$post = NULL with ajax request

Hi

I am building a filtering function that does an ajax request that uses get_posts to fetch correct posts from the database. My problem is that the $post variable is null which means i cant enter the loop to get my posts.

These functions are written in functions.php:

function ajax_filter_posts_scripts() {
  // Enqueue script
  wp_register_script('afp_script', get_stylesheet_directory_uri() . '/filter-ajax.js', false, null, false);
  wp_enqueue_script('afp_script');

  wp_localize_script( 'afp_script', 'afp_vars', array(
        'afp_nonce' => wp_create_nonce( 'afp_nonce' ),
        'afp_ajax_url' => admin_url( 'admin-ajax.php' ),
      )
  );
}
add_action('wp_enqueue_scripts', 'ajax_filter_posts_scripts', 100);

function ajax_filter_get_posts( $area ) {
global $post;
$cat_IDs 	= array(
   'coming' => 44,
   'reference' => 45,
   'sold' => 46,
   'sleeping' => 47,
   'for_sale' => 48
); 

  $area = $_POST['area'];

  // Verify nonce
  if( !isset( $_POST['afp_nonce'] ) || !wp_verify_nonce( $_POST['afp_nonce'], 'afp_nonce' ) )
    die('Permission denied');

  $for_sale_args = array(
	'post_type' 		=> 'objekt',
	'category' 		=>$cat_IDs['for_sale'],
	'posts_per_page'	=> -1,
	'meta_query'		=> array(
			array(
				'key' => 'gw_search_tag',
				'value' => $area,
				'compare' => 'LIKE'
			)
													),
	'order'	=> 'DESC'
);

// If area is not set, remove key from array and get all posts
 if( !$area ) {
    unset( $for_sale_args['meta_query'] );
}

if ( have_posts() ) : while ( have_posts() ) : the_post();
   $for_sale_posts = get_posts($for_sale_args);
   foreach ( $for_sale_posts as $post ) : setup_postdata( $post );?>
      <!-- html stuff -->
<?php
   endforeach; wp_reset_postdata();
endwhile;
endif;

die();
}
add_action('wp_ajax_filter_posts','ajax_filter_get_posts');
add_action('wp_ajax_nopriv_filter_posts', 'ajax_filter_get_posts');

And in my ajax-filter.js:

jQuery(document).ready(function($) {
    $body = $("body");

    $(document).on({
        ajaxStart: function() { $body.addClass("ajax-loading");    },
        ajaxStop: function() { $body.removeClass("ajax-loading"); }
    });
    $('#filter li').click( function(event) {
        // Prevent defualt action - opening tag page
        if (event.preventDefault) {
            event.preventDefault();
        } else {
            event.returnValue = false;
        }

        // Get tag slug from title attirbute
        var selected_area = $(this).attr('area');
        console.log(selected_area);
        $('#list').fadeOut();

        data = {
            action: 'filter_posts', // function to execute
            afp_nonce: afp_vars.afp_nonce, // wp_nonce
            area: selected_area, // selected tag
            };

        $.post( afp_vars.afp_ajax_url, data, function(response) {

            if( response ) {
                // Display posts on page
                $('#list').html( response );
                // Restore div visibility
                $('#list').fadeIn();
            }
        });
    });
});

I read somewhere that i needed to do

require_once('./wp-blog-header.php');

but that didn’t work either.

How can I retrieve the $post variable? Or is it something else that I am doing wrong?