How do you test WebSocket applications?

WebSocket testing operates at multiple levels: (1) Unit testing server handlers — test event handler functions in isolation by mocking the socket object; (2) Integration testing — use the ws library as a test client to send messages and assert responses: const client = new WebSocket('ws://localhost:8080'); client.on('message', msg => expect(JSON.parse(msg)).toEqual(expected)); (3) Socket.IO testing — use socket.io-client in Jest tests with the server running on a dynamic port; (4) Load testing — tools like Artillery (ws engine), k6 (native WebSocket support), and Gatling simulate thousands of concurrent WebSocket connections; (5) Manual testing — tools like Postman, Insomnia, and browser DevTools (Network tab → WS filter) allow manual WebSocket inspection; (6) End-to-end — Playwright and Cypress support WebSocket interception for E2E tests.