Bindings

To provide a more familiar experience when mocking third-party WebSocket clients, MSW uses bindings. A binding is a wrapper over the standard WebSocket class that encapsulates the third-party-specific behaviors, such as message parsing, and gives you a public API similar to that of the bound third-party library.

For example, here’s how to handle Socket.IO communication using MSW and a designated SocketIO binding:

import { ws } from 'msw'
import { toSocketIo } from '@mswjs/socket.io-binding'
 
const chat = ws.link('wss://chat.example.com')
 
export const handlers = [
  chat.addEventListener('connection', (connection) => {
    const io = toSocketIo(connection)
 
    io.client.on('hello', (username) => {
      io.client.emit('message', `hello, ${username}!`)
    })
  }),
]

Note that binding is not meant to cover all the public APIs of the respective third-party library. Unless the binding is shipped by that library, maintaining full compatibility is not feasible.