Is it possible to access a regular javascript variable from within a JQuery function?

Yes.

if it is in global scope, yes.

It should be, so long as the main.js loads first and if you set it as a global variable.

You make a global variable by creating it outside of a function. If needed, you can create it outside a function, then set it inside a function in the main.js file.

You can freely share variables between different JS files (a jQuery file is just a JS file) in several different ways:

1. Define the variables in the global scope, then they can be accessed anywhere.
2. Define variables on the window object. This makes them globally accessible even if your code defining the variables isn't in the global scope.
3. Define variables as properties on any object that you can get to from your code. So, if you have a global config object called myConfig, you could define properties on it like myConfig.count = 0; and you can then access myConfig.count from anywhere. This is often referred to as namespacing and creates only a single global object which you then add multiple properties to.
4. Define a globally accessible function that returns your data that you can call from anywhere.

When designing how this works, remember that it's generally better to introduce as few globally accessible symbols as possible because each one is an opportunity for a conflict with some other code in the page.