Cannot read property 'title' of undefined. Express

Question

Hi im developing nodejs express app. I am getting exception and dont know why. Everything seem to perfect for me. My exception is shown below: ``` 500 TypeError: C:\Nodejs\NodejsBlog\apps\blog/views/postdetail.jade:23
21| .col- md-12
22| .posts
> 23| h3= post.title
24| p=post.body
25| p tag:
26| i=post.tag

Cannot read property 'title' of undefined 21| .col-md-12 22| .posts > 23| h3= post.title 24| p=post.body 25| p tag: 26| i=post.tag Cannot read property 'title' of undefined at eval (eval at (C:\Nodejs\NodejsBlog ode_modules\jade\lib\jade.js:152:8), :221:59) at C:\Nodejs\NodejsBlog ode_modules\jade\lib\jade.js:153:35 at Object.exports.render (C:\Nodejs\NodejsBlog ode_modules\jade\lib\jade.js:197:10) at Object.exports.renderFile (C:\Nodejs\NodejsBlog ode_modules\jade\lib\jade.js:233:18) at View.exports.renderFile [as engine] (C:\Nodejs\NodejsBlog ode_modules\jade\lib\jade.js:218:21) at View.render (C:\Nodejs\NodejsBlog ode_modules\exp…

Accepted Answer

This is a race condition issue. The two functions that pull from MongoDB are asynchronous and so the call to `res.render()` happens before the DB returns the data in each function's respective callback. You need to nest each function so that they have access to the proper context. See below:

```
app.get('/Post/:id', function (req, res, next){
  Post.findOne({_id:req.params.id},function(err, postData){
    if (err) return next(err);

Comment.findOne({postid:req.params.id},function(err, comments){
      if (err) return next(err);

return res.render(__dirname+"/views/postdetail",{
        title: 'adfasdf',
        stylesheet: 'postdetail',
        post:postData,
        comments:comments
      });

});
  });
});
```

However, you can see how this can get pretty messy as you nest further and further. To prevent this, you can use a control flow library like caolan/async

Side Note:

You're Jade is looking to iterate over a `comments` array and you are returning a single doc from MongoDB (assuming you are using the `mongoose` module). You will want to change your Mongoose function from `findOne()` to simply `find()` so that `mongoose` can return an array of docs with the proper `postid`.

Edit:

Vinayak Mishra is also correct in pointing out that you can use Express' routing middleware as a way to impose control flow within a route. Here is an example:

```
// Use the app.param() method to pull the correct post doc from the database.
// This is useful when you have other endpoints that will require work on
// a Post document like PUT /post/:postid
app.param('postid', function (req, res, next, id) {

Post.findById(id, function (err, post) {
    if (err) return next(err);
    if (!post) return next('route');
    req.post = post;
  });

});

app.get('/post/:postid',
  // -- First route middleware grabs comments from post doc saved to req.post
  function (req, res, next) {
    Comment.find({ postid: req.post.id }, function (err, comments) {
      if (err) return next(err);
      req.comments = comments;
      next();
    });
  },
  // -- This route middleware renders the view
  function (req, res, next) {
    res.render('/postDetail', {
      // ... list locals here ...
    });
  }
);
```

Cannot read property 'title' of undefined. Express

Solution

Problem