Wordpress ajax returns 0 only - php

Question

I am using datatables in which I've implemented a <select> tag, I am trying to create a dependent dropdown but the problem is ajax returns only 0,
Also when I alert(data) in success function, the alert comes up with [object Object] reponse, not sure what's going wrong.
For testing purpose I've putted the echo "<option value='test'>Test</option>";
$('#data-modification-table tbody').on('change', 'td select', function(e) {
e.preventDefault();
let $tr = $(e.target).closest('tr');
$.ajax({
type: "POST",
url: ajaxurl,
dataType: "json",
data: {
action: "send_dropdown_account_data",
account_id: $tr.find('.user_account_name').val(),
project_id: $tr.find('.user_project_name').val(),
},
success: function(data) {
alert(data);
},
error: function(req, status, err) {
console.log('Something went wrong', status, err);
}
});
});
add_action('wp_ajax_nopriv_send_dropdown_account_dat', 'update_modification_dropdown_options');
add_action('wp_ajax_send_dropdown_account_dat', 'update_modification_dropdown_options');
function update_modification_dropdown_options()
{
echo "<option value='test'>Test</option>";
$output = ob_get_clean();
wp_send_json_success($output);
wp_die();
}
Inside inspect's network option I can see this response for admin-ajax.php: {"success":true,"data":"0"}
In console there is no error or message.

Answer 1

Instead of echoing the output you should just return an array of data instead directly to the wp_send_json_success method.
E.g.
$data = [
'input' => '<option value="test">Test</option>',
];
wp_send_json_success($data);
https://developer.wordpress.org/reference/functions/wp_send_json_success/
The response will be returned in that AJAX call as JSON.
alert(data.input)

Answer 2

First of all you can remove the type and dataType. Encode a variable into JSON using wp_json_encode(). Your function send_dropdown_account_data will end as following:
echo wp_json_encode([
'input' => '<option value="test">Test</option>',
]);
wp_die();
Now at ajax success function you will have to parse it before using.
success: function(data) {
data = JSON.parse(data);
alert(data.input);
},
Also check your action hook names as #DeepakSaini mentioned.

Answer 3

You are missing action name in add_action();
add_action('wp_ajax_nopriv_send_dropdown_account_data', 'update_modification_dropdown_options');
add_action('wp_ajax_send_dropdown_account_data', 'update_modification_dropdown_options');
Try this one in ajax function.