How to Mock a Function in Jest When Testing Another Function

Learn how to effectively mock a function being called from another function during unit testing with Jest. This guide breaks down concepts to help you test your code with confidence.
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How to Mock a Function in Jest When Testing Another Function

When working with unit tests in JavaScript, especially when using the Jest framework, it often becomes necessary to mock functions to isolate the code under test. A common scenario arises when you want to test how one function interacts with another function in the same file. This guide addresses a common issue encountered in such scenarios and provides you with strategies to effectively mock functions within Jest.

The Problem: Testing Functions Calling Other Functions

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When trying to test originalFunc, if we define anotherFunc as a regular function in the same file, calling originalFunc will actually invoke the real anotherFunc. This defeats the purpose of unit testing because we want to isolate originalFunc from its dependencies.

Example Test

The following code illustrates how you might write a test for originalFunc:

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The challenge here is that the real anotherFunc is invoked rather than the mocked version, which means that the test doesn't behave as expected.

The Solution: Keep a Reference for anotherFunc

Make sure you export anotherFunc:

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Test Output

When you run your test, you should see output confirming that the test passed and the mock was invoked correctly:

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Alternative: Using Dependency Injection

Another effective strategy is to use dependency injection, where you pass anotherFunc as a parameter to originalFunc. This can be done as follows:

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This way, you can easily pass a mocked version of anotherFunc during tests, making your code more flexible and test-friendly.

Conclusion

With these strategies, you're well-equipped to tackle function testing in your JavaScript projects with confidence!