gistfile1.js

function parseHeaders(data) {
    const headers = { };
    const lines = data.split('\n');
    for (let i = 0; i < lines.length; ++i) {
        const colonPos = lines[i].indexOf(':');
        const key = lines[i].slice(0, colonPos).toLowerCase().trim();
        const value = lines[i].slice(colonPos + 1).trim();
        if (key === '') {
            continue;
        }
        if (headers.hasOwnProperty(key)) {
            headers[key] += ', ' + value;
        } else {
            headers[key] = value;
        }
    }
    return headers;
}

class AjaxError extends Error { }
class AjaxAbortedError extends Error { }

function Request(method, url, headers, body) {
    this.method = method;
    this.url = url;
    this.headers = headers !== void 0 ? headers : { };
    this.body = body !== void 0 ? body : null;
}

function Response(status, headers, body) {
    this.status = status;
    this.headers = headers;
    this.body = body;
}

function ajax(request) {
    return new Promise(function(resolve, reject) {
        const ajax = new XMLHttpRequest();

        ajax.addEventListener('load', function() {
            const headers = parseHeaders(ajax.getAllResponseHeaders());
            const response = new Response(ajax.status, headers, ajax.responseText);
            resolve(response);
        });

        ajax.addEventListener('error', function() {
            reject(new AjaxError());
        });

        ajax.addEventListener('abort', function() {
            reject(new AjaxAbortedError());
        });

        for (let key in request.headers) {
            if (request.headers.hasOwnProperty(key)) {
                ajax.setRequestHeader(key, request.headers[key]);
            }
        }
        ajax.open(request.method, request.url);
        ajax.send(request.body);
    });
}