Navigator: onLine property

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, switching the browser to offline mode sends a false value. Until Firefox 41, all other conditions returned 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.

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

window.addEventListener("offline", (e) => {
  console.log("offline");
});

window.addEventListener("online", (e) => {
  console.log("online");
});