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‫In this video, let's create a special type

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‫of middleware called param middleware.

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‫So param middleware is middleware that only runs

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‫for certain parameters, so basically,

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‫when we have a certain parameter in our URL.

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‫Now in our example here, the only parameter

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‫that we might have in our route URL is the id, right?

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‫And so we can write middleware that only runs

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‫when this id is present in the URL,

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‫okay, so let me show you how to do it.

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‫It is quite simple actually, so it's on our router

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‫and then the param method, okay.

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‫And so here we specify first the parameter

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‫that we actually want to search for,

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‫so basically the parameter for which

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‫this middleware is gonna run, and it's called id,

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‫and then of course our actual middleware function.

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‫And as usual, we have access to the request

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‫and to the response object, and then

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‫senses a middleware function also

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‫to the next function, right?

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‫Now in a param middleware function,

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‫we actually get access to a fourth argument

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‫and that one is the value of the parameter in question.

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‫So we usually call that one val, which stands for value.

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‫And so now we can go ahead and simply log it

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‫to the console, just to take a look

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‫if this actually works, so let's say Tour id is

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‫and then id, right?

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‫Next up, we also have to call next, right?

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‫Because otherwise the request response cycle

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‫will get stuck in this middleware function

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‫and it's not gonna be able to move on

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‫to the next middleware in the stack, right?

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‫Which are gonna be these routes down here.

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‫All right, so let's give it a save and check it out now.

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‫And we want it to not for get all users

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‫but for get one Tour, right?

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‫Well let me show you first what happens

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‫if we do not have any id, okay, and so now,

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‫we don't see any log down in our console.

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‫But if I now sent the same request on this route

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‫where we have the id, let's see what happens then.

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‫Oh, it tells me that id is not defined,

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‫and indeed it's not id, so that was a stupid mistake.

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‫So it is val for value, right?

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‫So remember how I said that this value parameter

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‫is the one that will actually hold

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‫the value of the id parameter, and so of course

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‫that's the one that we need to then use here

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‫in order to get access to that id.

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‫So let's try that again, and now indeed

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‫we have Tour id is two, right?

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‫So that log came right from this function here.

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‫Now, what I also want to show you is that

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‫this middleware function is not going to

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‫run for any of the user routes.

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‫So let's say we call a user with a specific id,

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‫so let's go ahead and copy this one here,

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‫create a new request with an id,

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‫and let me actually save it also.

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‫So get user and save to user, and when I send it here

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‫we of course get our standard response of

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‫this route is not defined, but you also see

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‫that we have no log like we had before.

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‫And so that is of course because this middleware function

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‫is only specified in our tour router.

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‫So in this kind of local mini application,

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‫so again, that is the analogy that I like to make.

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‫So basically that each router is

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‫kind of a mini sub-application, one for each resource.

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‫And so since this middleware is only specified

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‫on this router, well then of course,

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‫it is only part of the middleware stack

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‫if we are actually inside of this sub-application.

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‫Makes sense?

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‫So let's suppose we have an incoming request on tours/id.

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‫So that request will then go through all these middleware,

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‫so first this middleware, then this one,

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‫then this middleware, then this one,

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‫so all of these are part of the middleware stack

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‫and then it will finally hit this middleware

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‫and since this is actually the route,

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‫it will then get into tourRouter middleware.

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‫Okay, and so from there it then goes right into

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‫this middleware, and so then this code will be run.

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‫And again, that's only because it has an id in the route.

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‫If not, well, then this would simply be ignored

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‫and it would move on right to the next middleware

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‫in the stack, so these down here, right?

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‫Cool, so that is how param middleware works;

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‫but for now it is not really that useful.

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‫But we can actually use it for

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‫a very practical use case here.

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‫So let's go to our handler functions here

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‫and you see that in all of the handler functions

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‫that actually use the id, we check if the id is valid.

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‫So we do it here in get tour, and we also do it

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‫in update tour, so here, and in delete tour.

