xhr-lib.js

function addGetParam(url, name, value) {
    return url + (/\?/.test(url) ? '&' : '?') + encodeURIComponent(name) + '=' + encodeURIComponent(value);
}

function createXHR(win = window) {
    if(typeof win.XDomainRequest === 'function') {
        return new win.XDomainRequest();
    }
    return new win.XMLHttpRequest();
}

function openXHR(params = {}) {
    const xhr = params.xhr || createXHR();
    const method = (params.method || 'GET').toUpperCase();
    const isAsync = typeof params.async === 'boolean' ? params.async : true;
    const headers = params.headers || {};
    const url = (params.url.startsWith('//') ? 'https:' + params.url : params.url);
    xhr.open(method, url, isAsync);
    xhr.withCredentials = true;
    for(let hdrs = Object.getOwnPropertyNames(headers), i = hdrs.length; i--;) {
        let header = hdrs[i];
        let value = headers[header];
        if(Array.isArray(value)) {
            for(let i = value.length; i--;) {
                if(typeof value[i] === 'string') {
                    xhr.setRequestHeader(header, value[i]);
                }
            }
        } else if(typeof value === 'string') {
            xhr.setRequestHeader(header, value);
        }
    }
    return xhr;
}

function getJSON(url, disableCredentials = false) {
    return new Promise((resolve, reject) => {
        const xhr = openXHR({url});
        if(disableCredentials) {
            xhr.withCredentials = false;
        }
        xhr.onreadystatechange = function() {
            if(xhr.readyState !== 4) { return; }
            if(xhr.status < 200 || xhr.status > 299) {
                const err = new Error('Response status: ' + xhr.status);
                err.statusCode = xhr.status;
                reject(err);
                return;
            }
            resolve(xhr.responseText);
        };
        xhr.send(null);
    }).then(responseText => JSON.parse(responseText));
}

var jsonpID = 0;
function getJSONP(url, timeout = 12000) {
    return new Promise((resolve, reject) => {
        const name = '_seedr_jsonp_' + jsonpID++;
        const script = document.createElement('script');
        function destroy() {
            document.head.removeChild(script);
            delete window[name];
        }
        const timeoutID = setTimeout(() => {
            destroy();
            reject(new Error('JSONP request timeout: ' + url));
        }, timeout);
        script.onerror = () => {
            clearTimeout(timeoutID);
            destroy();
            reject(new Error('JSONP request error: ' + url));
        };
        window[name] = function jsonpCB(data) {
            clearTimeout(timeoutID);
            destroy();
            resolve(data);
        };
        script.type = 'text/javascript';
        script.src = addGetParam(url, 'callback', name);
        document.head.appendChild(script);
    });
}

function getXML(url) {
    return new Promise((resolve, reject) => {
        const xhr = openXHR({url});
        xhr.onreadystatechange = function() {
            if(xhr.readyState !== 4) { return; }
            if(xhr.status < 200 || xhr.status > 299) {
                const err = new Error('Response status: ' + xhr.status);
                err.statusCode = xhr.status;
                reject(err);
                return;
            }
            resolve(xhr.responseXML);
        };
        xhr.send(null);
    });
}

module.exports = {
    createXHR,
    openXHR,
    getJSON,
    getJSONP,
    getXML
};

xhr-lib.test.js

const {
    createXHR,
    openXHR,
    getJSON,
    getJSONP,
    getXML
} = require('./xhr-lib.js');

// your jest test
// see https://jestjs.io/docs/ru/getting-started