Global variable troubles (Calling Variables Declared in Another Unit) in javascript

Ah, I forgot the Javascript scope can be a little weird. You won't actually be able to reference that var directly without it being a const. 

So you could approach it two different ways:

let fooString = ""

function setFooString(){
 
 fooString = "Test";
 
}

function getFooString() {
  
  return fooString;
}

module.exports.setFooString = setFooString;
module.exports.getFooString = getFooString;

// bar file

let foo = require("foo");

function barFunction() { 
  foo.setFooString();
  ShowMessage(foo.getFooString());
}

Or you could do a const object like this:

// foo file
const info = {
  fooString : ""
}

function setFooString(){
 
 info.fooString = "Test";
 
}

module.exports.setFooString = setFooString;
module.exports.info = info;

// bar file

let foo = require("foo");

function barFunction() { 
  foo.setFooString();
  ShowMessage(foo.info.fooString);
}

I personally would prefer the second way because it's much easier to expand on what you're acceessing from the first unit.