basic tampermonkey js to get video url

adownl.js

// ==UserScript==
// @name         teka artedownloader
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.arte.tv/de/videos/*
// @grant        GM_download
// ==/UserScript==

(function() {
    'use strict';
    console.log("----------- DWNL started ------------");
    let api_base = " https://api.arte.tv/api/player/v1/config/de/";
    var loc = window.location.pathname.split('/');
    var id = loc[3];
    var name = loc[4];

    var download_url = api_base + id;
    console.log("----------- download-url", download_url);

    window.onload = function () {
        var para = document.createElement("a");
        para.setAttribute('id', 'dwnl');
        para.setAttribute('style', 'color: #333');
        var node = document.createTextNode("Download");
        para.appendChild(node);
        document.getElementsByClassName('metas-infos')[0].insertBefore(para, null);
        document.getElementById('dwnl').addEventListener("click", function(){

//             var r = confirm("download " + name + "?");
//             if (r == true) {

                var getJSON = function(url, callback) {

                    var xhr = new XMLHttpRequest();
                    xhr.open('GET', url, true);
                    xhr.responseType = 'json';

                    xhr.onload = function() {

                        var status = xhr.status;

                        if (status == 200) {
                            callback(null, xhr.response);
                        } else {
                            callback(status);
                        }
                    };

                    xhr.send();
                };

                getJSON(download_url,  function(err, data) {

                    if (err != null) {
                        console.error(err);
                    } else {

                        var down_url = data.videoJsonPlayer.VSR.HTTPS_SQ_1.url;
                        console.log("will download: ", down_url);
                        document.getElementById('dwnl').textContent = down_url;
                        document.getElementById('dwnl').setAttribute('href', down_url);
                        GM_download(down_url, "xx"); // not working on my chromium  65.0.3325.181
                    }
                });
//             }
        });
    };

})();