Master Reliable Network Change Detection in JavaScript
Master Reliable Network Change Detection in JavaScript

Online and Offline Events Not Firing in JavaScript – Why and How to Fix It

JavaScript online/offline events unreliable? Learn why they don't always fire and how to reliably detect network changes.6 min


When building web apps, supporting offline mode is crucial. JavaScript provides handy browser events like online and offline for detecting changes in network connectivity.

You might’ve followed MDN’s example perfectly, yet run into a situation where these events just refuse to fire. It can be perplexing, especially if you’ve double (or triple!) checked your implementation.

Let’s break down why these online and offline events sometimes don’t fire as you’d expect—and how to reliably fix the issue.

Why Aren’t Online and Offline Events Triggering?

At first glance, adding these events seems straightforward—simply attaching event listeners like this should work seamlessly:

window.addEventListener('online', () => {
    console.log('Now online!');
});

window.addEventListener('offline', () => {
    console.log('Now offline!');
});

But here’s the catch: even if your event listeners appear correct, these events may remain mysteriously quiet during real-world testing. Let’s say you disconnect from Wi-Fi or unplug your LAN cable—your browser might just stare blankly back at you without firing those events.

Confusing, right?

In reality, browsers rely on the operating system or specific APIs to inform them about network states. Hence, there’s some inconsistency across implementations. You might have faced one of these scenarios:

  • Different browsers giving different results: Chrome may catch the events in one mode while Firefox might not respond at all.
  • Different ways of simulating offline: Disconnecting Wi-Fi or unplugging Ethernet cable doesn’t always trigger the events. Sometimes events fire only if you explicitly set offline mode inside browser developer tools.

Sound familiar?

Testing Across Different Browsers and Methods

If you’ve tested your events across several browsers and disconnected methods, you might’ve noticed some patterns:

  • Chrome and Safari: Typically, these behave similarly, where disconnecting your internet physically (by unplugging Ethernet cable or turning off Wi-Fi) might not trigger events reliably.
  • Firefox: Firefox is sometimes more responsive to physical network disconnections but also inconsistent at times.
  • Browser Developer Tools: Ironically, toggling the ‘offline’ checkbox in Chrome’s DevTools usually works as expected. But in real-world scenarios, users won’t have access to DevTools, so that’s not a practical approach for detecting offline scenarios.

The inconsistency stems from browser implementation details. Browsers rely on system-level APIs, network interfaces, and DNS resolutions—meaning, just because your network slows down—or your router drops connection temporarily—doesn’t guarantee the browser recognizes this as an “offline event.”

A Reliable Workaround: Checking with Fetch API

Although the built-in online/offline events offer convenience, we’ve established they’re not 100% reliable. A popular workaround is to periodically check network connectivity using JavaScript’s native Fetch API.

Here’s an example snippet you can use:

async function checkRealOffline(url) {
    try {
        const response = await fetch(url, { cache: "no-cache" });
        return response.ok; 
    } catch (error) {
        return false; 
    }
}

// Usage example:
setInterval(async () => {
    const online = await checkRealOffline('/ping.txt');
    if (!online) {
        console.log('You are offline!');
    } else {
        console.log('You are connected!');
    }
}, 5000); // Checks every 5 seconds

In this solution, we create a simple, lightweight file like ping.txt hosted on your server. The fetch method tries constantly reaching this endpoint for confirmation, ensuring true network availability.

But, Why Exactly are Online and Offline Events Unreliable?

These built-in browser events rely on indicators provided by your operating system or underlying network context. If the operating system doesn’t explicitly tell browsers that the internet is down, browsers will assume they’re still connected.

Think of it like your phone signal bars. Your device might show full bars because one tower is reachable, even though your mobile data isn’t truly functional because of network outages beyond your local tower.

Similarly, your browser may see your network is technically up (you’re connected to Wi-Fi), but your router’s actual internet service is down. Browsers can’t always distinguish between “connected to local network without internet” and truly offline scenarios.

Alternative Approaches to Detecting Offline Status

Aside from periodic fetch requests, other practical solutions include:

  • Heartbeat requests: Such as periodically querying low-resource endpoints (like we did above).
  • Service workers: Using service workers, you can intercept requests, cache them, and handle offline-first experiences more robustly.
  • Navigator.onLine Property: The navigator.onLine property is another indicator but is equally unreliable. Combining this property with fetch or service workers is a common strategy.

While services workers are advanced, and perhaps warrant their own article on JavaScript techniques, they’re powerful tools. For beginners, tackling the fetch API method might be more comfortable.

Exploring Further and Experimenting Yourself

To ensure robust application behavior, you’ll probably want to layer strategies. For instance, use the navigator events alongside periodic fetch checks.

Whenever possible, inform users carefully: “The connection looks unstable” instead of bluntly stating “You’re offline”. Consider network states might fluctuate, and offering gentle messaging is user-friendly.

If You Want to Dive Deeper…

You can further explore discussions on sites like Stack Overflow threads. Giving yourself a broader view helps you craft more robust app logic handling network connectivity gracefully.

To sum it up, if browser-provided online/offline events aren’t reliable in your testing scenarios, it’s likely due to browser inconsistencies, OS-level issues, or DNS behaviors. Switching to manual connectivity checks using fetch requests or service workers provides a reliable fallback.

Have you encountered other interesting online/offline quirks? Or perhaps you know alternative clever tricks—feel free to share your insights in the comments below.

Thanks for exploring this common yet tricky JavaScript scenario with me! Understanding these quirks helps you build a smoother offline experience, making your app more user-friendly and resilient.


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Shivateja Keerthi
Hey there! I'm Shivateja Keerthi, a full-stack developer who loves diving deep into code, fixing tricky bugs, and figuring out why things break. I mainly work with JavaScript and Python, and I enjoy sharing everything I learn - especially about debugging, troubleshooting errors, and making development smoother. If you've ever struggled with weird bugs or just want to get better at coding, you're in the right place. Through my blog, I share tips, solutions, and insights to help you code smarter and debug faster. Let’s make coding less frustrating and more fun! My LinkedIn Follow Me on X

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