Online and Offline Events with JavaScript

In my last post, I wrote about building offline first applications. One thing I did not cover was how to detect when a user is online or offline, so that your application can respond accordingly -- such as when you need to display connectivity status to a user or check whether a user is online before performing an action in your application.

Going all the way back to IE8, JavaScript has provided access to navigator.onLine, which is a simple little API that returns a boolean where true means the user is online, false meaning offline. Unfortunately, though, navigator.onLine is considered an unreliable API. In Chrome and Safari it only determines whether a device has connection to a network, not whether or not that network can actually reach the internet.

However, for some purposes, navigator.onLine may be enough to suit your needs. So let's take a quick look at how to use it.

Basic Detection

Let's say, for example, we want to display a message to our users when they're offline. To implement our status message, we could write:

function updateOnlineStatus() {
  document.getElementById('status').innerText = 'Online';

function updateOfflineStatus() {
  document.getElementById('status').innerText = 'Offline';

window.addEventListener('online',  updateOnlineStatus);
window.addEventListener('offline', updateOfflineStatus);

In the above code, we're just listening for the browser to fire the online or offline events and we're updating the contents of an element with an ID of status to reflect the current state.

Better Detection

If your application needs a more reliable mechanism for detecting user connectivity, there is hope! Enter Offline.js.

Offline.js listens for a request to fail with a network-related error, then triggers a custom offline status event. From there, it begins trying to fetch your /favicon.ico every few seconds until it is able to succesfully retrieve that asset, at which time it will trigger the online event.

Implementing Offline.js is actually super simple. Just include the script, a theme, and one of the language files and you're all done. You also have the option of just leveraging the underlying JS API, just by excluding the theme CSS file.

Further Reading