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Before we continue to implement the rest of the functionalities for our API, let's handle any errors

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that might occur in our application.

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If you have taken some of my courses, you can see that I use a third party package like Express Async

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Handler to do that.

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But for this we are going to create our custom error handler from scratch.

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So let's get going inside the server file.

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You can see we have a comment here called error handlers and you need to create your error handling

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below all your route.

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And experts make it pretty easy to be able to handle errors.

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We bring in app dot use and for this we pass in a callback function as that and inside the callback

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function we have access to four domains.

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The first one is the error request response and then next.

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So in the error that might work in our application, we have access on this object core error and the

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request contains the request object and then the response contains the response object and then the

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next function.

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So in any of our route, if we throw an error, we are going to have access to that error inside this

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object.

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On the error object, we have these properties.

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One is the actual message about the error.

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The next property is what we call the status.

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And then after that we also have what is called the status code.

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And lastly, we also have what is called the stack.

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The stack determines which file that a particular error OC and what line of code that that error.

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And I will show you as you move on.

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So for any error that might occur in our application, we are going to receive these properties.

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So we are going to create a function on its own.

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And then we require it and use it inside this app to use because we want to keep our server file with

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less code.

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So because of that, let's cut this one.

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And next is let's go to the folders here and let's create one folder called middleware.

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Here we go for the middle, where I'm going to have a couple of them.

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But for now, let's create one for error handling and I'll call this one as global error handler dot

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JS and here our paste what I copied and I need to assign to a function called context as global error

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handler is equal to that.

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So how are we going to send these messages?

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So let's see on the error because it's an object, we can add our own properties to the objects.

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So first of all, we are going to add a property called status code and then it's equal to error dot

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status code.

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If there is one, then go ahead and use it.

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Otherwise use what is called 500 and next case.

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Let's add a property called status.

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Equal to error dot status if there is one.

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Go ahead and use it.

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Otherwise we are going to provide our message.

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See error.

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It means that when you throw an error and we don't provide these values, we are going to use this default

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values.

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So next step is let's send the response as rez dot status.

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And for this one, we are going to use the error dot status code so we can assign to a variable cost

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as status code.

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And the same as the status cons as status is equal to that.

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So on the rest status, we are going to use the status code in case a user provide upon throwing an

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error.

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And after that we are going to use the JSON to send the message to the user.

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And here we go.

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On the status, we're going to use this message.

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And then for the message, it's going to be as error dot message.

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Or we can even assign to evacuate here as concert message is equal to error dot message.

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So here I can remove this one.

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Then this property that we want to send is going to be the stock trace.

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So we call this one as stuck and we have access to that on the error dot stock and our pass in here

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as the stock.

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So the next is let's export this middleware from this file as equal to global arrow handler.

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So inside our server file, let's go ahead and then require that middleware as global arrow handler.

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And here we go.

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Perfect.

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So now let's pass it inside this arrow middleware here, and that is it.

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Now we have configured our global middleware to receive any error that we are going to throw with this

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implementation.

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I can now throw or catch any error that might be OC.

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So let's get to the controllers and let's check it out.

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So let's go to the user's controllers and here we go.

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You can see that here we are checking if a user is found and if either is found, it means that this

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email exists.

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In that way.

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We are sending this arrow, but this is not the right way of sending arrow.

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That's why we have configured our global arrow handler.

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So let's see how we are going to throw an arrow to achieve that.

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We are going to pass next to the handler as that.

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So any error that might work, we are going to pass through the next middleware and then the next middleware

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will not handle to our global arrow handler.

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So let's check it out.

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So here, if a user is found, let's throw an arrow.

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So I would like to use this same message.

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But for this, I'm going to cut everything from here and let me break my cable here.

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So here we are going to connect and then you are going to pass in an arrow and together one are going

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to create instance of the error class.

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And for this we pass in the message, remember, we need to pass in the message and then the status

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code and then the status as well.

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But for this, let's pass in this message as that.

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So let's remove this one, let's save it.

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And now let's check it out.

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So inside the tender client, this user has been registered already.

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So as soon as I hit send and there we go, this is a valid error case.

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And in that we have a status code here which is 500 meaning internal server error and this is a message

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or the properties that we pass into the global arrow handler.

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The first one is a status as we specify.

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Now let me show you where it is inside the middleware.

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This is where we are referring to.

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So if a user refuses to pass in the status, we are going to use this one.

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And the next one is the message.

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And the message is what we pass in upon calling next as that.

