Fetch polyfill per Internet Explorer

var url = "/demo";
 
fetch(url, {
    method: "POST",
    mode: "cors", // parametro vincolante per le chiamate cross domain
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ value: 1 }),
})
    .then((response) => response.json())
    .then((data) => {
        console.log(data);
    })
    .catch((error) => {
        console.error(error);
    });

if (typeof fetch === 'undefined') {
    /**
     * implementazione PARZIALE di fetch per Internet Explorer 
     * parametro header.mode: non gestito, il default è 'cors'
     * @param {*} url 
     * @param {*} options es. {method: "POST", mode: "cors", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ value: 1 }) }
     * @returns un oggetto con metodi then(...) e catch(...)
     */
    var fetch = function (url, options) {
        console.log('fetch polyfill by https://www.sgart.it');

        var thenFunctions = [];   // non gestisce catene di then o catch
        var catchFunctions = [];

        var returnObject = {
            then: function (fn) {
                thenFunctions.push(fn);
                return returnObject;
            },
            catch: function (fn) {
                catchFunctions.push(fn);
                return returnObject;
            }
        };

        var xHttp = new XMLHttpRequest();
        xHttp.onreadystatechange = function () {
            if (this.readyState === 4) {
                this.ok = this.status >= 200 && this.status <= 299;
                if (this.status === 200) {
                    console.log('then');
                    this.json = function () {
                        return JSON.parse(this.responseText);
                    };
                    var data = this;
                    for (var i = 0; i < thenFunctions.length; i++) {
                        var fn = thenFunctions[i];
                        data = fn(data);
                    }
                } else {
                    var error = { status: this.status, message: this.responseText };
                    console.error('catch', error);
                    for (var i = 0; i < catchFunctions.length; i++) {
                        var fn = catchFunctions[i];
                        error = fn(error);
                    }
                }
            }
        };

        var method = options && options.method ? options.method : 'GET';
        xHttp.open(method, url, true);
        if (options && options.headers) {
            for (var key in options.headers) {
                var value = options.headers[key];
                xHttp.setRequestHeader(key, value);
            }
        }

        if (options && options.body) {
            var data = options.body;
            xHttp.send(data);
        } else {
            xHttp.send();
        }

        return returnObject;
    }
}