How to test Websocket Client with Jest and react-testing-library

Question

I'm using create-react-app, Jest and react-testing-library for the configuration of the chatbot project.

I have a React functional component that connects to a WebSocket server and DOM changes according to WebSocket messages, for example

const LiveChat = () => {
  const [socket, setSocket] = useState(null)

   useEffect(() => {
    setSocket(new WebSocket('ws://localhost:1234'))
   }, [])

  useEffect(() => {
    socket && socket.onmessage = message => { handleAgentMessages(message.data) }
  }, [socket])

  const handleAgentMessages = message => {
     const { messageContent, messageType, secureKey } = JSON.parse(message)
     if (messageType === TEXT && messageContent) {
       alert(messageContent)
       playChatMessageSound()
     }
       ...
   }

   return (
      <div className='live-chat' data-testid='live-chat'>
          ...
      </div>
   )
}

I want to test when a TEXT message comes, is alertbox appeared with conteining message etc. I have looked through the internet and I find jest-websocket-mock library, but it seems that I need to mock the client with this library as well, but I just want to mock server and expect the client to connect the mocked WebSocket server, do you have any ideas?

Answer 1

I am not sure if this is the right way or not. But this works for me,

    global.sendMsg = null;
    global.WebSocket = class extends WebSocket {
        constructor(url) {
            super("wss://test");
            global.sendMsg = null
        }
    
        addEventListener(event, cb) {
            if (event === "open") {
                cb();
            } else if(event === "message") {
                global.sendMsg = cb;
            }
        }
    };

    test("testing message",() => {
      render(<LiveChat />);
      global.sendMsg({data:"test-msg"});
 
      expect....
    })

Basically, I overwrote the WebSocket class and stored the message event callback to a constant, and trigger it on the tests to imitate the server message.