Cannot read property 'length' of undefined Node JS?

Question

MongoDB

What to do with template error

/home/bred/Рабочий стол/Папка/Blog/views/post/post.ejs:152 150| <h3>Комментарии</h3> 151| <div class="comment-list"> >> 152| <%- include('../layout/comments.ejs', {items: comments}) %> 153| <button class="btn btn-link" id="new">Ответить</button> 154| <form class="comment" action=""> 155| <textarea class="form-control" name="body" id="" rows="3"></textarea> /home/bred/Рабочий стол/Папка/Blog/views/layout/comments.ejs:12 10| <%= item.body %> 11| <span class="link" id="reply">ответить</span> >> 12| <%- include('./comments.ejs', {items: item.children}) %> 13| </li> 14| <% }) %> 15| /home/bred/Рабочий стол/Папка/Blog/views/layout/comments.ejs:3 1| Comments 2| >> 3| <% if (items.length) { %> 4| <ul> 5| <% } %> 6| Cannot read property 'length' of undefined

    	TypeError: /home/bred/Рабочий стол/Папка/Blog/views/post/post.ejs:152
    150|     <h3>Комментарии</h3>
    151|     <div class="comment-list">
 >> 152|       <%- include('../layout/comments.ejs', {items: comments}) %>
    153|       <button class="btn btn-link" id="new">Ответить</button>
    154|       <form class="comment" action="">
    155|         <textarea class="form-control" name="body" id="" rows="3"></textarea>

/home/bred/Рабочий стол/Папка/Blog/views/layout/comments.ejs:12
    10|           <%= item.body %>
    11|             <span class="link" id="reply">ответить</span>
 >> 12|             <%- include('./comments.ejs', {items: item.children}) %>
    13|         </li>
    14|         <% }) %>
    15| 

/home/bred/Рабочий стол/Папка/Blog/views/layout/comments.ejs:3
    1| Comments
    2| 
 >> 3| <% if (items.length) { %>
    4|   <ul>
    5|     <% } %>
    6| 

Cannot read property 'length' of undefined
    at eval (eval at compile (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:633:12), <anonymous>:11:18)
    at returnedFn (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:668:17)
    at include (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:666:39)
    at eval (eval at compile (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:633:12), <anonymous>:26:17)
    at Array.forEach (<anonymous>)
    at eval (eval at compile (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:633:12), <anonymous>:17:14)
    at returnedFn (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:668:17)
    at include (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:666:39)
    at eval (eval at compile (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:633:12), <anonymous>:60:17)
    at returnedFn (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:668:17)
    at tryHandleCache (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:254:36)
    at View.exports.renderFile [as engine] (/home/bred/Рабочий стол/Папка/Blog/node_modules/ejs/lib/ejs.js:485:10)
    at View.render (/home/bred/Рабочий стол/Папка/Blog/node_modules/express/lib/view.js:135:8)
    at tryRender (/home/bred/Рабочий стол/Папка/Blog/node_modules/express/lib/application.js:640:10)
    at Function.render (/home/bred/Рабочий стол/Папка/Blog/node_modules/express/lib/application.js:592:3)
    at ServerResponse.render (/home/bred/Рабочий стол/Папка/Blog/node_modules/express/lib/response.js:1012:7)

post.ejs

<% include ../layout/header %>



<% if (!user.id && !user.login) { %>

<!-- Posts { -->
    </div>
    <div class="col-md-8 order-md-1">

        <div class="card mb-2" style="">
          <div class="card-body">
            <h5 class="card-title"><%= post.title %></h5>
            <p class="card-text"><%= post.body %></p>
            <h6 class="card-subtitle mb-2 text-muted"><b>Создано: </b><%= post.createdAt %><br><b>Обновлено: </b><%= post.updatedAt %></h6>
            <span>
              Автор:<a href="#" class="card-link"> <%= post.author %></a>
            </span>
            <a href="#" class="card-link float-right">Комментарии</a>
          </div>
        </div>

    </div>
<!-- } -->

  <div class="comments" id="<%= post.id %>">
    <h3>Комментарии</h3>
    <div class="comment-list">
      <%- include('../layout/comments.ejs', {items: comments}) %>
      <button class="btn btn-link" id="new">Ответить</button>
      <form class="comment" action="">
        <textarea class="form-control" name="body" id="" rows="3"></textarea>
        <div class="buttons">
          <button class="btn btn-danger cancel">Закрыть</button>
          <button class="btn btn-primary send">Отправить</button>
        </div>
      </form>
    </div>
  </div>
        
    </div>
  </div>
</div>

<% include ../layout/footer %>

archive.js

router.get('/posts/:post', async (req, res, next) => {
  const url = req.params.post.trim().replace(/ +(?= )/g, '');
  const userId = req.session.userId;
  const userLogin = req.session.userLogin;

  if (!url) {
    const err = new Error('Not Found');
    err.status = 404;
    next(err);
  } else {
    try {
      const post = await models.Post.findOne({
        url
      });

      if (!post) {
        const err = new Error('Not Found');
        err.status = 404;
        next(err);
      } else {

        const comments = await models.Comment.find({
          post: post.id
        })
        console.log(comments)

        res.render('post/post', {
          post,
          comments,
          user: {
            id: userId,
            login: userLogin
          }
        });
      }
    } catch (error) {
      throw new Error('Server Error');
    }
  }
});

comment.js

router.post('/add', async (req, res) => {
  const userId = req.session.userId;
  const userLogin = req.session.userLogin;

  
  console.log(req.body)

  if (!userId || !userLogin) {
    res.json({
      ok: false,
    });
  } else {
    const post = req.body.post;
    const body = req.body.body;
    const parent = req.body.parent;

    try {

      if (!parent) {
        await models.Comment.create({
          post,
          body,
          author: userId
        })
      }

    } catch (error) {
      res.json({
        ok: false,
      });
    }
  }
});

Answer 1

if comments from the database gets null and then you map it to items, then items.length will obviously be equal to null.length, which is reflected in your error