Navigator.onLine

Returns the online status of the browser. The property returns a boolean value, with true meaning online and false meaning offline. The property sends updates whenever the browser's ability to connect to the network changes. The update occurs when the user follows links or when a script requests a remote page. For example, the property should return false when users click links soon after they lose internet connection.

Browsers implement this property differently.

In Chrome and Safari, if the browser is not able to connect to a local area network (LAN) or a router, it is offline; all other conditions return true. So while you can assume that the browser is offline when it returns a false value, you cannot assume that a true value necessarily means that the browser can access the internet. You could be getting false positives, such as in cases where the computer is running a virtualization software that has virtual ethernet adapters that are always "connected." Therefore, if you really want to determine the online status of the browser, you should develop additional means for checking.

In Firefox and Internet Explorer, switching the browser to offline mode sends a false value. Until Firefox 41, all other conditions return a true value; testing actual behavior on Nightly 68 on Windows shows that it only looks for LAN connection like Chrome and Safari giving false positives.

You can see changes in the network state by listening to the online and offline events.

A boolean.

To check if you are online, query window.navigator.onLine, as in the following example:

if (navigator.onLine) {
  console.log('online');
} else {
  console.log('offline');
}

If the browser doesn't support navigator.onLine the above example will always come out as false/undefined.

To see changes in the network state, use addEventListener to listen for the events on window.online and window.offline, as in the following example:

window.addEventListener('offline', function(e) { console.log('offline'); });

window.addEventListener('online', function(e) { console.log('online'); });

See Online/Offline Events for a more detailed description of this property as well as new offline-related features introduced in Firefox 3.