eliferjunior Yjs — CRDT Framework for Collaborative Editing
Overview
Yjs, the high-performance CRDT (Conflict-free Replicated Data Type) framework for building collaborative applications. Helps developers implement real-time document editing, offline-first sync, and peer-to-peer collaboration with automatic conflict resolution.
Instructions
Document and Shared Types
Create collaborative data structures that merge automatically:
// src/collaboration/document.ts — Set up a collaborative document with shared types
import * as Y from "yjs";
// A Y.Doc is the top-level container for all shared data
// Every connected client gets a copy that stays in sync
const doc = new Y.Doc();
// Y.Text — collaborative rich text (used with editors like Tiptap, ProseMirror)
const yText = doc.getText("document-content");
yText.insert(0, "Hello, ");
yText.insert(7, "world!");
// Result: "Hello, world!" — inserts merge correctly even if concurrent
// Y.Map — collaborative key-value store (like a shared object)
const yMap = doc.getMap("settings");
yMap.set("theme", "dark");
yMap.set("fontSize", 14);
// Two users setting different keys: both applied
// Two users setting the same key: last-write-wins (deterministic)
// Y.Array — collaborative ordered list
const yArray = doc.getArray("tasks");
yArray.push([{ id: "1", title: "Design mockup", done: false }]);
yArray.push([{ id: "2", title: "Implement API", done: false }]);
// Concurrent inserts at different positions: both preserved in correct order
// Y.XmlFragment — collaborative XML tree (for rich text editors)
const yXml = doc.getXmlFragment("rich-content");
// Used internally by editor bindings (Tiptap, ProseMirror, Slate)
// Nested structures — Y types can be nested arbitrarily
const yNestedMap = new Y.Map();
yNestedMap.set("status", "active");
yMap.set("project", yNestedMap); // Map inside a map
WebSocket Provider
Connect clients through a WebSocket server for real-time sync:
// src/collaboration/provider.ts — WebSocket-based real-time sync
import * as Y from "yjs";
import { WebsocketProvider } from "y-websocket";
const doc = new Y.Doc();
// Connect to a y-websocket server
// All clients in the same room sync automatically
const provider = new WebsocketProvider(
"wss://your-yjs-server.example.com", // WebSocket server URL
"document-room-123", // Room name — clients in same room sync
doc,
{
connect: true, // Auto-connect on creation
params: { token: "auth-token-here" }, // Auth params sent on connect
}
);
// Awareness — lightweight presence data (cursors, selections, user info)
// Unlike document state, awareness is ephemeral (not persisted)
const awareness = provider.awareness;
awareness.setLocalStateField("user", {
name: "Alice",
color: "#ff5733",
cursor: null,
});
// Listen to other users' awareness changes
awareness.on("change", () => {
const states = awareness.getStates(); // Map<clientId, state>
states.forEach((state, clientId) => {
if (clientId !== doc.clientID) {
console.log(`User ${state.user?.name} is connected`);
}
});
});
// Connection status
provider.on("status", ({ status }: { status: string }) => {
console.log(`Connection: ${status}`); // "connecting" | "connected" | "disconnected"
});
// Sync status — fires when initial sync with server is complete
provider.on("sync", (isSynced: boolean) => {
if (isSynced) console.log("Document fully synced with server");
});
Server-Side Setup
Run a y-websocket server for document persistence:
// server/yjs-server.ts — WebSocket server with persistence
import { WebSocketServer } from "ws";
import { setupWSConnection, setPersistence } from "y-websocket/bin/utils";
import * as Y from "yjs";
import { MongodbPersistence } from "y-mongodb-provider";
const wss = new WebSocketServer({ port: 1234 });
// Persist documents to MongoDB (survives server restarts)
const mdb = new MongodbPersistence(process.env.MONGODB_URL!, {
collectionName: "yjs-documents",
flushSize: 100, // Batch 100 updates before flushing to DB
multipleCollections: true, // Separate collection per document for performance
});
setPersistence({
bindState: async (docName: string, ydoc: Y.Doc) => {
// Load existing document state from MongoDB
const persistedDoc = await mdb.getYDoc(docName);
const persistedState = Y.encodeStateAsUpdate(persistedDoc);
Y.applyUpdate(ydoc, persistedState);
// Save updates as they happen
ydoc.on("update", (update: Uint8Array) => {
mdb.storeUpdate(docName, update);
});
},
writeState: async (docName: string, ydoc: Y.Doc) => {
// Called when all clients disconnect — final persistence
await mdb.flushDocument(docName);
},
});
wss.on("connection", (ws, req) => {
// Authenticate the connection
const token = new URL(req.url!, "http://localhost").searchParams.get("token");
if (!verifyToken(token)) {
ws.close(4001, "Unauthorized");
return;
}
setupWSConnection(ws, req);
});
console.log("Yjs WebSocket server running on port 1234");
Editor Integration (Tiptap)
Add collaborative editing to a Tiptap rich text editor:
