Added a countdown timer

site/mental-arithmatic/scripts/countdown.js

@@ -0,0 +1,35 @@
+import { Controller } from "https://unpkg.com/@hotwired/stimulus/dist/stimulus.js";
+
+export class CountdownController extends Controller {
+    static targets = ["countdown", "fakeinput"];
+
+    start() {
+        this.element.classList.remove('hidden');
+
+        this._countdown = 3;
+        this.countdownTarget.innerText = this._countdown;
+
+        this._tickInterval = window.setInterval(this._tick.bind(this), 1000);
+        window.setTimeout(() => {
+            this.fakeinputTarget.focus();
+        }, 1);
+    }
+
+    _tick() {
+        this._countdown -= 1;
+
+        if (this._countdown === 0) {
+            this.countdownTarget.innerText = "GO!";
+        } else if (this._countdown < 0) {
+            window.clearInterval(this._tickInterval);
+            this._startActualGame();
+        } else {
+            this.countdownTarget.innerText = this._countdown;
+        }
+    }
+
+    _startActualGame() {
+        this.element.classList.add('hidden');
+        window.dispatchEvent(new CustomEvent("startGame"));
+    }
+}