Welcome to the Pester Testing Playground! This repository provides a comprehensive, hands-on introduction to Pester, the testing and mocking framework for PowerShell. Whether you're new to testing or looking to improve your skills, this repo takes you from basic to expert-level testing patterns.

• What is Pester?
• Repository Structure
• Getting Started
• Learning Path
• Running Tests
• Best Practices
• Additional Resources

Pester is a testing and mocking framework for PowerShell that enables you to:

• Write unit tests and integration tests for your PowerShell code
• Mock external dependencies to isolate code under test
• Generate code coverage reports
• Integrate tests into CI/CD pipelines
• Validate configurations and deployments

Pester follows a simple file naming convention (*.Tests.ps1) and uses an intuitive DSL with keywords like Describe, Context, It, Should, and Mock.

The repository is organized into progressive learning levels:

pesterdevPlayground/
│
├── 01-Basic/                          # Level 1: Fundamentals
│   ├── Get-Greeting.ps1               # Simple string function
│   └── Get-Greeting.Tests.ps1         # Basic assertions and contexts
│
├── 02-Intermediate/                   # Level 2: Parameterized Testing
│   ├── Calculator.ps1                 # Arithmetic operations
│   └── Calculator.Tests.ps1           # TestCases and data-driven tests
│
├── 03-Advanced/                       # Level 3: Mocking & Dependencies
│   ├── UserValidation.ps1             # Functions with external calls
│   └── UserValidation.Tests.ps1       # Mocking and mock assertions
│
├── 04-Expert/                         # Level 4: Integration, File I/O & DateTime
│   ├── FileOperations.ps1             # Configuration file management
│   ├── FileOperations.Tests.ps1       # Setup/teardown and integration tests
│   ├── DateTimeParsing.ps1            # Robust date parsing functions
│   └── DateTimeParsing.Tests.ps1      # Comprehensive date format testing
│
└── README.md                          # This file

• PowerShell 5.1 or PowerShell 7+
• Pester 5.x (recommended)

If you don't have Pester installed, run:

# Install Pester (requires admin/elevated privileges)
Install-Module -Name Pester -Force -SkipPublisherCheck

# Verify installation
Import-Module Pester -PassThru

You should see output similar to:

ModuleType Version    Name
---------- -------    ----
Script     5.x.x      Pester

git clone https://github.com/yourusername/pesterdevPlayground.git
cd pesterdevPlayground

Follow these levels in order to progressively build your Pester skills:

Concepts Covered:

• BeforeAll block for setup
• Describe and Context for organizing tests
• It blocks for individual test cases
• Basic Should assertions (-Be, -BeLike, -Match)
• Testing default parameters and error conditions
• Pipeline support testing

What You'll Learn:

• How to structure a basic test file
• Dot-sourcing functions to test
• Writing clear, descriptive test cases
• Testing both happy paths and error scenarios

Run Tests:

Invoke-Pester -Path .\01-Basic\Get-Greeting.Tests.ps1 -Output Detailed

Concepts Covered:

• TestCases for parameterized testing
• Data-driven tests with multiple inputs
• Testing mathematical operations
• Edge case validation
• Integration tests combining multiple functions

What You'll Learn:

• How to reduce test duplication with TestCases
• Testing multiple scenarios efficiently
• Organizing related test cases
• Validating complex calculations

Run Tests:

Invoke-Pester -Path .\02-Intermediate\Calculator.Tests.ps1 -Output Detailed

Concepts Covered:

• Mocking functions and external dependencies
• Mock assertions with Should -Invoke
• Parameter filters for precise mock verification
• BeforeEach for per-test setup
• Testing code with external API calls
• Verifying call order and behavior

What You'll Learn:

• How to isolate code from external dependencies
• Mocking strategies for unit testing
• Verifying function interactions
• Testing error handling without hitting real APIs

Run Tests:

Invoke-Pester -Path .\03-Advanced\UserValidation.Tests.ps1 -Output Detailed

Concepts Covered:

• BeforeAll/AfterAll for test suite setup/teardown
• BeforeEach/AfterEach for test-level cleanup
• Testing file system operations
• Using $TestDrive for temporary test files
• Integration testing with real I/O
• Idempotency testing
• Complete workflow testing
• Explicit format DateTime parsing to prevent ambiguous date interpretation
• UTC conversion and timezone handling
• DateTimeOffset for timezone-aware operations
• Format validation before parsing
• Flexible format detection for multiple date formats
• Production-ready patterns addressing real-world date format issues

What You'll Learn:

• How to safely test file operations
• Proper test isolation and cleanup
• Integration test patterns
• Testing concurrent operations
• End-to-end workflow validation
• How to safely parse date strings with explicit formats
• Converting between local time, UTC, and DateTimeOffset
• Preventing production issues with ambiguous date parsing
• Testing leap years, end-of-year dates, and edge cases
• Handling international date formats (US vs European)
• Batch processing dates via pipeline
• Real-world integration scenarios (API timestamps, log parsing, database storage)

