Implement P2P polling app with Yjs and WebRTC

Uses Yjs CRDTs for conflict-free shared state and y-webrtc for
peer-to-peer data exchange. Users can add poll options and vote/unvote,
with all changes syncing in real-time across connected peers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.js

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+import * as Y from 'yjs';
+import { WebrtcProvider } from 'y-webrtc';
+
+// --- Peer ID (stable across reloads) ---
+
+function getOrCreatePeerId() {
+  let id = localStorage.getItem('peer-id');
+  if (!id) {
+    id = crypto.randomUUID();
+    localStorage.setItem('peer-id', id);
+  }
+  return id;
+}
+
+const peerId = getOrCreatePeerId();
+
+// --- Room name from URL ---
+
+function getRoomName() {
+  const params = new URLSearchParams(window.location.search);
+  return params.get('room') || 'default-poll';
+}
+
+const roomName = getRoomName();
+
+// --- Yjs setup ---
+
+const ydoc = new Y.Doc();
+const provider = new WebrtcProvider(roomName, ydoc);
+const yOptions = ydoc.getMap('poll-options');
+
+// --- Data operations ---
+
+function addOption(name) {
+  const id = crypto.randomUUID();
+  const optionMap = new Y.Map();
+  optionMap.set('name', name);
+  optionMap.set('votes', new Y.Map());
+  yOptions.set(id, optionMap);
+}
+
+function toggleVote(optionId) {
+  const optionMap = yOptions.get(optionId);
+  if (!optionMap) return;
+  const votes = optionMap.get('votes');
+  if (votes.has(peerId)) {
+    votes.delete(peerId);
+  } else {
+    votes.set(peerId, true);
+  }
+}
+
+// --- UI rendering ---
+
+const container = document.getElementById('poll-options');
+const emptyState = document.getElementById('empty-state');
+
+function render() {
+  container.innerHTML = '';
+
+  const entries = [];
+  yOptions.forEach((optionMap, id) => {
+    entries.push({
+      id,
+      name: optionMap.get('name'),
+      votes: optionMap.get('votes').size,
+      voted: optionMap.get('votes').has(peerId),
+    });
+  });
+
+  entries.sort((a, b) => b.votes - a.votes || a.name.localeCompare(b.name));
+
+  emptyState.className = entries.length > 0 ? 'hidden' : '';
+
+  for (const entry of entries) {
+    const div = document.createElement('div');
+    div.className = 'poll-option';
+
+    const nameSpan = document.createElement('span');
+    nameSpan.className = 'option-name';
+    nameSpan.textContent = entry.name;
+
+    const voteCount = document.createElement('span');
+    voteCount.className = 'vote-count';
+    voteCount.textContent = entry.votes;
+
+    const voteBtn = document.createElement('button');
+    voteBtn.className = entry.voted ? 'vote-btn voted' : 'vote-btn';
+    voteBtn.textContent = entry.voted ? 'Unvote' : 'Vote';
+    voteBtn.addEventListener('click', () => toggleVote(entry.id));
+
+    div.append(nameSpan, voteCount, voteBtn);
+    container.appendChild(div);
+  }
+}
+
+// Re-render on any change in the shared state
+yOptions.observeDeep(() => render());
+
+// --- Add option handlers ---
+
+const input = document.getElementById('option-input');
+const addBtn = document.getElementById('add-btn');
+
+addBtn.addEventListener('click', () => {
+  const name = input.value.trim();
+  if (name) {
+    addOption(name);
+    input.value = '';
+  }
+});
+
+input.addEventListener('keydown', (e) => {
+  if (e.key === 'Enter') addBtn.click();
+});
+
+// --- Connection status & peer count ---
+
+const statusEl = document.getElementById('connection-status');
+const peerCountEl = document.getElementById('peer-count');
+
+provider.on('synced', ({ synced }) => {
+  statusEl.textContent = synced ? 'Connected' : 'Connecting...';
+  statusEl.className = synced ? 'connected' : 'connecting';
+});
+
+function updatePeerCount() {
+  const count = provider.awareness.getStates().size;
+  peerCountEl.textContent = `${count} peer${count !== 1 ? 's' : ''}`;
+}
+
+provider.awareness.on('change', updatePeerCount);
+updatePeerCount();
+
+// --- Initial render ---
+render();

src/style.css

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+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+body {
+  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+  background: #f5f5f5;
+  color: #333;
+  min-height: 100vh;
+}
+
+#app {
+  max-width: 600px;
+  margin: 0 auto;
+  padding: 2rem 1rem;
+}
+
+header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-bottom: 2rem;
+}
+
+h1 {
+  font-size: 1.5rem;
+}
+
+#status {
+  display: flex;
+  gap: 1rem;
+  font-size: 0.85rem;
+  color: #666;
+}
+
+#connection-status::before {
+  content: "";
+  display: inline-block;
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  margin-right: 4px;
+  vertical-align: middle;
+}
+
+#connection-status.connected::before {
+  background: #4caf50;
+}
+
+#connection-status.connecting::before {
+  background: #ff9800;
+}
+
+#add-option {
+  display: flex;
+  gap: 0.5rem;
+  margin-bottom: 1.5rem;
+}
+
+#option-input {
+  flex: 1;
+  padding: 0.6rem 0.8rem;
+  border: 1px solid #ddd;
+  border-radius: 6px;
+  font-size: 1rem;
+  outline: none;
+}
+
+#option-input:focus {
+  border-color: #2196f3;
+}
+
+#add-btn {
+  padding: 0.6rem 1.2rem;
+  background: #2196f3;
+  color: white;
+  border: none;
+  border-radius: 6px;
+  font-size: 1rem;
+  cursor: pointer;
+}
+
+#add-btn:hover {
+  background: #1976d2;
+}
+
+.poll-option {
+  display: flex;
+  align-items: center;
+  background: white;
+  padding: 0.8rem 1rem;
+  border-radius: 8px;
+  margin-bottom: 0.5rem;
+  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.08);
+}
+
+.option-name {
+  flex: 1;
+  font-size: 1rem;
+}
+
+.vote-count {
+  font-size: 0.9rem;
+  font-weight: 600;
+  color: #666;
+  margin-right: 0.75rem;
+  min-width: 1.5rem;
+  text-align: center;
+}
+
+.vote-btn {
+  padding: 0.4rem 1rem;
+  border: 2px solid #2196f3;
+  border-radius: 6px;
+  font-size: 0.85rem;
+  cursor: pointer;
+  background: white;
+  color: #2196f3;
+}
+
+.vote-btn:hover {
+  background: #e3f2fd;
+}
+
+.vote-btn.voted {
+  background: #2196f3;
+  color: white;
+}
+
+.vote-btn.voted:hover {
+  background: #1976d2;
+}
+
+#empty-state {
+  text-align: center;
+  color: #999;
+  padding: 2rem;
+}
+
+#empty-state.hidden {
+  display: none;
+}