It seems that this is a common issue. I have googled and searched Stack Exchange about this but didn't find any solution that helps in my situation.

Problem: When trying to use Ajax with admin-ajax.php, I always get the error 400.

POST .php 400

This is what I'm trying to achieve:

I'm building a web site for my client with my custom made theme. I created custom post type with custom post status 'Archived'. I'm trying to add a new column with a button in the custom post listing. With this button the admin can set the custom post as archived with single click.

Here is the simplified version of the code. I removed unrelated codes here.

<?php
add_action( 'wp_ajax_set_post_archived', 'set_post_archived' );
add_action( 'admin_footer', 'set_post_archived_js' );

function set_post_archived_js() {
    ?>
    <script type="text/javascript">
        jQuery(document).ready(function($) {
            $('.set-archived-button').click(function(event){
                event.preventDefault();

                var data  = {
                    action: 'set_post_archived',
                    post_id: $(this).data('post_id'),   
                };
                
                jQuery.post(
                    ajaxurl, data,
                    function(response) {
                    alert('Got this from the server: ' + response);
                });
            });
        });
    </script>
    <?php
}

function set_post_archived()
{
  // The function code here...
  
  wp_die();
}

The button's code is

<a href="#" data-post_id="70" class="button set-archived-button">Archive</a>

Post id in data-post_id is created dynamically.

As mentioned, this code is on the backend, so wp_ajax_nopriv_{action_hook} shouldn't be required.

Background-info

I have tried both jQuery.post() and jQuery.ajax(). Seems that, it doesn't make difference. Here is how the code is being loaded if it matters.

In functions.php:

if ( is_admin() ) {
    // Only if on backend
    require get_template_directory() . '/admin/admin-functions.php';
}

In /admin/admin-functions.php:

global $pagenow;
if ( $pagenow == 'edit.php' || $pagenow == 'post.php' ) {
    require get_template_directory() . '/admin/machines-functions.php';
}

The actual code with Ajax is in the file /admin/machines-functions.php.

It seems that this is a common issue. I have googled and searched Stack Exchange about this but didn't find any solution that helps in my situation.

Problem: When trying to use Ajax with admin-ajax.php, I always get the error 400.

POST https://www.example/wp-admin/admin-ajax.php 400

This is what I'm trying to achieve:

I'm building a web site for my client with my custom made theme. I created custom post type with custom post status 'Archived'. I'm trying to add a new column with a button in the custom post listing. With this button the admin can set the custom post as archived with single click.

Here is the simplified version of the code. I removed unrelated codes here.

<?php
add_action( 'wp_ajax_set_post_archived', 'set_post_archived' );
add_action( 'admin_footer', 'set_post_archived_js' );

function set_post_archived_js() {
    ?>
    <script type="text/javascript">
        jQuery(document).ready(function($) {
            $('.set-archived-button').click(function(event){
                event.preventDefault();

                var data  = {
                    action: 'set_post_archived',
                    post_id: $(this).data('post_id'),   
                };
                
                jQuery.post(
                    ajaxurl, data,
                    function(response) {
                    alert('Got this from the server: ' + response);
                });
            });
        });
    </script>
    <?php
}

function set_post_archived()
{
  // The function code here...
  
  wp_die();
}

The button's code is

<a href="#" data-post_id="70" class="button set-archived-button">Archive</a>

Post id in data-post_id is created dynamically.

As mentioned, this code is on the backend, so wp_ajax_nopriv_{action_hook} shouldn't be required.

Background-info

I have tried both jQuery.post() and jQuery.ajax(). Seems that, it doesn't make difference. Here is how the code is being loaded if it matters.

In functions.php:

if ( is_admin() ) {
    // Only if on backend
    require get_template_directory() . '/admin/admin-functions.php';
}

In /admin/admin-functions.php:

global $pagenow;
if ( $pagenow == 'edit.php' || $pagenow == 'post.php' ) {
    require get_template_directory() . '/admin/machines-functions.php';
}

The actual code with Ajax is in the file /admin/machines-functions.php.

• ajax

• 1 I suspect this is caused by a common mistake, however, this mistake ssimply does not happen if you had used a REST API endpoint for your AJAX rather than the old admin-ajax.php api, and if something had happened it would have given you a plaintext error message in human readable language. Is there a specific reason you decided to use the older API? In fact on closer inspection of your Q I suspect what you want can be done with a request to the WP Core official endpoints that already exist! – Tom J Nowell ♦ Commented Feb 1, 2021 at 16:00
• No specific reason why I'm using admin-ajax.php. This is all new to me, so I'm learning all this right now and REST API is surely something I will look into in the near future :) Thank you for pointing this out. – Antti Tolvanen Commented Feb 1, 2021 at 16:56

1 Answer 1

It seems that you're adding the AJAX action only if $pagenow (the current admin page/file) is edit.php or post.php, and that's not going to work on admin-ajax.php, so you should instead add the action via the admin_init hook if you wish to add the action only on the admin side. E.g.

add_action( 'admin_init', function () {
    add_action( 'wp_ajax_set_post_archived', 'set_post_archived' );
} );