✅ How to Mock socket.io in Jest Like a Senior Developer

If you're writing unit tests in a Node.js application that uses socket.io, you've probably hit the "how do I mock this real-time madness?" wall. Fret not—this article is your guide to mocking socket.io with Jest like a pro.

🎯 Why Mock socket.io?

socket.io is great for real-time communication, but when it comes to testing, you don’t want actual WebSocket connections firing up during your test suite. That’s where mocking comes in:

• ✅ Avoid real network calls
• ✅ Speed up your tests
• ✅ Isolate logic for unit testing

Let’s break down how to mock socket.io on both client and server side.

📦 Prerequisites

Make sure you have the following installed:

npm install --save-dev jest
npm install socket.io

🧪 Scenario: Mocking socket.io Server in Jest

Imagine you have a server that looks like this:

// server.js
const { Server } = require('socket.io');
const http = require('http');

const server = http.createServer();
const io = new Server(server);

io.on('connection', (socket) => {
  console.log('A user connected');
  socket.on('ping', () => {
    socket.emit('pong');
  });
});

module.exports = server;

Now you want to write unit tests that mock socket.io.Server.

🧰 How to Mock socket.io.Server with Jest

Step 1: Mock the Module

// __mocks__/socket.io.js
const mSocket = {
  on: jest.fn(),
  emit: jest.fn(),
};

const mServer = {
  on: jest.fn(),
};

const Server = jest.fn(() => mServer);

module.exports = {
  Server,
  __mocks__: {
    mServer,
    mSocket,
  },
};

Step 2: Tell Jest to Use the Mock

jest.mock('socket.io');

Step 3: Write Your Test

// server.test.js
const http = require('http');
const { Server, __mocks__ } = require('socket.io');
const server = require('./server');

describe('Socket.IO Server', () => {
  it('should initialize the socket server and listen to connections', () => {
    expect(Server).toHaveBeenCalledWith(expect.any(http.Server));
    expect(__mocks__.mServer.on).toHaveBeenCalledWith('connection', expect.any(Function));
  });
});

Boom 💥 — you've now mocked socket.io.Server and verified that your server code registers connection listeners.

🧑‍💻 Bonus: Mocking socket.io-client in Jest

If you’re testing client-side code with socket.io-client, follow the same pattern:

// __mocks__/socket.io-client.js
const mSocket = {
  on: jest.fn(),
  emit: jest.fn(),
};

const io = jest.fn(() => mSocket);

module.exports = io;
module.exports.__mocks__ = { mSocket };

Then in your test:

jest.mock('socket.io-client');

const io = require('socket.io-client');
const { __mocks__ } = require('socket.io-client');

test('Client should emit event', () => {
  const socket = io();
  socket.emit('ping');
  expect(__mocks__.mSocket.emit).toHaveBeenCalledWith('ping');
});

🧠 Pro Tips

• For full control, consider using EventEmitter in your mock to simulate events.
• Separate integration tests from unit tests—don’t mix mocks with real sockets.
• Use socket.io-client's io() mock to simulate connections in frontend tests.

📚 TL;DR

🔚 Conclusion

Mocking socket.io in Jest doesn’t have to be rocket science. With the right setup, you can test your real-time code without spinning up a single WebSocket. Mock smart, test fast 🚀.

Looking for more Node.js testing tips? Bookmark this blog and stay tuned!