即席のinput要素が欲しいときはquicksettingsが便利そう - Qiita

class CounterApp { constructor(root) { this._count = 0; this.viewElement = document.createElement('div'); root.appendChild(this.viewElement); this.viewElement.innerText = this._count; } add(v) { this._count += v; this.viewElement.innerText = this._count; } sub(v) { this._count -= v; this.viewElement.innerText = this._count; } } const app = new CounterApp(document.querySelector("#app")); const inpu