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‫So all these three functions have this very similar code

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‫where they check if the id is valid

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‫and if not, they send back this Invalid ID response.

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‫So we have all this code in the same place

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‫and as you already know, it is not a good practice

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‫to repeat code and so what we can do here

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‫is to use the concept of param middleware;

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‫and perform this check here in an outside middleware

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‫that it's gonna run before the request

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‫even hits these handler functions.

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‫So let's go ahead and copy or actually cut

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‫the code from here and create a new middleware function

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‫called checkID, and of course I also need to export that.

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‫So checkID and we have access, request, response,

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‫next, and again, keep in mind that

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‫it is a param middleware and so the fourth argument

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‫will be the value of the parameter.

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‫Okay, paste it here, and then don't forget

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‫to call next at the end of the middleware, all right?

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‫And what's also very important is that

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‫we have this return statement here,

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‫because if we didn't have this return here,

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‫well, then express would send this response back

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‫but it would still continue running

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‫the code in this function.

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‫And so after sending the response,

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‫it will then still hit this next function

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‫and it would move on to the next middleware

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‫and will then send another response to the client.

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‫But that is really not allowed,

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‫so remember that we actually run into this error before,

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‫where it told us that we were not allowed

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‫to send headers after the response had already been sent.

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‫And so that's the kind of error

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‫that we would run into if we didn't

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‫have this return statement, okay.

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‫So again, just make sure that after sending this response,

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‫the function will return so that it will finish

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‫and it will never call this next.

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‫So never forget that, but of course

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‫we're gonna do this multiple times

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‫throughout the rest of the course

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‫and so you will get used to this kind of pattern.

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‫So let's go ahead and remove this repeated code

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‫from all of these other functions

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‫so here we have it again, and yeah.

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‫I also want this code here just to make sure

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‫that the function's actually running,

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‫and okay, so now we can get rid of this here

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‫and replace it with our newly created controller function.

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‫So tourController, checkID,

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‫so let's check that out now,

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‫and well again, just to make sure

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‫that it doesn't run where we

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‫do not have an id, so we have no log here,

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‫so everything is still working on that side.

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‫And now get tour with a regular id,

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‫so a valid id, and so let's see.

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‫Well, we get the Tour id is log,

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‫and so that means if it did actually run

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‫our checkID middleware, right?

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‫And if we now try an invalid id,

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‫well then we get our Invalid ID message,

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‫the 404 error code, and of course our tour id.

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‫Let's do the same with the patch,

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‫we already have an invalid ID here

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‫and so it also runs on this one.

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‫So, perfect, right?

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‫So we no longer have the checkID code

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‫in the update handler that we just invoked basically,

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‫but still our ID is checked because

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‫we have that middleware, so this here.

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‫We have that middleware in the stack

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‫before it actually hits the update tourController.

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‫So this middleware is now part of our pipeline

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‫as you can imagine it, now you might argue

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‫that we might simply create a simple function

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‫which could also check for the ID

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‫and I call that function inside of

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‫each of these tour function, and then call it inside

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‫each of these relevant tour controllers;

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‫but that would really go against

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‫the philosophy of express, where we should always work

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‫with the middleware stack, so with this pipeline

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‫as much as we can, okay, and so these functions here,

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‫they do not have to worry at all about validation.

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‫Each of these functions has only one purpose

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‫which is to do what they say,

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‫so this one just gets the tour,

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‫this one just creates a tour, this one just updates,

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‫and this one just deletes, it doesn't check,

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‫it doesn't have to worry about any of that.

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‫And if we would now add another controller here

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‫also depending on the id, well then that would automatically

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‫also check if the id is invalid

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‫without us having to do any additional steps.

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‫So this automatically will check for the id

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‫and so that's really handy and also how

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‫express apps should work, great.

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‫So we have another tool in our express toolbox that

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‫we can now use in order to write our express applications.