Javascript – JQuery Ajax – How to Detect Network Connection error when making Ajax call

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I have some Javascript JQuery code that does an Ajax call to the server every 5 mins, it's to keep the server session alive and keep the user logged in. I'm using $.ajax() method in JQuery. This function seems to have an 'error' property that I'm trying to use in the event that the user's internet connection goes down so that the KeepAlive script continues to run. I'm using the following code:

var keepAliveTimeout = 1000 * 10;

function keepSessionAlive()
{
    $.ajax(
    {
        type: 'GET',
        url: 'http://www.mywebapp.com/keepAlive',
        success: function(data)
        {
            alert('Success');

            setTimeout(function()
            {
                keepSessionAlive();
            }, keepAliveTimeout);
        },
        error: function(XMLHttpRequest, textStatus, errorThrown)
        {
            alert('Failure');

            setTimeout(function()
            {
                keepSessionAlive();
            }, keepAliveTimeout);
        }
    });
}

When I run it, I'll get 'Success' popup on the screen in an alert box every 10 seconds which is fine. However, as soon as I unplug the network cable, I get nothing, I was expecting the error function to get called and see a 'Failure' alert box, but nothing happens.

Am I correct in assuming that the 'error' function is only for non '200' status codes returned from the server? Is there a way to detect network connection problems when making an Ajax call?

Best Answer

// start snippet
error: function(XMLHttpRequest, textStatus, errorThrown) {
        if (XMLHttpRequest.readyState == 4) {
            // HTTP error (can be checked by XMLHttpRequest.status and XMLHttpRequest.statusText)
        }
        else if (XMLHttpRequest.readyState == 0) {
            // Network error (i.e. connection refused, access denied due to CORS, etc.)
        }
        else {
            // something weird is happening
        }
    }
//end snippet
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