ajax response error not working as expected - php

Question

My success/error handling is not working as expected. In this example my php runs as intended, however, the intent of the processing failed. Let's say I was looking up a username in the database... if I found the username I would return $ajax_result['success'] = 'success';, but if I did not find the username I would return nothing. The success call works fine, but the error call is not firing.
The error in firebug I am getting is TypeError: response is null and therefore the error alert I have set does not fire.
What is the best way to solve this without actually returning $ajax_result['success'] = 'whatever';
example return from php would be something like this when processing went as expected:
$ajax_result['success'] = 'success'; // add success info
echo json_encode($ajax_result); // return result array to ajax
for 'errors' I am simply returning :
echo json_encode($ajax_result); // which is null in this case
the ajax:
var showInfo = function() {
$('#show-user').on('click', function () {
var $form = $(this).closest('form');
$.ajax({
type: 'post',
url: '/spc_admin/process/p_delete_user_show.php',
data: $form.serialize(),
dataType : 'json'
}).done(function (response) {
if (response.success == 'success') {
alert('success');
}
else
{
alert('error');
}
});
});
}

Answer 1

You can not json_encode a null value, easiest and cleanest way to make this work would probably be to make success a bool and set it to either true or false depending on if it succeeded or not (This is ususaly what most user expects a 'success' or 'result' parameter in response to be, not a string).
//PHP:
header('Content-Type: application/json'); // To let the client know that its json-data.
$ajax_result['success'] = $wasSuccessfull; // true or false
die(json_encode($ajax_result)); // Exit the script with the json response.
//JavaScript:
if (response.success) {
alert('success');
} else {
alert('error');
}

Answer 2

try this one:
if (!response.success) {
alert('error');
}
else
{
alert('success');
}