recommended approach to generalising code with variables that must be assigned by the user

As other have said really, but just to be clear:

• https://www.espruino.com/Bangle.js+Custom is when you want to customise some part of the app before upload. To update the app it needs re-running, so it's worth using 'local storage' on the web browser to keep track of what was entered
  
• https://www.espruino.com/Bangle.js+Storage is when you want to be able to read/write from a data file after the app has been uploaded - so you could use that to set the credentials in a JSON file by from the web browser.