Converting a list to comma separated value using JavaScript

I need to turn a list (user defined) into a string of comma separated values that can then be copied to the clipboard, within Storyline 360.

Essentially, I want people to be able to paste a list of ID numbers and have that converted to a string of those numbers, separated with a comma.

My JavaScript knowledge is very basic and I am going round in circles trying to get anything to display.

I have the TextEntry variable setup where users can paste their numbers into, a button they press to perform the conversion and another variable OutputString setup to display the conversion. 

Any help will be greatly appreciated.