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In this video, we're now going to implement
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the global error handling middleware that we
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just talked about before.
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So remember that the goal is to write a
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middleware function, which is gonna be able
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to handle operational errors like this one.
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Okay, so when a user hits a URL that doesn't exist,
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while we can consider that an operational error,
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and we in this case handle it by sending back
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this response here, right?
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But again, the goal is to do that in one central place.
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For example, we have similar stuff up here.
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So all over the place here we have these snippets
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here, which handle the errors.
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So we have to strike etch block here,
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and if there is an error, well, then we handle it
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by sending that error to the client.
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And so again, in the end, we want to get rid
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of all of this and handle the error in one
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central middleware.
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And so let's now start by actually building
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that middleware function.
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And in Express, it's actually very easy.
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Remember how I told you that Express
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already comes with middleware handlers
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out of the box.
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So as always, we start by using app.use,
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okay, and then in here we define our middleware function.
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So, to define an error handling middleware,
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all we need to do is to give the middleware function
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four arguments and Express will then automatically
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recognize it as an error handling middleware.
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And therefore, only call it when there is an error.
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And so just like in many other cases, this middleware
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function is an error first function,
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which means that the first argument
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is the error,
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and then we have request, response, and next.
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So by specifying four parameters,
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Express automatically knows that this entire
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function here is an error handling middleware.
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So let's now implement the code for this function here
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and after that I will show you how we can actually
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create an error so that this middleware function
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is actually caught.
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So two steps, first we create the middleware,
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then in the second step we will actually
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create an error so that this function will get caught.
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And for now, let's keep it really simple here.
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So all we really want to do in order to handle
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this error is to send back a response to the client.
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So res.status,
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but now we don't really know which status code it is, right?
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So for example, in this case here it's a 404,
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but we have some errors here, which have other status codes.
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Like we have a 400 for example for bad request,
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or really all kinds of other codes.
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And so we actually want to read that status code
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from the error object, all right?
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So when we create that error a bit later
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in the second step, as I told you,
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we will define this status code on the error.
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So let's say status code,
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and now I want to define a default here.
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Because there will be errors that are not
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coming from us because there are gonna be errors
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without a status code, so errors that are not
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created by us, but for example some other places
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in the note application.
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And I know that sounds a bit confusing for now,
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but you will see throughout this section.
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For now let's just really define this default
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status code.
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So we say that the error.statuscode is equal
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to err.statuscode basically if it is defined
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or the code 500, which means again,
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internal server error, and so that's usually
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the standard that we use.
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And in the same way, we also define the status,
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so let's say error.status is equal to error.status
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if it is defined, and if not,
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it's error.
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So error, remember, is when we have a 500 status code
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and if it's a 400 status code, then it's a fail.
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So for example here in this 404 , status is fail.
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And so now here we can then use that,
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so error.statuscode, and then send some json,
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so very similar to what we did before.
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So we start with the status and read that from error.status
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and the message will be coming from the error as well.
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So err.message and I'm gonna show you in a second
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how that err.message property here is created.
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All right, but for now, this is our very simple
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error handling middleware.
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So very simple, but for now it works.
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And so now the second step, where we actually
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create an error.
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And so let's do that here.
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So right here in that function, which handles
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all the unhandled routes, so let me comment this
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one out here and instead we want to create an error.
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So let's say const err
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and it's a new error.
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So we basically use the built in error constructor
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in order to create an error.
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And now we can pass in a string and that string
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will then be the error message property.
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So what we just talked about down here.
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So that message should be this message.
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All right?
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And then it ought to say err.status
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which is fail,
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and then err.statusCode is equal to 404.
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So that's what I mentioned before that we can define
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the status code and the status on the error object.
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And so that's exactly what we're doing here.
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We're creating an error and we then
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define the status and status code properties on it
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so that our error handling middleware
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can then use them in the next step.
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But now, how do we actually read that next step?
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So that next middleware.
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Well, as always, we use next.
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But this time we use next in a special way.
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Because now we need to actually pass that error
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into next, so if the next function receives
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an argument, no matter what it is,
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Express will automatically know that there was an error
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so it will assume that whatever we pass into next
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is gonna be an error.
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And that applies to every next function in every
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single middleware anywhere in our application.
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So again, whenever we pass anything into next,
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it will assume that it is an error, and it will
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then skip all the other middlewares in the middleware
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stack and sent the error that we passed in
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to our global error handling middleware,
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which will then, of course, be executed.
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And so let's now pass in the error
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which will skip again, all other middlewares
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in the stack and go straight to this one.
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Now in this case, there is actually
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no other middleware in the middle, okay?
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So it's really just the next one here,
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but if we were to use next and pass in an error
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somewhere else, then of course it will work the same.
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And so let's now test all of this simply by
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trying to access a route that was not defined.
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And so that will then trigger all of this code here,
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jump to the error handling middleware,
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and then send back the response based on all
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of this logic here.
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So, here we still have our wrong or non-existing route
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and let's start by actually calling this one,
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which should still be working,
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okay so just to confirm
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that everything works just fine and it does,
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and now this one.
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And indeed we get can't find API slash tours on this server,
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and so our global error handling middleware is
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actually doing its job.
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Perfect, that's the first step in really implementing
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a way better error handling mechanism
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in our application.
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So we could now go ahead and try to implement
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this kind of stuff here, everywhere in all
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our handlers.
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For example, over here in all of these functions
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that we have here.
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So replacing everything that we have here
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with this kind of error.
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But what I want to do for now is to actually create
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our own error class.
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So that we don't have to write all of this code here
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and instead have a more like streamlined class
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of ourself.
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So that's a common practice, and so let's do that
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in the next video.
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