Javascript using global variables in another script

Script1.js is attached to both MyWebsite1.html and MyWebsite2.html, Script2.js is only attached to MyWebsite2.html. Script1.js is attached first in of MyWebsite2.html.

Script1.js

var global_var;
function open() {
    global_var = 1;
    window.open("MyWebsite2.html", "_self");
}

Script2.js

$(document).ready(function(){
    console.log(global_var);
});

When calling the open() function of my first script (via onclick of MyWebsite1.html) it sets global_var to 1 and opens MyWebsite2.html correctly. But for Script2.js global_var is still undefined.
If I set global_var to 1 outside of the open() function in Script1.js, then for Script2.js global_var is 1.

If you are wondering why I’m using $(document).ready(function(){}: because I want to append html code with it later to MyWebsite2.html.