diff --git a/script.js b/script.js
new file mode 100644
index 0000000..588bc8d
--- /dev/null
+++ b/script.js
@@ -0,0 +1,106 @@
+(function () {
+  'use strict';
+
+  // Storage helpers with safety guards
+  const storage = {
+    get(key, fallback = '0') {
+      try {
+        const v = localStorage.getItem(key);
+        return v === null ? fallback : v;
+      } catch (_) {
+        return fallback;
+      }
+    },
+    set(key, value) {
+      try {
+        localStorage.setItem(key, String(value));
+      } catch (_) {
+        // ignore storage errors (e.g., quota or privacy mode)
+      }
+    }
+  };
+
+  // Parse integer safely
+  function toInt(val, fallback = 0) {
+    const n = parseInt(String(val).replace(/[^0-9-]/g, ''), 10);
+    return Number.isFinite(n) ? n : fallback;
+  }
+
+  // DOM refs
+  const scoreEl = document.getElementById('score');
+  const bestEl = document.getElementById('best-score');
+  const newGameBtn = document.getElementById('new-game');
+
+  if (!scoreEl || !bestEl) {
+    // If markup isn't present, do nothing.
+    return;
+  }
+
+  const BEST_KEY = '2048:bestScore';
+
+  // Initialize UI from storage
+  let bestScore = toInt(storage.get(BEST_KEY, '0'), 0);
+  bestEl.textContent = String(bestScore);
+
+  // Keep a last known score to avoid redundant writes
+  let lastScore = toInt(scoreEl.textContent, 0);
+
+  function updateBestIfNeeded(current) {
+    if (current > bestScore) {
+      bestScore = current;
+      bestEl.textContent = String(bestScore);
+      storage.set(BEST_KEY, bestScore);
+    }
+  }
+
+  // Observe score changes anywhere in the app (robust even if other code mutates DOM directly)
+  const observer = new MutationObserver(() => {
+    const current = toInt(scoreEl.textContent, 0);
+    if (current !== lastScore) {
+      lastScore = current;
+      updateBestIfNeeded(current);
+    }
+  });
+
+  observer.observe(scoreEl, { childList: true, characterData: true, subtree: true });
+
+  // Also capture explicit custom events if the game dispatches them
+  // Consumers can do: window.dispatchEvent(new CustomEvent('score:set', { detail: { value: 128 } }))
+  window.addEventListener('score:set', (e) => {
+    const value = toInt(e?.detail?.value, 0);
+    scoreEl.textContent = String(value);
+  });
+
+  // Consumers can do: window.dispatchEvent(new CustomEvent('score:add', { detail: { delta: 4 } }))
+  window.addEventListener('score:add', (e) => {
+    const delta = toInt(e?.detail?.delta, 0);
+    const next = toInt(scoreEl.textContent, 0) + delta;
+    scoreEl.textContent = String(next);
+  });
+
+  // Expose a tiny API for convenience (optional)
+  window.ScoreTracker = {
+    get score() { return toInt(scoreEl.textContent, 0); },
+    set score(v) { scoreEl.textContent = String(toInt(v, 0)); },
+    get best() { return bestScore; },
+    resetScore() { scoreEl.textContent = '0'; },
+    resetBest() { bestScore = 0; bestEl.textContent = '0'; storage.set(BEST_KEY, 0); },
+  };
+
+  // New game typically resets current score but preserves best
+  if (newGameBtn) {
+    newGameBtn.addEventListener('click', () => {
+      // Ensure best is up-to-date before reset
+      updateBestIfNeeded(toInt(scoreEl.textContent, 0));
+      scoreEl.textContent = '0';
+    });
+  }
+
+  // If best score changes in another tab, sync it here
+  window.addEventListener('storage', (e) => {
+    if (e.key === BEST_KEY) {
+      bestScore = toInt(e.newValue, 0);
+      bestEl.textContent = String(bestScore);
+    }
+  });
+})();