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Learn how to mock functions in PowerShell using Pester 5 to test your scripts effectively without modifying live data.
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How to Effectively Test Your PowerShell Functions with Pester 5 Using Mocking Techniques

Testing PowerShell functions can sometimes be challenging, especially when these functions interact with external services or resources that you might not want to impact during your testing procedures. A common scenario developers encounter is using Pester, PowerShell's testing framework, to mock functions while testing script logic. In this post, we will explore a scenario where we need to test a PowerShell function called Remove-GraphUser, focusing on how to mock a command so that we can test our function effectively.

The Problem

You have written a PowerShell function called Remove-GraphUser, which is responsible for removing users from a graph based on their IDs. This function takes a collection of user objects as input and interacts with the Graph API by calling the Remove-MgUser command. However, when testing this function with Pester 5.3.1 in PowerShell 7, you encounter issues with the mocking feature:

Your mock does not seem to work unless defined in the BeforeAll section, leading to uncertainties in your tests.

You are unable to properly iterate over the results to validate the users that were meant to be removed.

The Solution

Understanding Pester's Lifecycle

Pester operates in phases: Discovery and Run. It's essential to understand how data is processed during these phases:

BeforeDiscovery: This runs at the start of the test execution, allowing you to prepare your environment (like importing CSV files).

BeforeAll: This runs before the actual tests start, where you can set up mocks.

It Blocks: This is where you define the actual tests, containing assertions and calls to your function.

The Updated Approach

To correctly test the Remove-GraphUser function, a few adjustments should be made. Here’s how:

Move the foreach Loop Inside the It Block: Since -ForEach is processed in the Discovery phase, instead, utilize a regular foreach statement within the It block.

Mock the Remove Function: Set up the mock for Remove-MgUser in the BeforeAll block, but manage your result checks inside the It block.

Here is the revised version of your testing script:

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

What This Accomplishes

No Real Changes in the Production Environment: By mocking the Remove-MgUser function, you ensure that your testing does not affect any live user data.

Successful Assertions: Now, you can comfortably validate that your Remove-GraphUser function correctly processes the user objects and that the assertions regarding their IDs hold true.

Conclusion

Testing your PowerShell functions with Pester does not have to be complicated. With a solid understanding of Pester’s phases and the proper structuring of your tests, you can achieve effective mocking that allows you to focus on ensuring your function logic is sound without impacting real-world data. So, the next time you test a function that interacts with an API, remember these tips for a smoother testing experience!