Can't declare methods in a script being run in PowerShell - powershell

Question

I'm invoking a script as described here and exemplified here. It's running and I can see the outputs to the console (both hazaa and shazoo). However, the method declared in it does not appear to be there, despite testing both versions below.
Invoke-Expression -Command $target
$target | Invoke-Expression
The contents of the file are like this.
Write-Host "Hazaa"
function TestPower { Write-Host "I got the power..." }
Write-Host "Shazoo"
When I execute the same function definition in the console, it's there, working fine. I'm not getting any errors on the manual execution nor the invokation from the other file. No warnings neither.
The weirdest part is that the function definitions done in the invoking script (i.d. the one that the invokes the execution not the one that in being targeted for the invokation).

Answer 1

To rephrase your issue - a function defined in a script invoked with Invoke-Expression isn't available after the expression has executed, i.e.:
test.ps1
Write-Host "Hazaa"
function TestPower { Write-Host "I got the power..." }
Write-Host "Shazoo"
Interactive
PS> Invoke-Expression -Command "C:\src\so\test.ps1"
PS> TestPower
TestPower : The term 'TestPower' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ testpower
+ ~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (testpower:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException
You can fix this by dot-sourcing the script in the expression instead:
PS> Invoke-Expression -Command ". 'C:\src\so\test.ps1'"
Hazaa
Shazoo
PS> TestPower
I got the power...
Note that the documentation for script scope and dot sourcing says:
Each script runs in its own scope. The functions, variables, aliases, and drives that are created in the script exist only in the script scope. You cannot access these items or their values in the scope in which the script runs.
Which explains why TestPower isn't available outside of your Invoke-Expression because the script is running in its own scope.
By contrast, using dot sourcing:
The dot sourcing feature lets you run a script in the current scope instead of in the script scope. ... After the script runs, you can use the created items and access their values in your session.