a.js

// modloader.js
load (module: constructor) {
    try {
      let mod = new module(this.game)

      this.registerMod(mod)

      this.log(`loaded mod: '${mod.name}'`)
    } catch (e) {
      this.log('Failed to load: ' + module.name)
    }
  }
  
// app.js
let Game = require('./game.js')
var game = new Game({'name': 'Haxel'})

// Mods
let engine = require('./engine.js')

// here engine is a constructor for class type Engine?
game.modLoader.load(engine)
game.init()


// engine.js
// @flow
let Mod = require('./mod.js')
let Game = require('./game.js')

class Engine extends Mod {
  constructor (game : Game) {
    var opts = {
      name: 'Engine',
      dependencies: []
    }

    super(game, opts)
    this.game = game
  }

}

module.exports = Engine

// mod.js
// @flow
let Game = require('./game.js')

class Mod {
  name : string
  game: Game
  enabled: boolean
  dependencies: Array<any>

  constructor (game: Game, opts: { name: string, dependencies: Array<any>}) {
    this.game = game

    this.name = opts.name
    this.dependencies = opts.dependencies

    this.enabled = false
  }
}

module.exports = Mod