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Learn how to effectively use Jest to `mock modules` in different ways across various tests within the same test file, ensuring greater flexibility and control over your unit tests.
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How to Mock Modules Differently in Jest Tests per Test Case

When working with unit tests in JavaScript, especially using Jest, you often need to mock modules to isolate the components you're testing. However, there might be scenarios where you want to return different values from the same module across various tests within the same test file. In this post, we'll explore how to achieve this using Jest effectively.

The Challenge: Customizing Hooks for Each Test

Let's say you have a custom React hook named useMyHook that returns different values based on the input parameter. Your goal is to have it return false by default, but for specific tests, you need it to return true based on certain keys. Here's a simplified version of what your hook looks like:

[[See Video to Reveal this Text or Code Snippet]]

In this case, you want to write tests where:

The hook returns false for both keys by default.

In some tests, it should return true for the foo key and in others for the bar key.

The Solution: Customizing Mocks in Jest

To handle this requirement in Jest, we can leverage the following strategies to mock the module behavior precisely as needed:

1. Using jest.resetModules()

The first approach is to use jest.resetModules() inside the beforeEach block. By resetting the module cache, you enable Jest to re-import the module after updating its mock implementation.

Implementation Steps:

Reset Modules: Before each test, reset the modules to clear the cache.

Do Mock in Each Test: Use jest.doMock() to customize the mock for each specific test.

Example Code:

[[See Video to Reveal this Text or Code Snippet]]

2. Using Jest Spies

A more flexible approach involves using Jest spies to mock the implementation of your hook dynamically, allowing you to specify the return values for each test case without the need for resetting modules.

Implementation Steps:

Mock with Jest Spy: Use jest.fn() to create a mock function that returns false by default.

Customize Behavior: In tests where you need the hook to return true, use the mockReturnValueOnce method.

Example Code:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By utilizing the techniques outlined above, you can effectively mock your custom React hooks differently in Jest tests, making it easier to test various scenarios within the same test file. Whether you choose to reset modules or use Jest spies for mocking will depend on the specific requirements of your tests.

By mastering these strategies, you'll enhance your unit testing practices and ensure that your components behave predictably in isolation. Happy testing!