If you asked me what one key takeaway I would like people to get from this book, it would be to plan for the future. Throughout this book, we have discussed things like creating functions and modules with your reusable code. By making things modular and extensible, you are giving yourself a great foundation. However, there will come a time when you may need to make changes to these foundational parts. If you have a function or module used across multiple automations, you need to ensure that your changes do not break existing functionality. The best way to do that is by creating unit and integration tests, and there is no better way to do that in PowerShell than by using Pester.
Pester is the predominant test and mock framework for PowerShell. It is one of PowerShell's most prominent community-owned extensions, with over 130 contributors and an Open Collective group. It has been such the gold standard for PowerShell testing that Microsoft includes it in the default PowerShell installation. Like many things in PowerShell, it is simple to get started but is also very powerful once you start diving deep into it.