--- name: yjs-93546fd52af0 description: >- Yjs is a high-performance CRDT (Conflict-free Replicated Data Type) framework for building collaborative applications: shared types whose concurrent edits merge automatically. Use when a user asks to add real-time collaborative editing, shared cursors and presence, offline-first sync, or peer-to-peer collaboration to an app, to run or secure a y-websocket server, to persist Yjs documents, or to make a Tiptap or ProseMirror editor collaborative. license: Apache-2.0 compatibility: "Yjs 13.x (14 is still in pre-release). Runs in browsers and Node.js; y-websocket 3 client, @y/websocket-server for the server, Tiptap 3 for the editor example" metadata: author: terminal-skills version: 1.1.0 category: development tags: "crdt, real-time, collaboration, text-editing, offline-first" repository: https://github.com/yjs/yjs --- # Yjs — CRDT Framework for Collaborative Editing ## Overview Yjs is a CRDT framework for collaborative applications. Data lives in shared types (`Y.Text`, `Y.Map`, `Y.Array`, `Y.XmlFragment`) inside a `Y.Doc`; every client edits its own copy and the copies converge without a central authority deciding conflicts. Providers move the updates around: `y-websocket` through a server, `y-webrtc` peer to peer, `y-indexeddb` to the browser's disk for offline work. ## Instructions ### Install ```bash npm install yjs # core library (13.x) npm install y-websocket # WebSocket client provider (3.x) npm install y-indexeddb # browser offline persistence npm install y-webrtc # peer-to-peer sync, needs a signaling server # Server (it left the y-websocket package in v3) npm install @y/websocket-server@0.1.1 ws npm install y-mongodb-provider # MongoDB persistence (community package); y-leveldb for LevelDB # Tiptap 3 editor binding npm install @tiptap/react @tiptap/starter-kit @tiptap/extension-collaboration @tiptap/extension-collaboration-caret @tiptap/y-tiptap ``` The current `@y/websocket-server` (0.1.5) depends on the Yjs 14 pre-release. Loaded next to Yjs 13 it prints "Yjs was already imported" and persistence code fails, so stay on 0.1.1, the last release with a Yjs 13 peer dependency, while the app uses Yjs 13. ### Document and Shared Types Create collaborative data structures that merge automatically: ```typescript // 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; every connected client holds a copy const doc = new Y.Doc(); // Y.Text — collaborative text (rich text via yText.format and deltas) const yText = doc.getText("document-content"); yText.insert(0, "Hello, "); yText.insert(7, "world!"); // "Hello, world!" — concurrent inserts are both kept // Y.Map — collaborative key-value store const yMap = doc.getMap("settings"); yMap.set("theme", "dark"); // Different keys set concurrently: both applied. Same key: one write wins on every client. // Y.Array — collaborative ordered list const yArray = doc.getArray("tasks"); yArray.push([{ id: "1", title: "Design mockup", done: false }]); // Y.XmlFragment — XML tree used by editor bindings (Tiptap, ProseMirror) const yXml = doc.getXmlFragment("rich-content"); // Shared types nest: a map inside a map const project = new Y.Map(); project.set("status", "active"); yMap.set("project", project); // Group changes into one update; the second argument is the transaction origin doc.transact(() => { yMap.set("fontSize", 14); yMap.set("lineHeight", 1.5); }, "settings-form"); // React to changes yMap.observe((event) => console.log("keys changed:", [...event.keysChanged])); // Undo/redo for one user: undoManager.undo() reverts only local edits to yText const undoManager = new Y.UndoManager(yText); ``` ### WebSocket Provider Connect clients through a WebSocket server for real-time sync: ```typescript // src/collaboration/provider.ts — WebSocket-based real-time sync import * as Y from "yjs"; import { WebsocketProvider } from "y-websocket"; const doc = new Y.Doc(); // All clients that join the same room on the same server sync automatically const provider = new WebsocketProvider("ws://localhost:1234", "handbook-onboarding", doc, { params: { token: sessionToken }, // your app's session token, sent as ?token=... on every (re)connect }); // Awareness — presence data (cursors, selections, user info); never persisted const awareness = provider.awareness; awareness.setLocalStateField("user", { name: "Alice", color: "#ff5733" }); awareness.on("change", () => { awareness.getStates().forEach((state, clientId) => { if (clientId !