It is best practice to not clutter the global scope. Especially since other frameworks or drop-in scripts can pollute or overwrite your vars.
Create a namespace for yourself
https://www.geeksforgeeks.org/javascript-namespace/
More here: https://stackoverflow.com/search?q=namespace+javascript+global
Some examples using different methods of setting the vars
myOwnNS = {}; // or window.myOwnNS
myOwnNS.counter = 0;
myOwnNS["page1"] = { "specificForPage1":"This is page 1"}
myOwnNS.page2 = { "specificForPage2":"This is page 2", "pagenumber":2}
myOwnNS.whatPageAmIOn = function { return location.href.substring(location.href.lastIndexOf('page')+4)}