I Need a Event to Detect Internet Connect/Disconnect

I need a event to detect Internet connect/disconnect

This is all covered (including the difference between being on the network and having the network connect you to the Internet) at http://msdn.microsoft.com/en-us/library/ee264321(VS.85).aspx. I hope you meant to put that Windows 7 tag on your post, because all this is pretty new.

The key is INetworkListManager.get_IsConnectedToInternet() which pretty much does what it says on the tin. You have to jump around a bit to register for the events etc. The Code Pack wraps some of that up for you and has a network sample you can adapt.

How to sense a network connection and trigger an event

You can register to NetworkChange.NetworkAvailabilityChanged event for receiving events when the availability of the network changes

From MSDN:

The NetworkChange class raises NetworkAvailabilityChanged events when the availability of the network changes. The network is available when at least one network interface is marked "up" and is not a tunnel or loopback interface.

To have a NetworkChange object call an event-handling method when a NetworkAvailabilityChanged event occurs, you must associate the method with a NetworkAvailabilityChangedEventHandler delegate, and add this delegate to this event.

The NetworkAvailabilityChanged event is supported on Windows 2000 and later.

For more information
http://msdn.microsoft.com/en-us/library/system.net.networkinformation.networkchange.networkavailabilitychanged.aspx

Detect internet disconnection via socket

You have to understand heartbeat interval and heartbeat timeout. Earlier is the time when server emits a heartbeat each 25 seconds (by default). And later is timeout for server to keep that specific client in memory. If client receives the heartbeat within heartbeat timeout (60 seconds by default) it tells the server that I'm alive. If client doesn't receive a heartbeat from server within this timeout then server flushes that client out of memory and disconnect event is fired automatically on client side. This timeout is set when client connects to server. So you can set these properties in your Node.js server like this:

var socket = require('socket.io').listen(80,{
  'heartbeat interval': 5,
  'heartbeat timeout' : 10
});

Now server will emit heartbeat every 5 seconds and if client doesn't receive a heartbeat in 10 seconds, disconnect event will be fired. But remember that heartbeat interval should be less than heartbeat timeout. Hope this will work!

Check the internet connection for each and every time of submission in JavaScript

You can use navigator.onLine to check the either there is internet connection or not. You can also use 2 event listener to notify when network connection is connected or disconnected.

console.log(navigator.onLine);
// OR
window.addEventListener('online', () => {   //console.log('Online');});
window.addEventListener('offline', () => {        //console.log('Offline');  });

Detect the Internet connection is offline?

You can determine that the connection is lost by making failed XHR requests.

The standard approach is to retry the request a few times. If it doesn't go through, alert the user to check the connection, and fail gracefully.

Sidenote: To put the entire application in an "offline" state may lead to a lot of error-prone work of handling state.. wireless connections may come and go, etc. So your best bet may be to just fail gracefully, preserve the data, and alert the user.. allowing them to eventually fix the connection problem if there is one, and to continue using your app with a fair amount of forgiveness.

Sidenote: You could check a reliable site like google for connectivity, but this may not be entirely useful as just trying to make your own request, because while Google may be available, your own application may not be, and you're still going to have to handle your own connection problem. Trying to send a ping to google would be a good way to confirm that the internet connection itself is down, so if that information is useful to you, then it might be worth the trouble.

Sidenote: Sending a Ping could be achieved in the same way that you would make any kind of two-way ajax request, but sending a ping to google, in this case, would pose some challenges. First, we'd have the same cross-domain issues that are typically encountered in making Ajax communications. One option is to set up a server-side proxy, wherein we actually ping google (or whatever site), and return the results of the ping to the app. This is a catch-22 because if the internet connection is actually the problem, we won't be able to get to the server, and if the connection problem is only on our own domain, we won't be able to tell the difference. Other cross-domain techniques could be tried, for example, embedding an iframe in your page which points to google.com, and then polling the iframe for success/failure (examine the contents, etc). Embedding an image may not really tell us anything, because we need a useful response from the communication mechanism in order to draw a good conclusion about what's going on. So again, determining the state of the internet connection as a whole may be more trouble than it's worth. You'll have to weight these options out for your specific app.

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