foo1.js

(function () {

// export the API, but also create a global called "foo"
foo = module.exports = function (s) {
  console.log('foo1 (global): ' + s);
};
    
})();

foo2.js

module.exports = function (s) {
  console.log('foo2: ' + s);
  
  // note we can access the global from the server.js
  foo1('from foo2.js');
};

// oh look, I can access the foo global variable
foo('foo2.js being loaded');

server.js

foo1 = require('./foo1'); // note - global from our server
var foo2 = require('./foo2');

foo1('from server.js');
foo2('from server.js');

// this global "foo" leaks from foo1
foo('from server.js');