"Not a constructor", but the types check out

Question

Yes, this has been asked to death before, but none of these work for me.

I'm getting a TypeError: Config is not a constructor when calling the Config constructor. Going through other SO questions and MDN, it appears the usual cause of this error is either shadowing the constructor or calling a non-callable type, but neither of these check out in my project.

This is the call:

var Server = require("./server.js").Server;
var Config = require("./config.js").Config;

new Server(new Config("app/config.json")).run();

In ./config.js:

var fs = require("fs");

exports.Config = file => {
  var json;

  if (fs.existsSync(file) && !fs.lstatSync(file).isDirectory()) {
    json = JSON.parse(fs.readFileSync(file));
  }
  else {
    throw new ReferenceError("File doesn't exist: can't load config");
  }

  this.has = key => {
    return json.hasOwnProperty(key);
  };

  this.get = key => {
    return json[key] || null;
  };

  this.set = (key, value, write) => {
    json[key] = value;
    if (write) {
      fs.writeFileSync(file, JSON.stringify(json));
    }
  };
};

Logging the type of Config before calling it reveals that it's a Function, so it's almost certainly the same function as is defined in config.js. So, why is Node telling me it's not a constructor?

Answer 1

So, why is Node telling me it's not a constructor?

Because it's not a constructor. :-) Arrow functions are never constructors, they close over this and have no prototype property, and so cannot be used as constructors (which need to have a specific this set when they're called via new, and need to have a prototype property so it can be used to set the [[Prototype]] of the object created via new).

Either 1. Make it a function function, or 2. Make it a class.

Here's the one line change for #1:

exports.Config = function(file) {