== doc.clientID) console.log(`${state.user?.name} is here`); }); }); provider.on("status", ({ status }) => console.log(status)); // "connecting" | "connected" | "disconnected" provider.on("sync", (isSynced: boolean) => isSynced && console.log("Synced with server")); // Fired when the server closed the connection with a 4400–4499 code: no more reconnects provider.on("closed", ({ code, reason }) => console.log(`Rejected: ${code} ${reason}`)); ``` Node.js 22 and later have a global `WebSocket`; on older versions pass `WebSocketPolyfill: require("ws")` in the options. ### Server-Side Setup For development, `HOST=localhost PORT=1234 npx y-websocket` (from `@y/websocket-server`) starts an in-memory server; with 0.1.1 and `y-leveldb` installed, `YPERSISTENCE=./yjs-data` makes it persist to LevelDB. For a database of your choice, write the server yourself: ```javascript // server/yjs-server.mjs — WebSocket server with MongoDB persistence import http from "node:http"; import { WebSocketServer } from "ws"; import * as Y from "yjs"; import { setupWSConnection, setPersistence } from "@y/websocket-server/utils"; import { MongodbPersistence } from "y-mongodb-provider"; const mdb = new MongodbPersistence(process.env.MONGODB_URL, { collectionName: "yjs-documents", flushSize: 100, // merge stored updates into one record after 100 of them }); setPersistence({ provider: mdb, bindState: async (docName, ydoc) => { // Called once when the first client opens a document: load it, then store every update const persisted = await mdb.getYDoc(docName); await mdb.storeUpdate(docName, Y.encodeStateAsUpdate(ydoc)); Y.applyUpdate(ydoc, Y.encodeStateAsUpdate(persisted)); ydoc.on("update", (update) => mdb.storeUpdate(docName, update)); }, // Called when the last client disconnects writeState: async (docName) => { await mdb.flushDocument(docName); }, }); const server = http.createServer((_req, res) => { res.writeHead(200); res.end("okay"); }); const wss = new WebSocketServer({ server }); wss.on("connection", setupWSConnection); // room name = URL path server.listen(Number(process.env.PORT ?? 1234), process.env.HOST ?? "127.0.0.1"); ``` ### Editor Integration (Tiptap) Add collaborative editing to a Tiptap 3 rich text editor: ```tsx // src/components/CollaborativeEditor.tsx — Tiptap with Yjs collaboration import { useEffect, useState } from "react"; import { useEditor, EditorContent } from "@tiptap/react"; import StarterKit from "@tiptap/starter-kit"; import Collaboration from "@tiptap/extension-collaboration"; import CollaborationCaret from "@tiptap/extension-collaboration-caret"; import * as Y from "yjs"; import { WebsocketProvider } from "y-websocket"; type Collab = { doc: Y.Doc; provider: WebsocketProvider }; interface Props { documentId: string; userName: string; userColor: string } export function CollaborativeEditor({ documentId, userName, userColor }: Props) { const [collab, setCollab] = useState(null); // Create and destroy doc and provider in the same effect (safe under StrictMode) useEffect(() => { const doc = new Y.Doc(); const provider = new WebsocketProvider("ws://localhost:1234", documentId, doc); setCollab({ doc, provider }); return () => { provider.destroy(); doc.destroy(); }; }, [documentId]); return collab ? : null; } function Editor({ collab, user }: { collab: Collab; user: { name: string; color: string } }) { const editor = useEditor({ extensions: [ StarterKit.configure({ undoRedo: false }), // Collaboration brings its own undo/redo Collaboration.configure({ document: collab.doc }), CollaborationCaret.configure({ provider: collab.provider, user }), // carets via awareness ], }); return ; } ``` Tiptap 2 used `@tiptap/extension-collaboration-cursor` (`CollaborationCursor`) and `StarterKit.configure({ history: false })`; both were renamed in v3. ### Offline Support and Sync Handle offline editing with automatic merge on reconnect: ```typescript // 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(); // Saves the document in the browser; use the room name as the database name const indexedDb = new IndexeddbPersistence("handbook-onboarding", doc); indexedDb.on("synced", () => console.log("Local data loaded from IndexedDB")); // Syncs with other clients when online. Offline edits stay in IndexedDB and merge on reconnect const wsProvider = new WebsocketProvider("ws://localhost:1234", "handbook-onboarding", doc); // origin tells where an update came from: null for local edits (or the value passed to // doc.transact), the provider instance for updates it applied doc.on("update", (update: Uint8Array, origin: unknown) => { console.log(origin === wsProvider ? "Remote change received" : "Local change"); }); ``` ## Examples ### Example 1: Merge two copies that were edited offline **User request:** "Two devices changed the same task list while offline. Show me that Yjs merges them and how little has to be sent." ```javascript // merge.mjs import * as Y from "yjs"; const laptop = new Y.Doc(); const