Infinite Scroll ability with plain JavaScript (no jQuery)

InfiniteScroll.js

/*
 * Infinite Scroll js, no jQuery reliance
 *
 */

var activeCall = false, triggerElem, div = document.querySelector('#dataParentDiv > .row'), skip = 21, last = false;
document.addEventListener('DOMContentLoaded', function () {
    if (document.querySelectorAll('.product-item').length <= 21)
        last = true; // don't attempt to inifinite scroll for less than initial results
    else
        getTriggerElement();

    window.addEventListener('scroll', function () {
        if (last || activeCall)
            return;
        var bounds = triggerElem.getBoundingClientRect();
        if (bounds.top < (window.innerHeight + 300) && !activeCall && !last) {
            activeCall = true; // prevents multiple calls all at once
            minAjax({
                url: '/api/url',
                type: 'GET',
                data: {
                    categories: [1,2,3,4],
                    skip: skip
                },
                success: function (data) {
                    var elems = JSON.parse(data);
                    skip = skip + elems.length;
                    last = elems.length < 15;
                    for (var i = 0; i < elems.length; i++) {
                        div.insertAdjacentHTML('beforeend', elems[i]); // assuming API endpoint simply returns pre-built HTML
                        // other work can be done here for anything else
                    }
                    getTriggerElement();
                    activeCall = false;
                }
            });
        }
    });
});
function getTriggerElement() {
    var elements = document.querySelectorAll('.product-item');
    triggerElem = elements[elements.length - 3];
}

// minAjax: https://github.com/flouthoc/minAjax.js