How to define the return type / OutputType of a function

Why is the following changing type?

function SomeFunction($SomeParameter){
    return $SomeParameter
}

I guess I need to set a return type, but how?

An example is using:

$NewFolder=Join-Path $CurrentFolder -ChildPath $FolderName
$Tmp=SomeFunction($NewFolder)

Now $Tmp is an array and not just a path

Your issue is per "design". PowerShell will return an array in chunks so that it can be forwarded the PowerShell pipeline.

Example:

 SomeFunction -SomeParameter @(1,2,3,4) | Where-Object { $_ -gt 2 }

Without this behavior pipelining the output of the function to another function/cmdlet won't be possible.

If you want to return an array you can change to code to:

 function SomeFunction($SomeParameter){
    <#
     # Through the unary operator we can return an array with one entry.
     # This entry contains the original array.
     #>
    ,$SomeParameter
}

Another option would be the use of @() when at the calling side:

function SomeFunction($SomeParameter){
    # return to pipelin
    $SomeParameter
}

$array = @(SomeFunction -SomeParameter 1,2,3,4)

There is also this reddit answer explaining the behavior in more detail.

Hope that helps.