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Learn how to effectively unit test a function with scope variables in Jest or Vitest by controlling randomness and mocking functionalities.
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Accessing Function Scope Variables in Unit Testing with Jest/Vitest

When it comes to unit testing, understanding how to access variables that exist within the scope of a function can be tricky. This is especially true when your code makes use of unpredictable outcomes, such as random values. So, how can we handle a scenario where we want to test a function but need to access its internal variables, particularly when randomness is involved? In this guide, we will explore a practical solution using Jest or Vitest.

The Problem

Consider the following example of a function we want to test, defined in myFunction.js:

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In this function, foo is determined by the value of x, which is generated randomly using Math.random(). If we want to test this function using Jest or Vitest, the challenge arises: Is it possible to access the variable foo after executing the function?

The Answer and Approach

Unfortunately, the answer is no – you cannot directly access local variables from within a function, such as foo, once it has executed. However, we can employ a workaround to control randomness and enable meaningful testing. Here's how you can approach this problem:

Modify the Function

We’ll start by modifying myFunction to allow the injection of a custom Math object. This makes it possible to control what value Math.random() returns during tests. Here’s the refactored function:

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Prepare the Test

Once the function is modified, we can write our test using Jest or Vitest. We will mock Math.random() to return predetermined values, allowing us to verify that anotherFunction is called with the expected value of foo.

Here’s an example of how to do this in a test file (myFunction.test.js):

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Explaining the Code

Mocking Math.random(): We created a helper function, getMathRandomMock, which returns an object with a random method that always returns the result provided. This bypasses the unpredictable nature of Math.random().

Using Mock Functions: anotherFuncMock acts as a stand-in for any function passed to myFunction. By using Jest’s fn() method, we can track how this mock function is called during the execution of our test.

Assertions: After invoking myFunction, we assert that anotherFuncMock was called with the expected argument, enabling us to check what value foo was based on the predetermined random value we provided.

Conclusion

By following this approach, you can effectively control the randomness in your tests, allowing for deterministic and robust unit tests with Jest or Vitest. The key takeaway is to modify the function to accept a math injection that can be mocked during testing, enabling you to gain access to internal logical outcomes without directly needing to access local scope variables.

With this solution, not only do you ensure your unit tests are thoroughly validated, but you also gain better control over unpredictable outcomes.