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Now, as a final step here,

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in the getSinglePost action in the post-controller,

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I wanna add a little bit of error handling here

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when we create a post.

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As mentioned, in the Node + MongoDB section,

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way earlier in the course,

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there is a problem with Express.js

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related to errors caused by asynchronous operations.

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Those don't reach the default error-handling middleware

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out of the box,

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which is a pity.

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It would be better if it would work like this,

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but it doesn't.

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Therefore, theoretically, whenever you are working

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with asynchronous code and with asynchronous operations,

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you should add your own error handling

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and force Express to trigger its error-handling middleware

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if things go wrong.

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Now, here I'll only do it

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for this getSinglePost case, though,

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because there I can easily show you

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where our application would break otherwise.

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If I do log in real quick here with a valid user

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and I view a post,

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that, of course, works.

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But if I mess with the post id

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and I add a couple of characters,

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then this fails and the server crashes.

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And the reason for this

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is that we don't even make it to this check

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where we would return a 404 page

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because when we create the post here already,

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we actually try to convert the id to this ObjectId thing.

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And it turns out that this ObjectId constructor

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throws an error if it receives an id

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that does not follow the MongoDB id format

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which my self-entered id doesn't.

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Now, normally, when code throws an error,

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the default Express error-handler would become active.

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But since in the post-controller,

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getSinglePost is an async function,

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this actually is all wrapped into a promise,

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and this overall is an asynchronous operation,

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and there, unfortunately, errors

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generated by or inside of such async operations

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are not handled by the Express error-handling middleware.

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To still ensure we reach the error-handling middleware,

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we instead have to manually handle the error first

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and then call next and forward the error.

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So, therefore, we can use try-catch here

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to try creating a post.

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And for that, I'll create a variable named post

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to which I'll assign a value inside of this try block.

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And then I catch any errors that might be thrown

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by the operation that lives in the try block,

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in this case the creation of the post

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where this conversion of the id will actually happen.

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If that generates an error, then we can catch it here.

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And then what we can do here

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is we can use this third parameter

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which we get in all middleware functions,

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and route handlers are just middleware functions at the end,

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which is this next function.

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We can call next and pass the error to it,

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and this will then reach

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the Express default error-handling middleware,

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but we have to do it manually.

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I also will return this

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or return thereafter, doesn't make a difference,

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because I wanna ensure that this other code doesn't execute

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because it wouldn't work

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if we already failed to create an id.

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With that, if I now save that,

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if I now reload this invalid id, I get An error occurred!

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If I instead wanna get my 404 page,

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then I can, instead of calling next, simply manually render

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this 404 page like this.

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So that would be an option.

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It's up to you what you wanna do if this error occurs.

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So if I do that and reload, I still get An error occurred!,

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because actually here I don't have a 404 page anymore.

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If I quickly create a 404.ejs file, though,

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and I say Not found! in here,

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Could not find the resource,

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then you will see,

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maybe I'll also set the title to Not found!,

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then you will see that if I save this,

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I added this template, now if I reload, we see that.

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So that's how we can then handle the error ourselves.

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Again, theoretically, you should consider doing that

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for all your tasks,

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for all the actions where you reach out to the database,

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because all those things can theoretically fail.

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However, to keep that code fairly structured,

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I will not do it here.

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We will again see proper error handling

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in the next course section, though,

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which is about that main course project

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where we build this online shop.

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But this is a specific case you should be aware of.