How to Check if URL exists using javascript

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>

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