jquery - Wordpress admin-ajax.php returns 0 even if using wp_die()

I'm trying to filter my posts with a simple ajax call, but cant get it to work because my admin-ajax.php call always returns 0. There are many issues facing this problem but they are mostly fixed by adding wp_die(), which is not solving my issue.

Ajax Call:

$.ajax({
  type: 'POST',
  url: '/wp-admin/admin-ajax.php',
  dataType: 'html',
  data: {
    action: 'filter_projects',
  },
  success: function(res) {
    $('.project-tiles').html(res);
    console.log(res);
} 
})

PHP function:

function filter_projects() {
  echo "<p>test</p>";
  wp_die();
}
add_action('wp_ajax_filter_projects', 'filter_projects');
add_action('wp_ajax_nopriv_filter_projects', 'filter_projects');

The PHP function is registered correctly as I'm getting a 200 on it, but it's always returning 0 and 'success'.

What I already tried:

• WP_DEBUG=1 --> no Error
• Check dev console --> no Error
• Check network --> 200 on admin-ajax.php call but received "0"
• Tried die() and exit instead of wp_die()

I dont know how to debug this any further as it is not returning any errors.

Does anyone have an idea?

Solution:

instead of echo directly your HTML, try manipulating the results with JSON so you can get better results for evaluating your test. How would I do the same test you posted (using direct JS instead of JQUERY):

                  let xhr = new XMLHttpRequest();
                   
                  xhr.open('POST', "/wp-admin/admin-ajax.php", true);

                  xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');

                  var params = 'action=filter_projects';
                  
                  xhr.onreadystatechange = () => {

                    if(xhr.readyState == 4) {

                      // RETURN OK
                      if(xhr.status == 200) {

                         console.log(xhr.responseText); // TEXT RESPONSE
                         console.log(JSON.parse(xhr.responseText));  // JSON RESPONSE
                         
                      // HTTP ERROR 
                      }else{
                        
                        console.log("SOME ERROR HTTP");
                        console.log(xhr.responseText);

                      }

                    }
                }; 

                xhr.send(params);

PHP CODE:

function filter_projects() {

$content = "test";

$data = array('success' => "200", 'content' => $content);

$json_string = json_encode($data, JSON_PRETTY_PRINT);

echo $json_string; 

wp_die();

}

add_action('wp_ajax_filter_projects', 'filter_projects');
add_action( 'wp_ajax_nopriv_filter_projects', 'filter_projects' );