Testing with Pester 5.3.3

I have written a function to write the errors that occur in my functions in a csv file, this function is called in the catch block of my functions. I would like to write a Test in Pester to check that my function works correctly, but to be honest I don't know where to start, I have tried some things but they don't work for me, I have also been reading in the documentation but I am still not clear, I would appreciate any help/comments.

Here is the function for which I want to write a Test in Pester:

function Write-Logs {
    param (
        [ValidateSet("Error")]$MessageType,
        [string][Parameter(Mandatory=$true)]$Message,
        $LogFilePath,
        $Source
    )
    $CSVLogPath = Join-Path -Path $PSScriptRoot -ChildPath ".\errorslog.csv"
    $CSVLogObject = [PSCustomObject] @{
        Date = Get-Date
        Message = $Message
        MessageType = $MessageType
        Source = $Source
    }
    $CSVLogObject | Export-Csv -Path $CSVLogPath -NoTypeInformation -Encoding UTF8 -Append
}

and so I´m calling the function in the catch block:

    catch {
        Write-Logs -LogFilePath:$CSVLogPath -Message:$Error[0].Exception.Message `
        -Source:"FunctionName()" -MessageType:"Error"
        return
    }

Solution

Continuing from my comments, here is some code.

First make the function testable, by actually using the -LogFilePath parameter. This way you can write the log to a temporay file during testing. Due to the default value you can still use it without -LogFilePath when calling it from normal code.

function Write-Logs {
    param (
        [ValidateSet("Error")]$MessageType,
        [string][Parameter(Mandatory=$true)]$Message,
        $LogFilePath = (Join-Path -Path $PSScriptRoot -ChildPath ".\errorslog.csv"),
        $Source
    )
    $CSVLogObject = [PSCustomObject] @{
        Date = Get-Date
        Message = $Message
        MessageType = $MessageType
        Source = $Source
    }
    $CSVLogObject | Export-Csv -Path $LogFilePath -NoTypeInformation -Encoding UTF8 -Append
}

Test code:

BeforeAll {
    . $PSCommandPath.Replace('.Tests.ps1','.ps1')
}

Describe "Write-Logs" {
    BeforeEach{
        # Mock Get-Date so it returns a constant value suitable for testing
        $expectedDate = [DateTime]::new( 2022, 06, 28, 12, 36, 21 )
        Mock Get-Date { return $expectedDate }
    }   

    It "writes the expected CSV" {
    
        # You might read this from a file using Import-Csv
        $expectedCsv = [PSCustomObject]@{
            Date = $expectedDate
            Message = 'test message'
            MessageType = 'Error'
            Source = 'test source'
        }

        # Write log to temp file (Pester cleans it automatically, when It block ends)
        $testLogPath = "TestDrive:\test.log"
        Write-Logs -LogFilePath $testLogPath -MessageType $expectedCsv.MessageType -Message $expectedCsv.Message -Source $expectedCsv.Source

        $actualCsv = Import-Csv $testLogPath

        # Test if $expectedCsv equals $actualCsv
        Compare-Object $expectedCsv $actualCsv -Property Date, Message, MessageType, Source | Should -BeNullOrEmpty
    }
}

TestDrive: is a temporary drive created by Pester for each script block. It is very convenient for writing temporary files because Pester cleans it automatically when the script block ends. See Pester docs.
Once you got some basic tests working, you might want to improve your test code by using data-driven tests. This avoids duplication as you only need a single test, that can be fed from different data sets. See Pester docs, especially section "Providing external data to tests".