JavaScript HTML DOM Events: online and offline

The online and offline events trigger when the browser's network connection status changes (connecting or disconnecting).
EXAMPLE
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>online – Real and Simulated Detection</title>
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
    
    <style>
        body {
            font-family: Arial, sans-serif;
            padding: 20px;
        }
        .box {
            max-width: 100%;
            margin: 30px auto;
            border: 1px solid #e5e7eb;
            border-radius: 8px;
            padding: 16px;
        }
        h2 {
            margin-top: 0;
        }
        button {
            display: inline-block;
            margin: 4px 4px 4px 0;
            padding: 8px 16px;
            border: 1px solid #ccc;
            border-radius: 6px;
            background: #60a5fa;
            color: #fff;
            font-size: 1rem;
            cursor: pointer;
        }
        button:hover {
            background: #2563eb;
        }
        button#goOffline {
            background: #ef4444;
        }
        button#goOffline:hover {
            background: #dc2626;
        }
        .log {
            margin-top: 12px;
            padding: 8px;
            border: 1px solid #e5e7eb;
            max-height: 300px;
            overflow: auto;
            font-family: monospace;
            font-size: 13px;
            background: #f3f4f6;
            color: #333;
        }
        .status {
            margin-top: 12px;
            font-size: 16px;
            padding: 10px;
            border-radius: 4px;
            background: #eee;
            color: #333;
        }
    </style>
</head>
<body>
    <div class="box">
        <h2>The <code>online</code> and <code>offline</code> Events</h2>
        <p>This example detects connection status in two ways: via browser events and via active server 
           checking.</p>

        <button id="goOffline">Simulate Offline</button>
        <button id="goOnline">Simulate Online</button>

        <div class="status" id="statusBox">
            Current status: <strong id="state">checking...</strong>
        </div>
        
        <div class="log" id="log"></div>
    </div>

    <script>
        const logEl = document.getElementById("log");
        const stateEl = document.getElementById("state");
        const statusBox = document.getElementById("statusBox");

        // Keep track of the last known state to avoid spamming the log
        let isCurrentlyOnline = navigator.onLine;

        function log(msg){
            const line = document.createElement("div");
            line.textContent = `${new Date().toLocaleTimeString()} – ${msg}`;
            logEl.appendChild(line);
            logEl.scrollTop = logEl.scrollHeight;
        }

        function updateUI(isOnline){
            stateEl.textContent = isOnline ? "ONLINE (Connected)" : "OFFLINE (No Connection)";
            stateEl.style.color = isOnline ? "green" : "red";
            statusBox.style.border = isOnline ? "2px solid green" : "2px solid red";
            
            isCurrentlyOnline = isOnline;
        }

        updateUI(navigator.onLine);
        log(`Start: Browser says you are ${navigator.onLine ? "online" : "offline"}.`);

        // 1. Browser Events
        window.addEventListener("online", () => {
            log("EVENT: Browser reports 'online'.");
            updateUI(true);
        });

        window.addEventListener("offline", () => {
            log("EVENT: Browser reports 'offline'.");
            updateUI(false);
        });

        // 2. Simulation
        document.getElementById("goOffline").addEventListener("click", () => {
            window.dispatchEvent(new Event("offline"));
            log("SIMULATION: You clicked 'Offline'.");
        });

        document.getElementById("goOnline").addEventListener("click", () => {
            window.dispatchEvent(new Event("online"));
            log("SIMULATION: You clicked 'Online'.");
        });

        // 3. Active Checking (Connection Check)
        async function checkInternet(){
            try {
                // Random parameter prevents caching
                const url = "https://www.google.com/favicon.ico?t=" + new Date().getTime();
                await fetch(url, { method: "HEAD", mode: "no-cors" });
                
                // If we are here, internet is working.
                // If our system thought we were offline, update to online.
                if (!isCurrentlyOnline) {
                    log("CHECK: Internet is working again! (Detected via connection)");
                    updateUI(true);
                }

            } catch (e) {
                // If we are here, internet is NOT working.
                // If our system thought we were online, update to offline.
                if (isCurrentlyOnline) {
                    log("CHECK: No connection to server! (Detected via connection)");
                    updateUI(false);
                }
            }
        }

        // Check every 5 seconds
        setInterval(checkInternet, 5000);
    </script>
</body>
</html>
RESULT