In the header of your page, place this javascript code:

        <script type="text/javascript">

    

        // Creates an object which can read files from the server

        var reader = new XMLHttpRequest();

        

        var checkFor = "fileToCheckFor.html";

        

        // Opens the file and specifies the method (get)

        // Asynchronous is true

        reader.open('get', checkFor, true);

                    

        //check each time the ready state changes

        //to see if the object is ready

        reader.onreadystatechange = checkReadyState;

        

        function checkReadyState() {

        

            if (reader.readyState === 4) {

                

                //check to see whether request for the file failed or succeeded

                if ((reader.status == 200) || (reader.status == 0)) {

                

                //page exists -- redirect to the checked

                //checked for page

                document.location.href = checkFor;

                

                }

                else {

                

                //does nothing and quits the function

                //if the url does not exist

                return;

                

                }

            

            }//end of if (reader.readyState === 4)

        

        }// end of checkReadyState()

        

        // Sends the request for the file data to the server

        // Use null for "get" mode

        reader.send(null);



    </script>

This allows you to check whether or not a page exists on your server and redirect to it if it does. If the page does not exist, the javascript code does nothing and allows the current page to load.

Edit: Fixed bugs and rewrote some of the code for clarity, appearance, and practicality.