WordPress Ajax URL for function in functions.php

I have a desktop app and I use $.getJSON to get data from my wordpress site using the plugin the WP REST API. This works fine and so I assume I can also make an Ajax call to a function in the functions.php? How do I do this? None of the examples I have seen supply the ajaxURL. How do I find out what this is?

I have the following code:

Functions.php (in wordpress site on server)

//Called by Ajax from App - list the contents of the folder so they can be downloaded and stored locally
function folder_contents() {

    $folderPath = $_POST['path'] ;
    $files = array_diff(scandir($path), array('.', '..'));
    print_r($files);
    return $files;

    die(); 
}

add_action('wp_ajax_my_action', 'folder_contents');

app.js (run using node webkit locally from machine)

//ajax call 
var data = {
    action: 'path',
    path: datafolder;
};
var ajaxurl = baseurl + '';  //WHAT IS THIS?!?!
jQuery.post(ajaxurl, data, function(response) {
    alert('Got this from the server: ' + response);
    console.log(response);
});

——————-EDIT———————-

After the answer below I have tried the below code based on the advice but I get

“index.html:324 Uncaught ReferenceError: ajax_params is not defined”

Functions.php

//add code to use ajax
function add_ajax_script() {

    wp_localize_script( 'ajax-js', 'ajax_params', array( 'ajax_url' => admin_url( 'admin-ajax.php' ) ) );

}

//Called by Ajax from App - list the contents of the folder so they can be downloaded and stored locally

function folder_contents() {

    $folderPath = $_POST['path'] ;
    $files = array_diff(scandir($path), array('.', '..'));
    wp_send_json($files);

    die(); 
}

add_action('wp_ajax_folder_contents', 'folder_contents');
add_action('wp_ajax_nopriv_folder_contents', 'folder_contents');


add_action( 'wp_enqueue_scripts', 'add_ajax_script' );

App.js

//ajax call 
        var data = {action: 'powerpoint_folder_contents',path: datafolder};
        var ajaxurl =  ajax_params.ajax_url;
        console.log(ajaxurl);
        jQuery.post(ajaxurl, data, function(response) {
            alert('Got this from the server: ' + response);
            console.log(response);
        });

If I remove the add_ajax_script() function and simply use the below it returns 0.

var ajaxurl = baseurl + ‘wp-admin/admin-ajax.php’;

Please help…

//ajax call 
var data = {
    action: 'folder_contents',
    path: datafolder
};
var ajaxurl = baseurl + '';  //WHAT IS THIS?!?!
jQuery.post(ajaxurl, data, function(response) {
    alert('Got this from the server: ' + response);
    console.log(response);
});

And your PHP in functions.php

//Called by Ajax from App - list the contents of the folder so they can be downloaded and stored locally
function folder_contents() {

$folderPath = $_POST['path'] ;
$files = array_diff(scandir($path), array('.', '..'));
print_r($files);
return $files;

die(); 
}

add_action('wp_ajax_folder_contents', 'folder_contents');
add_action('wp_ajax_nopriv_folder_contents', 'folder_contents');

The above add your function as an ajax action. You then call the function name in your AJAX.

See https://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_(action)