vanilla javascript loading html content async

crosspromo.js

function loadBlocks()
{
    var crosspromoSites = [
        'elle',
        'marieclaire',
        'apetitonline',
        'svetzeny'
    ];

    var parentElement = document.getElementById('crosspromo');

    crosspromoSites.forEach(function(magazine, index)
    {
        var request = new XMLHttpRequest();
        request.open('GET', 'http://promo.burdamedia.cz/crosspromo/marianne/' + magazine + '/1', true);
        request.onload = function (e) {

            if (request.readyState === 4)
            {
                if (request.status === 200)
                {
                    var wrapper = document.createElement('div')
                    wrapper.id = 'crosspromo-' + index;
                    wrapper.innerHTML = request.responseText;
                    parentElement.appendChild(wrapper);
                }
                else
                {
                    console.error(request.statusText);
                }
            }
        };
        
        request.send(null);
    });
}

loadBlocks();