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Dynamic routes are

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an extremely important feature,

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which you will also see again and again

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throughout of this course,

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but also in any website you're going to build.

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Now, what happens though, in this dynamic route,

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if we try to get a restaurant

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with an ID that doesn't exists?

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So what happens if I manually change this ID

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here in the URL to R100, which is not a valid ID,

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I mean, it's maybe a valid ID,

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but it doesn't exist in our data set.

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If I hit enter here,

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then you'll note that this keeps on spinning.

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So we don't load a page and eventually

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this will time out and crash.

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Now, why does that happen?

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Why will we eventually time out here?

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Well, we will time out because in this route,

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which handles this request, we only send back a response

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if we did find a restaurant by that ID.

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Otherwise we don't send back a response.

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And I mentioned earlier in the course already

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that if you have a route

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where you don't send back a response,

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ultimately it will time out

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and the browser will show an error eventually,

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because it never receives a response from the server.

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And your server should always send back a response

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even if it's just an error response

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that tells the browser that something went wrong.

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But everything is better than just timing out

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and not knowing what's going on

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as it's currently the case here.

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We don't wanna wait until this eventually times out.

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No user will stick around that long.

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So therefore here, we also wanna send back a response

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if we don't find a restaurant.

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And how could we make that happen now?

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Well, you could think that you may be at

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a else case here in this if block

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and you send back a different response

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if you make it in here.

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But this would be wrong, and why would that be wrong?

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Well, keep in mind that this if statement

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is evaluated for every restaurant

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in this storedRestaurants array,

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and we won't have a matching condition here

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for the vast majority of all restaurants.

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There will only be one restaurant at most

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that matches with its ID

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because we'll never have more than one restaurant

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with that ID we're looking for,

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and in some cases we'll have no restaurant at all.

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So if we return an error response

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whenever this condition is not met,

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then we have a lot of cases

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where we don't meet the condition.

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And we might not even reach the restaurant

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where the condition might've been met.

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So else is not a good solution here

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because we might send back an error too early.

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Now, therefore the proper way of handling this

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is to wait until we exhausted our list of restaurants

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and we went through this entire list of restaurants.

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So after this for-loop.

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At this point, we had a look at every restaurant

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and we compared all the IDs.

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And we now know that there is no restaurant

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with a matching ID.

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And therefore now is the time to send back an error response

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where we let the user know that we didn't have a match.

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Now, how should this error response look like?

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That is up to you.

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You could send back this restaurant-detail template,

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maybe with some alternative content.

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In this restaurant-detail template,

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we could also add a if check

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and then maybe check if we have any restaurant data,

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in which case we output that.

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And if we don't have any data, we output something else.

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That could be one option.

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An alternative would be to prepare

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a dedicated 404 error page.

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And we have all seen these kinds of pages

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where you visit some part of a website

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and all of a sudden you get this 404 error page

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that tells you that this resource couldn't be found.

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And I wanna indeed add such a page.

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So I'll add a brand new template, 404.ejs.

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And in there I'll grab the general skeleton

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from about.ejs let's say.

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So I'll copy that and parse that in here.

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But then here in this h1 tag on the 404.ejs page

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on this template, I'll say page not found.

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And then maybe we add some explanatory text below

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that where we say, unfortunately we weren't able to,

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whoops, we weren't able to find this page.

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And of course you could add

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a more detailed error message here,

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but I'll keep it fairly simple deliberately here.

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So now that's my 404 error page, which I wanna show

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if we don't find a restaurant.

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And to make that work, back in app.js

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after this for-loop as discussed,

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we can now again render a response,

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but now it's the 404.ejs file.

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And we don't need to parse any data into that

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because this template, in my case here at least,

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doesn't use any dynamic data.

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We also don't need to return here

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because previously I returned to force a stop

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of this function execution.

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Now, here we are at the end of the function anyways,

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there is no code after that, so we could return,

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but it really doesn't make a difference.

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The function execution stops after this line anyways,

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because this function in which we are here is done.

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And hence this is enough.

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And now with that, if you save this,

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if you reload this unknown ID,

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you now get this 404 content,

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which is better than timing out,

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and which clearly informs the user

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that something went wrong.