script.php

<?php
function my_plugin_enqueue_scripts() {
    wp_enqueue_script('my-plugin', plugins_url('/assets/js/init.js', __FILE__), [], '', true);
    wp_localize_script('my-plugin', 'myAjaxVar', [
        'ajaxurl' => admin_url('admin-ajax.php'),
        'url' => 'https://ckp.ie'
    ]);
}

add_action('wp_enqueue_scripts', 'my_plugin_enqueue_scripts');

function save_to_database() {
    $array = $_POST['array'];

    // Do stuff

    wp_die();
}
add_action('wp_ajax_save_to_database', 'save_to_database');
add_action('wp_ajax_nopriv_save_to_database', 'save_to_database');


// Javascript
function apiPostXhr(url, action, data) {
    var request = new XMLHttpRequest();

    request.open('POST', url, true);
    request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
    request.onload = function (response) {
        if (this.status >= 200 && this.status < 400) {
            // Response success
        } else {
            // Response error
        }
    };
    request.onerror = function() {
        // Connection error
    };
    request.send('action=' + action + data);
}

// Call the function
apiPostXhr(myAjaxVar.ajaxurl, 'save_to_database', '&array=' + myArray);