// src/components/CollaborativeEditor.tsx — Tiptap with Yjs collaboration
import { useEditor, EditorContent } from "@tiptap/react";
import StarterKit from "@tiptap/starter-kit";
import Collaboration from "@tiptap/extension-collaboration";
import CollaborationCursor from "@tiptap/extension-collaboration-cursor";
import * as Y from "yjs";
import { WebsocketProvider } from "y-websocket";
interface Props {
documentId: string;
userName: string;
userColor: string;
}
export function CollaborativeEditor({ documentId, userName, userColor }: Props) {
const doc = useMemo(() => new Y.Doc(), []);
const provider = useMemo(
() => new WebsocketProvider("wss://yjs.example.com", documentId, doc),
[doc, documentId]
);
const editor = useEditor({
extensions: [
StarterKit.configure({
history: false, // Disable default history — Yjs handles undo/redo
}),
Collaboration.configure({
document: doc, // Bind editor content to Yjs document
}),
CollaborationCursor.configure({
provider, // Share cursor positions via awareness
user: { name: userName, color: userColor },
}),
],
});
// Clean up on unmount
useEffect(() => {
return () => {
provider.destroy();
doc.destroy();
};
}, [doc, provider]);
return (
<div className="editor-container">
<EditorContent editor={editor} />
<ConnectionStatus provider={provider} />
</div>
);
}
function ConnectionStatus({ provider }: { provider: WebsocketProvider }) {
const [status, setStatus] = useState("connecting");
useEffect(() => {
const handler = ({ status }: { status: string }) => setStatus(status);
provider.on("status", handler);
return () => provider.off("status", handler);
}, [provider]);
return (
<div className={`status-badge ${status}`}>
{status === "connected" ? "🟢 Connected" : "🔴 Reconnecting..."}
</div>
);
}
Offline Support and Sync
Handle offline editing with automatic merge on reconnect:
// src/collaboration/offline.ts — IndexedDB persistence for offline support
import * as Y from "yjs";
import { IndexeddbPersistence } from "y-indexeddb";
import { WebsocketProvider } from "y-websocket";
const doc = new Y.Doc();
// IndexedDB provider — saves document locally in the browser
// Changes made offline are preserved and synced when reconnected
const indexedDb = new IndexeddbPersistence("my-app-docs", doc);
indexedDb.on("synced", () => {
console.log("Local data loaded from IndexedDB");
});
// WebSocket provider — syncs with other clients when online
const wsProvider = new WebsocketProvider("wss://yjs.example.com", "doc-123", doc);
// The two providers work together:
// 1. Online: changes sync via WebSocket AND save to IndexedDB
// 2. Offline: changes save to IndexedDB only
// 3. Reconnect: IndexedDB state syncs with server, merging all changes
// Observe document changes (from any source: local, remote, or loaded from DB)
doc.on("update", (update: Uint8Array, origin: any) => {
if (origin === "local") {
console.log("Local change");
} else {
console.log("Remote change received");
}
});
Installation
# Core library
npm install yjs
# Providers (pick what you need)
npm install y-websocket # WebSocket sync
npm install y-indexeddb # Browser offline persistence
npm install y-webrtc # Peer-to-peer sync (no server)
# Editor bindings
npm install @tiptap/extension-collaboration @tiptap/extension-collaboration-cursor
# Server persistence
npm install y-mongodb-provider # MongoDB
npm install y-leveldb # LevelDB (lightweight)
Examples
Example 1: Setting up Yjs with a custom configuration
User request:
I just installed Yjs. Help me configure it for my TypeScript + React workflow with my preferred keybindings.
The agent creates the configuration file with TypeScript-aware settings, configures relevant plugins/extensions for React development, sets up keyboard shortcuts matching the user's preferences, and verifies the setup works correctly.
Example 2: Extending Yjs with custom functionality
User request:
I want to add a custom websocket provider to Yjs. How do I build one?
The agent scaffolds the extension/plugin project, implements the core functionality following Yjs's API patterns, adds configuration options, and provides testing instructions to verify it works end-to-end.
Guidelines
- Choose the right shared type — Y.Text for documents, Y.Map for settings/state, Y.Array for lists; don't force everything into one type
- Keep documents small — Large Y.Docs (>10MB) impact performance; split content into multiple documents
- Use awareness for ephemeral data — Cursors, selections, and typing indicators belong in awareness, not document state
- Always add offline persistence — y-indexeddb prevents data loss on disconnect; it's one line of code
- Authenticate at the provider level — Validate tokens in the WebSocket server before allowing sync
- Batch observations — Use
doc.transact()to group multiple changes into one update event - Garbage collect — Call
doc.gc = trueto enable garbage collection of deleted content - Test concurrent edits — Open multiple browser tabs and edit simultaneously; verify merge behavior