Simple session timeout check for client side applications.

Client Session Timeout Check

// Non-jquery example from http://stackoverflow.com/questions/333634/http-head-request-in-javascript-ajax

/**
 * The idea here is to poll against a route that requires authentication,
 * in this case /session_health_check, on an interval.
 * The important part is  the header in the response. Because
 * the current session goes with the request, polling against the endpoint
 * can either trigger a refresh on the page which should initiate the auth
 * flow, or you can popup a dialogue that says that you need to 
 * re-authenticate with a ?next= to send the use back to the page.
 * 
 * There are hundreds of ways I've seen this implemented and this is just one
 * quick and dirty way of managing client sessions. You could put the status 
 * check on the ajax calls for updates to client side view model, you can set
 * an initial time against your .Net timeout settings to warn a user their 
 * session is about to expire and refresh with calls to clear the interval,
 * have numerous ways of telling the user or simply refresh the page and rely
 * on the application authentication flow to take care of the rest.
 */

function sessionCheck( callback )
{
    var http = new XMLHttpRequest();
    http.open('HEAD', '/session_health_check');
    http.onreadystatechange = function() {
        if (this.readyState == this.DONE) {
            if (this.status != 200) {
                callback();
            }
        }
    };
    // Set a timeout for if the server doesn't respond at all
    http.timeout = 10000;  // Or whatever is an appropriate response time from the server
    http.ontimeout = function() {
        callback();  // or do stuff
    };
    http.send();
}

/**
 * Inside your init, setup a timeout to check on whatever interval
 * you feel is appropriate. reAuthFunction is where you start your flow,
 * anything from a popup to a window.location.reload()
 */
setInterval( function() { sessionCheck(reAuthFunction) }, 3000 );