nf3-submit-listener.js

// Create a new object for custom validation of a custom field.
var mySubmitController = Marionette.Object.extend( {
  
  initialize: function() {
    this.listenTo( Backbone.Radio.channel( 'forms' ), 'submit:response', this.actionSubmit );
  },

  actionSubmit: function( response ) {
    // Do stuff here.
  },

});

// On Document Ready...
jQuery( document ).ready( function( $ ) {

    // Instantiate our custom field's controller, defined above.
    new mySubmitController();
});

I've tried this now on two different WordPress sites, and on a clean WP install and i get this error on console:
Uncaught ReferenceError: Marionette is not defined

Do anybody have any clue or idea what has changed?

I get the same error

Any news on this error?

You likely have included your script before the <?php wp_footer(); ?> tag in your footer.php template - you need to include it after this tag (i.e., after the Marionette object has been injected by Ninja Forms).

You likely have included your script before the <?php wp_footer(); ?> tag in your footer.php template - you need to include it after this tag (i.e., after the Marionette object has been injected by Ninja Forms).

Adding to this: even if you've added your custom script that holds this gist in the proper way - meaning through wp_enqueue_script and wp_footer - the error may still show up. It's important to have your custom script be included after any dependencies, and Wordpress allows us to set this up like so:

wp_enqueue_script( 'my-custom-scripts', get_template_directory_uri() . "/my-folder/my-script.js", array('jquery', 'nf-front-end'), false, true );

This assures your custom script runs after 'nf-front-end', which in turn depends on 'nf-front-end-deps' which contains Marionette. Including your custom script like this makes sure the code runs after both Backbone and Marionette are loaded, solving the issue.

Add your code inside this form ready function because once the form is loaded then the error will be solved
jQuery(document).on( 'nfFormReady', function( e, layoutView ) {
Add your code here...
});

You can simply move all of the code below "jQuery( document ).ready( function( $ ) {" and it should work.