mini_backend.js

let jsonFromServer = {};
let BASE_SERVER_URL;

const backend = {
    setItem: function(key, item) {
        jsonFromServer[key] = item;
        return saveJSONToServer();
    },
    getItem: function(key) {
        if (!jsonFromServer[key]) {
            return null;
        }
        return jsonFromServer[key];
    },
    deleteItem: function(key) {
        delete jsonFromServer[key];
        return saveJSONToServer();
    }
};
window.onload = async function() {
    downloadFromServer();
}

async function downloadFromServer() {
    let result = await loadJSONFromServer();
    jsonFromServer = JSON.parse(result);
    console.log('Loaded', result);
}

function setURL(url) {
    BASE_SERVER_URL = url;
}

/**
 * Loads a JSON or JSON Array to the Server
 * payload {JSON | Array} - The payload you want to store
 */

async function loadJSONFromServer() {
    let response = await fetch(BASE_SERVER_URL + '/nocors.php?json=database&noache=' + (new Date().getTime()));
    return await response.text();

}

function loadJSONFromServerOld() {
    return new Promise(function(resolve, reject) {
        let xhttp = new XMLHttpRequest();
        let proxy = determineProxySettings();
        let serverURL = proxy + BASE_SERVER_URL + '/nocors.php?json=database&noache=' + (new Date().getTime());




        xhttp.open('GET', serverURL);

        xhttp.onreadystatechange = function(oEvent) {
            if (xhttp.readyState === 4) {
                if (xhttp.status >= 200 && xhttp.status <= 399) {
                    resolve(xhttp.responseText);
                } else {
                    reject(xhttp.statusText);
                }
            }
        };

        xhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
        xhttp.send();

    });
}





/**
 * Saves a JSON or JSON Array to the Server
 */
function saveJSONToServer() {
    return new Promise(function(resolve, reject) {
        let xhttp = new XMLHttpRequest();
        let proxy = determineProxySettings();
        let serverURL = proxy + BASE_SERVER_URL + '/save_json.php';
        xhttp.open('POST', serverURL);

        xhttp.onreadystatechange = function(oEvent) {
            if (xhttp.readyState === 4) {
                if (xhttp.status >= 200 && xhttp.status <= 399) {
                    resolve(xhttp.responseText);
                } else {
                    reject(xhttp.statusText);
                }
            }
        };

        xhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
        xhttp.send(JSON.stringify(jsonFromServer));

    });
}


function determineProxySettings() {
    return '';

    if (window.location.href.indexOf('.developerakademie.com') > -1) {
        return '';
    } else {
        return 'https://cors-anywhere.herokuapp.com/';
    }
}