Once and for all, what does navigator.onLine do?

Question

I've seen a thousand posts where someone is asking how to detect internet connectivity in javascript and there is always one person who says to use navigator.onLine and another that says it is unreliable and only tells you if a connection is available, not if you're connected.

Bottom line, will navigator.onLine tell me if the user is unable to contact there server due to poor coverage even if 3G/4G and WiFi are turned on and they are not in airplane mode or anything? Just your average dead zone.

Answer 1

From the MDN article about navigator.onLine:

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. All other conditions return a true value