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And then this one going to be the stack trace, which is this one and here is the stack trace.

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And for a stack trace it gives us more details about the error, where it works and which file.

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So if you read the file, this is where it works on the user controller line number 11, that is where

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the error occurred.

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So in our route we can throw any arrow and also we can catch any error that might be OC inside the cache

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block.

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So let's see for the registration, let me collapse this one.

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And before I continue, we need to return from this function so that the function will exit.

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Good.

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So the next point is that inside the cache we can code next and pass in the error.

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So now here we go.

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We call next and next is we pass a new and then the error class and you pass in the entire error.

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So now we are also handling any error that might be okay from MongoDB.

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So let's go ahead and then remove these properties and now let's hit st and let's see the error and

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this is a message we have which is status quo 200 main everything is okay, meaning that where we are

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throwing this error, which is this part, meaning that a user needs to provide a valid data so this

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is the right way of handling errors.

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So let's remove this one now.

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Let's go ahead and make the request again.

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And let's send and.

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Here we go.

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And this is a status code 500.

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Which means that is a valid error case.

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Let's also handle this arrow nicely.

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So I would like to use the same message.

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So here I'm going to remove the response and I'm going to bring in the next middleware and pass in new.

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And then the error.

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Then I'll pass in the message.

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All right.

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But we have one problem.

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We can see that any time you want to send an arrow, we need to use new arrow than it has in the message,

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and we can pass in the status code as well.

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So we can refactor this one by creating a class or a function for this.

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So let's get that one done.

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So inside the utils, you are going to create a file called App Error.

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Dark jazz, and we can create a class or a function for this.

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For the meantime, let me show you how we are going to create a class for that as app error, and then

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we are going to extend the error class from JavaScript as that.

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And inside here we are going to call the constructor function.

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Then you pass in the message and then the status code.

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Great.

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And inside that we are going to call support and you pass in the message.

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And lastly, we need to add the status code as this DOT status code is equal to the status code.

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And then for the status tool, we can add as failed or error.

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So here we go.

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This dot status is equal to fail grade.

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Now this is our error class, so I can go ahead and then export this one as module dot export.

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For this one to.

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I can create a function for this one instead of using a class.

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So if you're not familiar with classes in JavaScript, then you can use the function way.

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So here we go.

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Const as app error is equal to a function and this function are going to pass in the message and then

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they start as code and here we go.

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So we're going to create an instance of error for JavaScript error class as equal to name and error.

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Then you pass in the message.

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Remember, the error class accepts a single core message.

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That's why we pass in the message to the super of our class.

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Then on the object I can add our custom messages to it.

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So what we want to add is going to be the status code as error dot status code is equal to status code.

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So let me change this one to let.

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All right.

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So lastly, I need to retain the entire error object from this function and I can export it as that.

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Great.

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So now inside the users root, let's require the error class and then the app error function.

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So first let's use the error class.

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So here we go.

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So now we are going to refactor this one a little bit by using the class.

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So here we go.

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I would like to use the same message.

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So I'll cut this one and I remove everything from here.

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Remember, because it's a class we need to use new keyword here and then the app error.

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And for this one you pass it the message and then the status code as 400.

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So now with this one let's send the properties as full name is equal to Aram and then the email as Aram

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adds gmail.com and then the password.

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It's going to be one, two, three, four, five.

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So let's hit send.

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And you can see that we have the same message.

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But instead you provided our custom status code as 400 and everything remains the same.

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So with this one, if I refuse to provide the email, let's see, let's send the requests.

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And you can see that we have the error message from MongoDB.

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So this is one way of using it.

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Or if we want to use the function to why not, we can go ahead and then use it.

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So let's see how we can use the function or so for that one.

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We are going to remove the new and use the function and everything remains the same.

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So let's save it.

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And now let's make the request.

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And you can see everything remains the same.

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So let's bring back the email also and let's check and here you go.

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User already exists, but just so we can improve the code a little bit.

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But I think this one is okay for our likeness.

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So for this case, we need to pass in three arguments because the way we are handling errors in our

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application.

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So let's look through inside the log in.

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Also, we have some use cases here where we are sending a message which is an error.

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So here I would like to use the same message here.

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So let's remove this one and let's break next.

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And for the next I'm going to use the error plus and that app error.

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Then I'll pass in the message and then the status code ask 400.

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And for this one too, we are going to use the same error case and down here we are going to use next

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and then I'm going to pass in the error.

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I start and there we go.