如何在敲除js中绑定可观察数组的最后一个索引值

function Comment(NoteText, CreateDate) {
    this.commentText = ko.observable(NoteText);
    this.createDate = ko.observable(CreateDate);
}

function CommentList() {
    var nm = this;
    nm.newComment = ko.observable();
    nm.allComments = ko.observableArray([{ commentText: 'Hellow world', createDate: 'some date' }]);
    nm.AddNewComment = function () {
        nm.allComments.push(new Comment(this.newComment(), Date()));
        this.newComment('');
    }
    ko.applyBindings(nm, document.getElementById("comments"));
}

    var comments = new CommentList();
    $("#allcomments").hide();
    $("#btnViewAll").click(function () {
        $("#allcomments").show();
    });

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.2.3/jquery.min.js"></script>

<div id="comments">
        <input type="text" data-bind="value:newComment" /><button data-bind="click: AddNewComment">Add New Comment</button><br />
        <hr />
        Last Comment: <strong> <span data-bind="text: allComments[allComments.length-1]"></span> </strong><button id="btnViewAll">View ALL</button>
        <br />
        <div id="allcomments">
            <h2>All Comments</h2>
            <table>
                <thead>
                    <tr>
                        <td>Comment Text</td>
                        <td>Date</td>
                    </tr>
                </thead>
                <tbody data-bind="foreach: allComments">
                    <tr>
                        <td><span data-bind="text: commentText"></span></td>
                        <td><span data-bind="text: createDate"></span></td>
                    </tr>
                </tbody>
            </table>
        </div>
    </div>