Wordpress Frontend Ajax with wp_localize_script Error: ajaxurl is not defined

I'm trying to create markers on a map via ajax on a wp theme. After some struggle I found out that I can't use any php file to get data via ajax, I have to use the admin-ajax.php file.

Accordingly to many examples, this is my code

in functions.php

add_action( 'wp_enqueue_scripts', 'add_frontend_ajax_javascript_file' );
function add_frontend_ajax_javascript_file()
{
   wp_localize_script( 'frontend_ajax', 'frontendajax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));
    wp_enqueue_script( 'ajax_custom_script',  get_stylesheet_directory_uri() . '/includes/ajax-javascript.js', array('jquery') );

}

add_action( 'wp_ajax_get_post_information', 'get_post_information' );
add_action( 'wp_ajax_nopriv_get_post_information', 'get_post_information' );


function get_post_information() 
{ 

$get_this= $_GET['this'];
$get_that= $_GET['that'];

...my select...

echo json formatted data
}

The js file is loaded and working, it does other stuff before the ajax call, where it stops for an error in this line:

$.getJSON(frontendajax.ajaxurl, data, function(result) {

I tryed also like this:

$.ajax({ 
        url:frontendajax.ajaxurl,
        data: data,
        success: function(data) {

But I always Have the same error:

Reference Error: frontendajax.ajaxurl is not defined

where is my error?

PS: I use get_stylesheet_directory_uri() because I am in a child theme.