ajax responseText'undefined'

i'm using ajax in my codeigniter project but whenever i send data i get error and the response will be 'Undefined'. i built the simplest code i've ever imagine but nothing works. would you please tell me where is the wrong ??

<script type="text/javascript">
    $(document).ready(function() {  
        var content = "test";
        var datastring = 'content='+ content;
        jQuery.ajax({
            type:"POST",
            url:"<?php echo site_url('test/test'); ?>",
            data: datastring, 
            dataType: "html",
            async: false,
            success: function(data) { alert("succsess"); },
            error: function(ts) { alert(ts.responseText); },
            onComplete: function(data) {alert(data); }
        });
    });
</script>

and my codeigniter code is :

function test() {
    echo $this->input->post('content');
}

ErrorText is the last parameter of error callback.

error: function(xhr, status, errorText) {
  alert(errorText);
}

You should use complete in the place of oncomplete. The re-written code will be

<script type="text/javascript">
    $(document).ready(function() {  
        var content = "test";
        var datastring = 'content='+ content;
        jQuery.ajax({
            type:"POST",
            url:"<?php echo site_url('test/test'); ?>",
            data: datastring, 
            dataType: "html",
            async: false,
            success: function(data) { alert("succsess"); },
            error: function(ts) { alert(ts.responseText); },
            complete: function(data) {alert(data); }
        });
    });
</script>