phone = new Y.Doc(); laptop.getArray("tasks").push([{ id: "t1", title: "Design mockup" }]); Y.applyUpdate(phone, Y.encodeStateAsUpdate(laptop)); // both start from the same state // Offline: each device edits its own copy laptop.getArray("tasks").push([{ id: "t2", title: "Implement API" }]); phone.getArray("tasks").insert(0, [{ id: "t3", title: "Write tests" }]); // Back online: each side sends only what the other is missing const forPhone = Y.encodeStateAsUpdate(laptop, Y.encodeStateVector(phone)); const forLaptop = Y.encodeStateAsUpdate(phone, Y.encodeStateVector(laptop)); Y.applyUpdate(phone, forPhone); Y.applyUpdate(laptop, forLaptop); console.log(laptop.getArray("tasks").toJSON().map((t) => t.id)); console.log(phone.getArray("tasks").toJSON().map((t) => t.id)); console.log(forPhone.byteLength, "bytes sent to the phone"); ``` `node merge.mjs` prints the same order on both devices (the byte count varies by a byte or two between runs, because client IDs are random): ``` [ 't3', 't1', 't2' ] [ 't3', 't1', 't2' ] 47 bytes sent to the phone ``` ### Example 2: Reject clients whose token is no longer valid **User request:** "Users with an expired session keep reconnecting to our collaboration server every few seconds. Make the server turn them away for good." Refusing the HTTP upgrade with 401 does not help: browsers hide that status, the provider sees close code 1006 and retries. Accept the socket, then close it with a code in the 4400–4499 range, which y-websocket 3.1 treats as permanent: ```javascript // server/auth-server.mjs import http from "node:http"; import { WebSocketServer } from "ws"; import { setupWSConnection } from "@y/websocket-server/utils"; const server = http.createServer((_req, res) => { res.writeHead(200); res.end("okay"); }); const wss = new WebSocketServer({ noServer: true }); server.on("upgrade", (req, socket, head) => { wss.handleUpgrade(req, socket, head, (ws) => { const token = new URL(req.url, "http://localhost").searchParams.get("token"); if (token !== process.env.COLLAB_TOKEN) return ws.close(4401, "invalid token"); setupWSConnection(ws, req); }); }); server.listen(1234, "127.0.0.1"); ``` A client with a wrong token logs, through the `closed` handler shown above, `Rejected: 4401 invalid token` and stops reconnecting. After refreshing the session, set `provider.params = { token: newToken }` and call `provider.connect()`. Replace the string comparison with real token verification (JWT signature, session lookup). ## Guidelines 1. **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 2. **Keep documents small** — a Y.Doc is loaded and synced as a whole; split independent content into separate documents (one room each) or subdocuments 3. **Use awareness for ephemeral data** — Cursors, selections, and typing indicators belong in awareness, not document state 4. **Add offline persistence in the browser** — y-indexeddb keeps unsynced edits across reloads and disconnects; it's one line of code 5. **Authenticate on the server** — check the token before calling `setupWSConnection`; the room name comes from the URL, so also check that this user may open that document 6. **Batch changes** — Use `doc.transact()` to group multiple changes into one update event 7. **Garbage collection is on by default** — deleted content is compacted; create the doc with `new Y.Doc({ gc: false })` only if you need snapshots or version history 8. **One copy of Yjs** — two versions in one bundle or process break `instanceof` checks ("Yjs was already imported"); check with `npm ls yjs` 9. **`@y/websocket-server` is a starting point** — it keeps documents in memory in one process and does not scale horizontally; the y-websocket README lists Hocuspocus, y-redis and y-sweet as compatible backends 10. **Use `wss://` in production** — tokens in the query string and document updates travel in clear text over `ws://` 11. **Test concurrent edits** — Open multiple browser tabs and edit simultaneously; verify merge behavior --- Source attribution: TerminalSkills Original project: https://github.com/TerminalSkills/skills/blob/3bfbf7f76459d89422eddc0e5f572ad0e6c393c9/skills/yjs/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: TerminalSkills Original project: https://github.com/TerminalSkills/skills/blob/3bfbf7f76459d89422eddc0e5f572ad0e6c393c9/skills/yjs/SKILL.md Attribution basis: Source repository record Fields of work: Design, Development & Engineering, Product Management Roles: Backend Developer, Frontend Developer, Full-Stack Developer, Product Manager, UX Designer Specialties: Backend, Cross Platform, Frontend Tools: Yjs Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: b32c2ee7fd9c620d11213d19509710cc04c104f18f0da6e4baf2a965dab7b73a