Okay so this like wants to be an object but isn't... it refers to things like `this.fileStoreUsersDir` later on, but no where in this file is that property set... oh right and `this` in that context is essentially an alias for `module.exports`... I'd suspect that to be incredibly confusing for newer programmers, or just anyone unfamiliar with javascript/node-isms.

I would suggest actually using object orientation here, and subsuming `config_file.js` here as a constructor:
```javascript
function FileStore(storeDir, usersDir) {
  // NOTE: a more typical / scalable pattern is to take a "params" or "options" argument here... but two positionals barely warrants it
  this.storeDir = storeDir;
  this.usersDir = usersDir;
}

FileStore.prototype.setup = function(callback) {
  /* the async setup code suggested in config_file.js could go here */
}

FileStore.prototype.XXX = YYY; // all the previous module.exports functions on prototype now

module.exports = function(s, d, callback) {
  var store = new FileStare(s, d); // I'm being lazy, don't actually call these s and d
  store.setup(callback);
};
```