Navigator.onLine

The onLine property of the Navigator interface returns whether the device is connected to the network, with true meaning online and false meaning offline. The property’s value changes after the browser checks its network connection, usually 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. When its value changes, an online or offline event is fired on the window.

Browsers and operating systems leverage different heuristics to determine whether the device is online. In general, connection to LAN is considered online, even though the LAN may not have Internet access. For example, the computer may be running a virtualization software that has virtual ethernet adapters that are always “connected”. On Windows, the online status is determined by whether it can reach a Microsoft home server, which may be blocked by firewalls or VPNs, even if the computer has Internet access. Therefore, this property is inherently unreliable, and you should not disable features based on the online status, only provide hints when the user may seem offline.