File Operations Functions:

• New-ConfigurationFile - Create configuration files
• Get-ConfigurationFile - Read configuration files
• Update-ConfigurationFile - Modify configuration files
• Remove-ConfigurationFile - Delete configuration files
• Backup-ConfigurationFile - Backup configuration files
• Test-ConfigurationFile - Validate configuration files

DateTime Parsing Functions:

• ConvertTo-DateTime - Parse with explicit format specification
• ConvertTo-DateTimeUtc - Parse and convert to UTC
• ConvertTo-DateTimeOffsetUtc - Parse with timezone offset preservation
• Test-DateTimeFormat - Validate date format before parsing
• ConvertFrom-DateTime - Convert DateTime to ISO 8601 string
• ConvertTo-DateTimeFlexible - Automatic format detection from common formats

Run Tests:

# Run all expert level tests (file operations + datetime)
Invoke-Pester -Path .\04-Expert\ -Output Detailed

# Run specific test files
Invoke-Pester -Path .\04-Expert\FileOperations.Tests.ps1 -Output Detailed
Invoke-Pester -Path .\04-Expert\DateTimeParsing.Tests.ps1 -Output Detailed

To run all tests in the repository:

Invoke-Pester -Output Detailed

# Basic tests
Invoke-Pester -Path .\01-Basic\ -Output Detailed

# Intermediate tests
Invoke-Pester -Path .\02-Intermediate\ -Output Detailed

# Advanced tests
Invoke-Pester -Path .\03-Advanced\ -Output Detailed

# Expert tests (includes both file operations and datetime parsing)
Invoke-Pester -Path .\04-Expert\ -Output Detailed

Invoke-Pester -Path .\02-Intermediate\Calculator.Tests.ps1 -Output Detailed
Invoke-Pester -Path .\04-Expert\FileOperations.Tests.ps1 -Output Detailed
Invoke-Pester -Path .\04-Expert\DateTimeParsing.Tests.ps1 -Output Detailed

$config = New-PesterConfiguration
$config.Run.Path = '.\02-Intermediate\'
$config.CodeCoverage.Enabled = $true
$config.CodeCoverage.Path = '.\02-Intermediate\Calculator.ps1'
$config.Output.Verbosity = 'Detailed'

Invoke-Pester -Configuration $config

# Run with minimal output (only failed tests)
Invoke-Pester

# Run with detailed output (all tests)
Invoke-Pester -Output Detailed

# Run specific tests by name
Invoke-Pester -FullNameFilter "*adds*"

# Run tests and stop on first failure
Invoke-Pester -Stop

# Generate test results in NUnit format (for CI/CD)
$config = New-PesterConfiguration
$config.TestResult.Enabled = $true
$config.TestResult.OutputPath = 'TestResults.xml'
Invoke-Pester -Configuration $config

Based on the Pester documentation and examples in this repo:

• Place test files next to the code they test
• Use the *.Tests.ps1 naming convention
• Keep functions in separate .ps1 files
• Use dot-sourcing in BeforeAll to import functions

• Use Describe for the function or module being tested
• Use Context to group related scenarios
• Use It for individual test cases
• Keep test names descriptive and readable

• Use BeforeAll for expensive setup (runs once per block)
• Use BeforeEach for per-test setup
• Use AfterAll and AfterEach for cleanup
• Leverage $TestDrive for temporary file testing

• Use specific assertions (-Be, -BeExactly, -BeLike, -Match)
• Test both positive and negative cases
• Use Should -Throw for error testing
• Use parameter filters for precise expectations

• Mock external dependencies to isolate code
• Use Should -Invoke to verify mock calls
• Use parameter filters to verify correct arguments
• Test both with and without mocks when appropriate

• Use -TestCases to reduce duplication
• Provide descriptive test names with placeholders
• Include edge cases and boundary conditions
• Keep test data close to the test

• Aim for high coverage, but focus on meaningful tests
• Don't just test for coverage percentage
• Cover error paths and edge cases
• Review coverage reports to find gaps

• Pester Documentation
• Pester GitHub Repository
• PowerShell Gallery

• Pester Book
• PowerShell Testing with Pester
• Pester Gitter Chat

• PowerShell.org Forums
• PowerShell Discord - #testing
• Stack Overflow - Pester Tag

This is a learning repository! Feel free to:

• Add more examples
• Improve existing tests
• Fix bugs or typos
• Add documentation
• Share your experiences

This project is provided as-is for educational purposes. Feel free to use, modify, and share!

After completing all four levels:

1. Practice - Write tests for your own PowerShell scripts
2. Integrate - Add Pester to your CI/CD pipeline
3. Explore - Learn about code coverage and test-driven development (TDD)
4. Share - Help others learn Pester!

Happy Testing! 🎉

If you found this helpful, consider starring the repository and sharing it with others learning PowerShell testing.