index.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Minimal working example</title>
</head>

<body>
    <ul id="events"></ul>

    <script src="/socket.io/socket.io.js"></script>
    <script>
        const $events = document.getElementById('events');

        const newItem = (content) => {
            const item = document.createElement('li');
            item.innerText = content;
            return item;
        };

        const socket = io();

        socket.on('connect', () => {
            $events.appendChild(newItem('connect'));
        });

        let counter = 0;
        setInterval(() => {
            ++counter;
            socket.emit('hey', {
                counter
            }); // the object will be serialized for you
        }, 1000);
    </script>
</body>

</html>