How to display error returned by $.ajax call?

Question

I have a code that takes forever to load, and finally when I put error handler it shows alert, but I need to know what error it returned? How can I know?

EDIT: I get requested url not found, but I am certain the url: is a valid URL on my host, what could be wrong? I can even access it directly in browser.

// process logging in a user from sidebar
$("#login-form").submit(function(event) {
    $('input:submit').attr("disabled", true);
    $("p.form-result").empty();
    $('p.form-submit').after('<p class="loading"><img src="<?php bloginfo('template_directory'); ?>/img/loading.gif" alt="" /></p>');
    $.ajax({
        url: '<?php bloginfo('template_directory'); ?>/ajax/login.php',
        type: 'POST',
        data: $(this).serialize(),
        dataType: 'json',
        success: function(data){
            $('.loading').remove();
            $('input:submit').attr("disabled", false);
            if (data.status) {
                // success
                $("p.form-result").html('<span class="success">' + data.message + '</span>');
                window.setTimeout(function(){location.reload()},3000);
            } else {
                // error
                $("p.form-result").html('<span class="error">' + data.message + '</span>');
            }
        },
        error: function(data){
            alert('error');
        }
    });
    return false;
});

Answer 1

The error event of the jQuery function $.ajax receives 3 arguments

error : function(jqXHR, textStatus, errorThrown){

}

This is the jQuery documentation for this event :

A function to be called if the request fails. The function receives three arguments: The jqXHR (in jQuery 1.4.x, XMLHttpRequest) object, a string describing the type of error that occurred and an optional exception object, if one occurred. Possible values for the second argument (besides null) are "timeout", "error", "abort", and "parsererror". When an HTTP error occurs, errorThrown receives the textual portion of the HTTP status, such as "Not Found" or "Internal Server Error." As of jQuery 1.5, the error setting can accept an array of functions. Each function will be called in turn. Note: This handler is not called for cross-domain script and JSONP requests. This is an Ajax Event.

You'll we be able to know what error is with the parameter textStatus

Answer 2

Error event in ajax call get executed when ajax call has some invalid arguments in it. Following function will help you to understand error code by throwing an error and displaying error definition in alert.

error: function(xhr, ajaxOptions, thrownError){
                    alert(xhr.status);
                },