Unit Testing within Modules

Let's say you have code like this inside a script module (.psm1 file):

MyModule.psm1

function BuildIfChanged {
    $thisVersion = Get-Version
    $nextVersion = Get-NextVersion
    if ($thisVersion -ne $nextVersion) { Build $nextVersion }
    return $nextVersion
}

function Build ($version) {
    Write-Host "a build was run for version: $version"
}

# Actual definitions of Get-Version and Get-NextVersion are not shown here,
# since we'll just be mocking them anyway.  However, the commands do need to
# exist in order to be mocked, so we'll stick dummy functions here

function Get-Version { return 0 }
function Get-NextVersion { return 0 }

Export-ModuleMember -Function BuildIfChanged

You wish to write a unit test for this module which mocks the calls to Get-Version and Get-NextVersion from the module's BuildIfChanged command. In older versions of Pester, this was not possible. As of version 3.0, there are two ways you can perform unit tests of PowerShell script modules. The first is to inject mocks into a module:

For these example, we'll assume that the PSM1 file is named MyModule.psm1, and that it is installed on your PSModulePath.

Import-Module MyModule

Describe "BuildIfChanged" {
    Context "When there are Changes" {
        Mock -ModuleName MyModule Get-Version { return 1.1 }
        Mock -ModuleName MyModule Get-NextVersion { return 1.2 }

        # Just for giggles, we'll also mock Write-Host here, to demonstrate that you can
        # mock calls to commands other than functions defined within the same module.
        Mock -ModuleName MyModule Write-Host {} -Verifiable -ParameterFilter {
            $Object -eq 'a build was run for version: 1.2'
        }

        $result = BuildIfChanged

        It "Builds the next version and calls Write-Host" {
            Assert-VerifiableMock
        }

        It "returns the next version number" {
            $result | Should -Be 1.2
        }
    }

    Context "When there are no Changes" {
        Mock -ModuleName MyModule Get-Version { return 1.1 }
        Mock -ModuleName MyModule Get-NextVersion { return 1.1 }
        Mock -ModuleName MyModule Build { }

        $result = BuildIfChanged

        It "Should not build the next version" {
            Assert-MockCalled Build -ModuleName MyModule -Times 0 -ParameterFilter {
                $version -eq 1.1
            }
        }
    }
}

Notice that in this example test script, all calls to Mock and Assert‐MockCalled have had the -ModuleName MyModule parameter added. This tells Pester to inject the mock into the module's scope, which causes any calls to those commands from inside the module to execute the mock instead.

When you write your test script this way, you can mock commands that are called by the module's internal functions. However, your test script is still limited to accessing the public, exported members of the module. If you wanted to write a unit test that calls Build directly, for example, it wouldn't work using the above technique. That's where the second approach to script module testing comes into play. With Pester 3.0's InModuleScope command, you can cause entire sections of your test script to execute inside the targeted script module. This gives you access to non-exported members of the module. For example:

Import-Module MyModule

Describe "Unit testing the module's internal Build function:" {
    InModuleScope MyModule {
        $testVersion = 5.0
        Mock Write-Host { }

        Build $testVersion

        It 'Outputs the correct message' {
            Assert-MockCalled Write-Host -ParameterFilter {
                $Object -eq "a build was run for version: $testVersion"
            }
        }
    }
}

Notice that when using InModuleScope, you no longer need to specify a -ModuleName parameter when calling Mock or Assert-MockCalled for commands within that module. You are also able to directly call the Build function, which the module does not export.