fetch.js

window.fetch = window.fetch || function (url, options) {
        return new Promise(function (resolve) {
            const xhr = new XMLHttpRequest();

            xhr.onloadend = function() {
                resolve(Object.assign({}, xhr, {
                    ok: xhr.status === 200,
                    status: xhr.status,
                    statusText: xhr.statusText
                }));
            };

            xhr.open(options.method || 'get', url);
            for (const key in options.headers)
                xhr.setRequestHeader(key, options.headers[key]);
            options.body && xhr.send(options.body);
        });
    };