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Now, we talked a lot about

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404

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errors.

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That's just one kind of error that could occur, though.

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It's an error caused by the user of this website

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because the user entered an invalid URL.

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It could also be an error introduced by us as a developer,

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if we had some invalid link somewhere on our website.

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But typically, it's caused by the user.

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Now, there is a different kind of error,

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which is not caused by the error,

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and which we can't always avoid as a developer,

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but which we still should be prepared to handle, at least.

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And, that's a server side error.

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And in this simple website,

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we probably won't have too many issues,

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but one thing that could fail

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is reading or writing to this restaurants.json file.

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Of course it typically works,

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especially for testing this locally.

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But theoretically, if this would be a real website

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hosted on some remote machine,

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then maybe if too many visitors

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are visiting this page at the same time,

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there might be multiple conflicting

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read or write operations.

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And hence, one of these operations could fail.

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Now, that is actually a scenario which we can prevent

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as a developer by using a proper database,

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which we'll do very soon in this course.

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But still, it is one possible scenario

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where things can go wrong.

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And in general, in web development, things can go wrong.

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You should never assume that everything just works.

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Now, to simulate data reading or writing fails here,

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I will rename this file.

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Now, that's of course not a realistic scenario,

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but it allows us to test what users will see

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if reading or writing does fail.

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And since I renamed this now, that will fail

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because I tried to read a file named restaurants.json,

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whilst the file actually is named restaurant.json.

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And hence, if we now, for example

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go to browse restaurants, we get this error.

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And that's a very ugly error,

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which we don't wanna show to our users.

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Instead, we again,

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typically want to show a more customized error message.

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Now, we could prepare a standard server side error page,

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just as we have a standard 404.ejs file,

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or,

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we simply send back different responses if we get errors.

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Now, I will go for the standardized error page here,

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because we'll dive into more specific,

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customized error handling later.

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Now, to have a standardized page,

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which can often be very useful,

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I'll add a 500

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.ejs file here.

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Five-hundred is the standard error code

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for server side errors.

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A 500 status code

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means that something went wrong on the server.

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Now, here I'll copy that 404.ejs content and paste that in.

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And then, I'll just say, "Something went wrong!"

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And here, we could now add some extra explanation

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where we said,

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"Unfortunately, something went wrong on the server -

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we're...

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doing

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our best...

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to fix it

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as quickly

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as possible!"

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Something like that.

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Now, I wanna send back that page,

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that template here,

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whenever something does go wrong on the server.

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And how could we achieve this now?

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Now, there are different ways of achieving this

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as always in development,

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but it's such a standard scenario

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that something goes wrong in one of your routes

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that Express actually has built-in support for that,

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and makes it very easy for you to handle such errors.

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We can, again, work with our own middleware,

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just not with a standard middleware,

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like we have it here for the 404 page.

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But instead, a middleware that will only execute

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if an error occurred on your server.

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Because that's what's happening here,

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and Express will detect this.

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And then, if you added your own error handling middleware,

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Express will call,

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will execute that middleware for you.

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For this, we can add another app.use method call here,

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and pass a function to app.use.

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But this function now works a bit differently,

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because this function

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will now not just receive request and response,

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but instead four parameters,

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and it must receive four parameters,

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because that signals to Express

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that this is this special default

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error handler middleware function

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that should be invoked by Express

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if some error occurs anywhere in your Express application.

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And the four parameters which you should accept here,

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is an error object.

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Then, request and response as we have it here.

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And then, a fourth parameter, which is called next.

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Now, this error object is generated

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and populated automatically by Express.

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So, by that third-party library.

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And it simply contains more information

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about the error that occurred.

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You could use that extra information

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to handle different errors in different ways.

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For the moment, we won't use it,

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but we need to accept all four parameters here.

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So, we need to write this function like this

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for Express to understand

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that this is this special error handling function.

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Next is actually a parameter which you could accept

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in all your routes and the standard middleware here as well.

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But we haven't done this up to this point

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because it's a bit more advanced and hard to explain

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if you don't have a use case for it yet,

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which we don't have at this point.

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But we'll see next later in the course again.

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In the end, next allows you to have multiple middlewares

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that will work together.

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And when you call next inside of a middleware,

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it allows the request to move on

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to the next middleware or route handler in line.

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But we'll see it later.

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For the moment, we just need to accept it.

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And then, here we can render this "500" template

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that we prepared.

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And with that, if you save everything

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and we added this default error handler,

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if you now reload this page,

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which crashes at the moment,

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you get this standard 500 page.

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And that's huge,

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because now,

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to your end user,

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your site doesn't crash.

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Sure, something went wrong,

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but we are still on our page

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and we can still generally use it,

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and just parts of the page are not working.

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And that is something we typically wanna do.

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We wanna provide a good user experience

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and if something goes wrong,

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we at least wanna handle it

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with a custom page and message.

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Of course here,

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the entire scenario was simulated,

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because, of course,

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you typically don't rename files that you shouldn't rename,

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but it allowed me to show you

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this server side error scenario,

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and how you can handle that.

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And that is also something we'll encounter later again,

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when we dive into bigger course projects again,

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when we build our online shop later,

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because there,

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we of course all do wanna handle cases

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where things go wrong.

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And hence, I'll bring back the "S" here,

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but I'll keep that server side error handling in place.

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Now, of course everything works again, though.