main.js

var el = document.querySelector('main');
let g1 = new Game();
let g2 = new Game();
let g3 = new Game();

let layout = new StackLayout(el);
layout.addItem('g1', g1);
layout.addItem('g2', g2);
layout.addItem('g3', g3);

stacklayout.js

export default class StackLayout {
  constructor(el) {
    this.el = el;
    this.items = {};
  }

  setActiveItem(itemId) {
    this.items[itemId].render(this.el);
  }

  addItem(id, item) {
    this.items[id] = item;
  }

  getItemIds() {
    return Object.keys(this.items);
  }
}