The navigator.onLine property returns a Boolean for whether the browser is connected to some network (though not necessarily the internet). The online and offline events fire when the connection state changes.

Learning resources on MDN

• Navigator: onLine property
• Window: offline event
• Window: online event
• WorkerGlobalScope: offline event
• WorkerGlobalScope: online event
• WorkerNavigator: onLine property

Browser support (view on caniuse.com)

• Chrome 2 Released on 2009-05-21
• Chrome Android 18 Released on 2012-06-27
• Edge 12 Released on 2015-07-29
• Firefox 1.5 Released on 2005-11-29
• Firefox for Android 4 Released on 2011-03-29
• Safari 4 Released on 2009-06-08
• Safari on iOS 3.2 Released on 2010-04-03

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