cmd batch script function with return values

I found string functions like toUpper for batch files here, and also on stackoverflow, but I am unable to figure out how to use them. I found several examples and instructions, some of which are contradictory, but none of them worked for me, for some reason. How would I use a function like this in a .bat file? Here is a sample code for toUpper:

@echo off
setlocal enabledelayedexpansion

:: Main script starts here
set "arg=exampleString"

:: Output the original
echo Original: %arg%

:: Call the toUpper function to convert %arg% to uppercase
call :toUpper arg

:: Output the result
echo Uppercase: %arg%

EXIT /b 1


:toUpper str -- converts lowercase character to uppercase
if not defined %~1 EXIT /b
for %%a in ("a=A" "b=B" "c=C" "d=D" "e=E" "f=F" "g=G" "h=H" "i=I"
            "j=J" "k=K" "l=L" "m=M" "n=N" "o=O" "p=P" "q=Q" "r=R"
            "s=S" "t=T" "u=U" "v=V" "w=W" "x=X" "y=Y" "z=Z" "ä=Ä"
            "ö=Ö" "ü=Ü") do (
    call set %~1=%%%~1:%%~a%%
)
EXIT /b

What I get is this:

Original: exampleString
Uppercase: "ü=Ü"rg:ü=Ü

What's going on here?

EDIT: added missing return statement (see comments to answer)

nearly...

Add a goto :eof before the subroutine, else the code will run through it unintended.

Second: you already enabled delayed expansion, so use it:

goto :eof
:toUpper str -- converts lowercase character to uppercase
if not defined %~1 EXIT /b
for %%a in ("a=A" "b=B" "c=C" "d=D" "e=E" "f=F" "g=G" "h=H" "i=I"
            "j=J" "k=K" "l=L" "m=M" "n=N" "o=O" "p=P" "q=Q" "r=R"
            "s=S" "t=T" "u=U" "v=V" "w=W" "x=X" "y=Y" "z=Z" "ä=Ä"
            "ö=Ö" "ü=Ü" "ß=SS") do (
    set "%~1=!%~1:%%~a!"
)
EXIT /b 

The output is not quite what you want because of the Umlaute. Add a chcp 65001 at the top of your script to switch to a codepage which can handle them.

I also suggest adding "ß=SS" for proper grammatics.