This will list the completed Reminders and output it to a text file

List Completed Reminders Tasks.applescript

##############################################
# Title: List Completed Reminders
##############################################

# Iain Dunn
# Logic2Design
# www.logic2design.com
# [email protected]

# Last update: 11 April 2022
# Version: 1

# Contributors and sources
# 

##############################################
# Configuration
##############################################

##############################################
# Code
##############################################
tell application "Reminders"
	set lName to name of every list
	set dName to name of default list
end tell
tell me to activate
set lName to choose from list lName with prompt "Select Reminder List" default items {dName} without empty selection allowed
if lName is false then
	return 1
else
	set lName to lName as string
end if

set RemindersList to lName as text

# list Completed Reminders
tell application "Reminders"
	tell list RemindersList
		set reminderCompleted to name of reminders whose completed is true
	end tell
end tell
# create String
set theBody to list2string(reminderCompleted, "
")
#Display option
display dialog theBody
#Clipboard option
set the clipboard to theBody

# Text File Option
set myFile to open for access (choose file name) with write permission
write theBody to myFile
close access myFile
##############################################
# Functions
############################################### Select Reminders List
#Function
on list2string(theList, theDelimiter)
	set theBackup to AppleScript's text item delimiters
	set AppleScript's text item delimiters to theDelimiter
	set theString to theList as string
	set AppleScript's text item delimiters to theBackup
	return theString
end list2string