how can I jest test the following Vue method?

    getRealtimeData(conn) {
      this.socket = io.connect(this.connection);
      this.socket.on(conn, fetchedData => {
        this.addData(fetchedData);
      })
    },

I tried to mock the server as in this link https://socket.io/docs/v3/testing/ and I also tried via jest.fn ():

    jest.mock('socket.io-client', () => {
            const mSocket = {
                on: jest.fn(() => mockData),
                connect: jest.fn( )
            };
            return jest.fn(() => mSocket);
        });

but I don't reach the part inside fetchedData, Thx