may give a false sense of security because they do not validate component logic, behavior, or user interaction. Snapshot tests should supplement, not replace, other testing techniques (e.g. unit tests, integration tests).

Best Practices for Snapshot Testing

To maximize the effectiveness of snapshot testing and avoid common pitfalls, consider the following best practices:

1. Use Selective Snapshot Testing: Don’t snapshot everything. Be selective about which components or parts of the DOM to snapshot. Focus on areas where the DOM structure is important and expected to remain stable over time.

2. Pair Snapshot Testing with Functional Testing: Snapshot tests shouldn’t be the only type of test you write. Pair them with functional or integration tests to cover a broader range of scenarios, particularly for more complex or dynamic components.

3. Review and Understand Changes: When snapshots fail, take the time to review the changes. Blindly updating snapshots defeats the purpose of having them, as it can allow unintended changes to slip through.

4. Keep Snapshots Readable: Keep your snapshot files small and readable. If your snapshot files are large or unreadable, it becomes difficult for developers to understand the changes. Snapshots should ideally represent a manageable portion of the UI, rather than the entire DOM.

5. Use toMatchInlineSnapshot() for Smaller Components: For small, easily readable components, consider using inline snapshots instead of external files. This keeps the test code and the expected output in one place, improving readability.

Alternative Testing Approaches:

• Unit Tests:
  • Instead of snapshots, test the component’s logic and behavior directly by asserting that the output matches the expected result based on props, state, or context.
• Integration Tests:
  • Test multiple components working together to verify the overall behavior and interactions between different parts of the UI.
• End-to-End (E2E) Tests:
  • These tests simulate real user interactions and verify that the application behaves as expected from the user's perspective.
• Visual Regression Testing:
  • Tools like Percy or Chromatic can capture actual screenshots of components and detect pixel-level changes, offering a more precise visual validation than snapshots.

Conclusion

Snapshot testing with tools like Jest and Vitest is beneficial when used appropriately, particularly for static or simple components. However, it's not a one-size-fits-all solution. Hence, it should be complemented with other forms of testing to cover component behavior, user interaction, and logic. Avoid relying on snapshot testing for complex, dynamic, or frequently changing components, as it can lead to fragile tests and increase maintenance effort. So, snapshot testing can be a vital tool in the testing suite, complementing other testing strategies and contributing to an overall culture of quality and reliability.

I hope this was useful. Feel free to drop your comments below.

- Ayush 🙂