We can check internet connection status in JavaScript by using below code.

<!-- JavaScript code -->

<script type="text/javascript">
    window.addEventListener('online',  updateOnlineStatusFunction);
    window.addEventListener('offline', updateOnlineStatusFunction);

    function updateOnlineStatusFunction(event) {
        var currentInternetConnectionStatus = navigator.onLine ? "online" : "offline";
        var color = '#00ff00';

        if(currentInternetConnectionStatus == 'offline') {
            color = '#ff0000';
        }

        $('#internet-connection-status').css('color', color).html('You are ' + currentInternetConnectionStatus + ' now.');
    }
</script>

<!-- HTML Code -->

<div class="row">
    <div class="col-lg-12">
        <p>Please try to disconnect internet.</p>
        <p style="margin-top: 25px; font-size: 50px;font-weight: 600; color: #00ff00;" id="internet-connection-status">You are online now</p>
    </div>
</div>

Result: