您需要在控制器和视图上进行的更改很少，我在下面提到了这两项更改

There are few things you'll have to change on your controller and view, I have mentioned both changes below

在您的控制器上

exports.addPost = (req, res, next) => {
    var form = {
        titleholder: req.body.title,
        excerptholder : req.body.excerpt,
        bodyholder: req.body.body
    };
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
            req.flash('danger', errors.array())
            //req.session.save(() => res.redirect('../addpost'));
            res.render('admin/addpost',{
                layout: 'admin/layout',
                website_name: 'MEAN Blog',
                page_heading: 'Dashboard',
                page_subheading: 'Add New Post',
                form:form});
    } else {

后面的代码与您已经拥有的代码相同，我添加了一个form对象，并将您的res.redirect更改为res.render

Code after else is same as you already have, I have added a form object and changed your res.redirect to res.render

这将是您的视图的代码

<div class="col-sm-7 col-md-8 col-lg-9">
    <div class="card">
        <div class="card-header d-flex px-2">
            <h6 class="m-0"><%= page_subheading %></h6>
        </div>
        <div class="card-body p-2">
            <form action="./post/add" method="POST" class="mb-0">               
                <div class="form-group">
                        <input type="text" class="form-control" name="title" value="<%= typeof form!='undefined' ? form.titleholder : '' %>" placeholder="Title" />
                </div>

                <div class="form-group">
                        <input type="text" class="form-control" name="excerpt" value="<%= typeof form!='undefined' ? form.excerptholder : '' %>" placeholder="Excerpt" />
                </div>

                <div class="form-group">
                        <textarea rows="5" class="form-control" name="body" placeholder="Full text"><%= typeof form!='undefined' ? form.bodyholder : '' %></textarea>
                </div>

                <div class="form-group mb-0">
                    <input type="submit" value="Add Post" class="btn btn-block btn-md btn-success">
                </div>
        </form>
  </div>
    </div>
</div>