JestJS - Multiple websocket connections hangs Jest

Question

I'm using this function to test my server that creates as many websocket connections and checks when my game starts. However no matter the timeout I assign, it hangs on JestJS. On the browser - Firefox, Edge Chromium it works perfectly fine.

function checkGameStart(numberOfBots) {
    return new Promise((resolve, reject) => {
        let clients = [];
        let connection = [];
        for (let i = 0; i < numberOfBots; i++) {
            clients.push(new WebSocket('ws://127.0.0.1:8080'));
            connection.push(false);
            clients[i].onmessage = (msg) => {
                let data = JSON.parse(msg.data);
                if (data.title === "gameStarted") {
                    connection[i] = true;
                    checkAllClientsReady();
                }
            }
            clients[i].onerror = (err) => reject(err);
        }
        function checkAllClientsReady() {
            if (!(connection.includes(false))) {
                resolve(true);
                closeAllConnections();
            }
        }
        function closeAllConnections() {
            for (let i = 0; i < clients; i++) {
                clients[i].close()
            }
        }
    });
}

Does anyone know why it happens what I can do to make sure it doesn't happen again.

Test code;

test('Check the game starts', () => {
    return expect(checkGameStart(4)).resolves.toBe(true);
});

user_2075151 · Answer

I refactored your code a little bit and added a WebSocket server using ws NPM package on the test setup:

const { WebSocketServer } = require('ws')

const port = 8080
const wss = new WebSocketServer({ port })

beforeAll(() => {
  wss.on('connection', (ws) => {
    ws.send(JSON.stringify({ title: 'gameStarted' }))
  })
})
afterAll(() => {
  wss.close()
})

async function checkGameStart(numberOfBots) {
  await Promise.all(
    new Array(numberOfBots).fill(null)
      .map(() => new Promise((resolve, reject) => {
        const ws = new WebSocket(`ws://localhost:${port}`)

        ws.onmessage = ({ data }) => {
          const { title } = JSON.parse(data)
          if (title === 'gameStarted') {
            ws.close()
            resolve()
          }
        }

        ws.onerror = (err) => {
          ws.close()
          reject(err)
        }
      }))
  )
  return true
}

test('Check the game starts', async () => {
  await expect(checkGameStart(4)).resolves.toBe(true);
});

$ npx jest
 PASS  ./websocket.test.js
  ✓ Check the game starts (64 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.708 s, estimated 1 s
Ran all test suites.

This only works if Jest is also configured to use a jsdom test environment:

// jest.config.js
module.exports = {
  testEnvironment: "jsdom",
};

Otherwise, the WebSocket constructor will be undefined since it is only available on web browser environments and, by default, Jest runs in node environment.

JestJS - Multiple websocket connections hangs Jest

1 Answers