How to define global variable in Google Apps Script

Question

I see most examples from Google is they use only functions in a single giant script.

e.g. https://developers.google.com/apps-script/quickstart/macros

But in

Answer 1

I use this: if you declare var x = 0; before the functions declarations, the variable works for all the code files, but the variable will be declare every time that you edit a cell in the spreadsheet