let _library,
    _categories;

getCategories(){ 
    return _categories;
}

loadCategories(){
    let categs = _categories

    let htmlElems = "";
      for (var i = 0; i < categs.length; i++) {
           htmlElems += "<div class='cat-box cat-num-" + i + "'data-target='#modal-id' data-toggle='modal'>" + categs[i].Title + "</div>";
           $(".modal-title").eq(i).append(categs[i].Title);
      }

     let container = document.querySelector("div.top-training");
     container.innerHTML = htmlElems;
     console.log(container);

} 
     
$(document).on("click", '.cat-box', function(val) {
    console.log('.cat-box clicked')
    let cat  = $(this).data('category'),
        desc = $(this).find("span").last().text();

// irrelevant code 
     
<div class="modal" id="modal-id" role="dialog">
<!-- ...etc -->
<div class="modal-title" id="exampleModalLabel"></div>