问未捕获的长度:无法读取node.js上未定义的属性‘TypeError’

EN

function showNewsfeed() {
    var ajax = new XMLHttpRequest();
    ajax.open("POST", "/getNewsfeed", true);

    ajax.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {

            var response = JSON.parse(this.responseText);

            var html = "";
            for (var a = 0; a < response.data.length; a++) {
                var data = response.data[a];

                html += '<div class="central-meta item">';
                    html += '<div class="user-post">';
                        html += '<div class="friend-info">';

                            html += '<figure>';
                                html += '<img src="' + mainURL + "/" + data.user.profileImage + '" style="width: 45px; height: 45px; object-fit: cover;">';
                            html += '</figure>';

                            html += '<div class="friend-name">';
                                html += '<ins>';
                                    if (data.type == "post")
                                    {
                                        html += '<a href="/user/' + data.user.username + '">';
                                            html += data.user.username;
                                        html += '</a>';
                                    }
                                    else
                                    {
                                        html += data.user.name;
                                    }
                                html += '</ins>';

                                var createdAt = new Date(data.createdAt);
                                var date = createdAt.getDate() + "";
                                date = date.padStart(2, "0") + " de " + months[createdAt.getMonth()] + ", " + createdAt.getFullYear();

                                html += '<span>Publicado em: ' + date + '</span>';
                            html += '</div>';

                            html += '<div class="post-meta">';

                                html += '<div class="description">';
                                    html += '<p>';
                                        html += data.caption;
                                    html += '</p>';
                                html += '</div>';

                                if (data.image != "") {
                                    html += '<img src="' + mainURL + "/" + data.image + '">';
                                }

                                if (data.video != "") {
                                    html += '<video style="height: 359px; width: 100%;" controls src="' + mainURL + "/" + data.video + '"></video>';
                                }
                            
                                html += createUpvotesSection(data);
                            html += '</div>';
                        html += '</div>'
                    html += '</div>';
                html += '</div>';
            }
            document.getElementById("newsfeed").innerHTML = html;
        }
    };

function createUpvotesSection(data) {

                var isUpvoted = false;
                for (var b = 0; b < data.upvoters.length; b++) {
                    var upvoter = data.upvoter[b];
                    if (upvoter._id == window.user._id) {
                        isUpvoted = true;
                        break;
                    }
                }

                var html = "";

                html += '<div class="we-video-info">';
                    html += '<ul>';

                        html += '<li>';
                            var className = "";
                            if (isUpvoted) {
                                className = 'upvote';
                            } else {
                                className = "none";
                            }

                            html += '<span class="' + className +'" onclick="toggleUpvotePost(this);" data-id="' + data._id +'">';
                                html += '<i class="ti-thumb-up"></i>';
                                html += '<ins>' + data.upvoters.length + '</ins>';
                            html += '</span>';

                        html += '</li>';

                        html += '<li>';
                            html += '<span class="comment" title="Comments">';
                                html += '<i class = "fa fa-comments-o"></i>';
                                html += '<ins>' + data.comments.length + '</ins>';
                            html += '</span>';
                        html += '</li>';

                        html += '<li>';
                            html += '<span class="share" onclick="sharePost(this);" data-id="' + data._id +'">';
                                html += '<i class="ti-share"></i>';
                                html += '<ins>' + data.shares.length + '</ins>';
                            html += '</span>';
                        html +=  '</li>';

                    html += '</ul>';
                html += '</div>';

                return html;
            }