Navigator.onLine - Web APIs 编辑

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. To learn more, see the HTML5 Rocks article, Working Off the Grid.

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 for the events on window.ononline and window.onoffline.

Syntax

online = window.navigator.onLine;

Value

online is a boolean true or false.

Examples

Basic usage

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.

Listening for changes in network status

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'); });

Specifications

Browser compatibility

BCD tables only load in the browser

Notes

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

See also

• HTML5 Rocks: Working Off the Grid With HTML5 Offline
• HTML5 Rocks: "Offline": What does it mean and why should I care?
• Mozilla Blog: Offline Web Applications