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In this video, let's implement what is called validation.

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For example, if we are accepting only images by the end, user is trying to upload a PDF file or if

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we are accepting a specific size of a file, let's say one megabyte, but a user is trying to upload

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ten megabytes.

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How are we going to prevent that?

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Okay.

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Let me show you inside the motor configuration here.

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We are going to pass an additional property called Limits.

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And for this one, we are going to tell our package that, hey, we want to accept a file with specific

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file limit.

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So here you provide file size and specify the size of the file that you want to upload.

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In our case, we want to accept a file, not more than one megabytes.

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So here we go.

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You provide one and then six zeros, and this is equivalent to one megabytes.

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So with this one, let's go ahead and then upload a file which is more than one megabytes.

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Inside a folder, you're going to have an image which is three megabytes.

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So let's see if you're going to get some error.

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So now back to Postman.

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Let's go ahead and select the file called Large Image and let me send and you can see that more error

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that is file two large.

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You see how we are preventing that.

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Now, this is one use case.

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Another use case is how can we prevent a specific file type?

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So let's say that we are receiving all the images, but a user is trying to upload a PDF file.

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How are we going to prevent that?

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Let's see.

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Inside here we are going to pass an additional attribute called file filter as a function call.

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And is that inside the function?

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We have access to the requests, the file and then the callback.

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The request contains the request objects and the file contains the file the user is trying to upload.

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And the callback is a function that we are going to call if the upload was successful or not.

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So for the meantime, let's accept only images with the file extension call JPEG or P and G.

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So now how are we going to get it inside this function?

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We have access to the file the user is uploading on the file.

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So if you console log file like that, let's see what you're going to get.

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Now let's get back to Postman and upload a file which is less than one megabytes.

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And I get that if I check my terminal, you can see that I have access to the object.

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Now we can check whether the file is PDF or PNG or JPEG.

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So now back to our function here.

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So inside this function, let's say we are accepting.

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So how can we implement that?

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We are going to use JavaScript to get it done.

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Before we continue on, let me show you how we are going to call callback here.

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We are going to call callback twice.

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First is we are going to call when everything is successful and then we are going to call if there are

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some errors.

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Now let's start with the success.

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One for the success we are going to call callback with the first arguments being null, meaning that

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there is no error.

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Then then the second arguments are going to be true, meaning proceed to the next module and then if

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there is some error, we are going to call callback.

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And the first element is going to be a text that we want to specify for this one.

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Let's say that please upload image with extension dot P and G.

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Now let's see how we are going to implement that.

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So let's comment this one.

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We will come back to it very soon.

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So let's check the conditions here before calling the callback.

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So now have a look.

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We are going to make use of if statements and how can we get the file.

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Remember, we can get the file on this property and we be able to know the extension on what is called

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the original name.

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We can see that we have dot pdf so now we can make use of this one to check the file type a user is

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uploading.

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So on the original name we are going to make use of and way.

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So file dot original name dot ends with if it ends with with extension dot, p and g, then let's see

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how we are going to call the callback.

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We are going to call this success.

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So our code is one and put it here and remove the comments.

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And then this one I start.

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What about error?

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They're going to make use of errors.

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And next is I'll cut this one and then put it inside the errors as that.

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Good.

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So now let's have a look.

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Well, we need to finish up this one.

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It takes into argument the first one going to be the message and then the next one going to be as false.

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Meaning there is an error, so don't proceed to the next minute.

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So now we are done with the implementation.

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Let's get back to Postman and now we are sending Pdaf.

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So let's see.

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Now you can see that.

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Please upload image with extension dot p and g.

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If we change the extension here to let's say JPEG, let's say JPG, let's select this image that is

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dot jpg and let's say we can see that upload successful.

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And if I check my folder, you can see that I have all the images being uploaded for this one.

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We can add the extension to it, but unless we write some code, get it done.

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But like I said, you are going to make the saving or you are going to store the images into cloud nine.

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But to be able to understand file upload, you need to know how more thought works.

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That is why I'm showing this code.

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Good.

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So let's say that we also want to support, let's say, PNG and JPEG.

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We can make use of all and then provide the file our original name ends with and then let's say P and

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G.

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So here we can pass in the type of file that you want to support and you'll be good to go.

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So this is the basic flow of this, how Malta works.

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In the next video, I'll show you how we can make it simpler ordinary to upload file easily as compared

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to this one.

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But understanding this one will help you how file upload works in express.