File validation pipe in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct decorator for file upload in NestJS.

NestJS

import { [1] } from '@nestjs/common';

Drag options to blanks, or click blank then click option'

AUploadedFile

BUseInterceptors

CController

DGet

Attempts:

3 left

💡 Hint

Common Mistakes

Using @UseInterceptors instead of @UploadedFile

Importing from wrong package

Confusing with @Controller or @Get decorators

The @UploadedFile() decorator is used to access the uploaded file in a controller method.

2fill in blank

medium

Complete the code to apply a file validation pipe in the controller method parameter.

NestJS

uploadFile(@UploadedFile(new [1]()) file: Express.Multer.File) { return file; }

Drag options to blanks, or click blank then click option'

AFileValidationPipe

BValidationPipe

CParseFilePipe

DParseIntPipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using ValidationPipe which does not handle files

Using a non-existent FileValidationPipe

Using ParseIntPipe which is for numbers

The ParseFilePipe is the built-in NestJS pipe for validating uploaded files.

3fill in blank

hard

Fix the error in the pipe options to allow only PNG files.

NestJS

new ParseFilePipe({ validators: [ new FileTypeValidator({ fileType: '[1]' }) ] })

Drag options to blanks, or click blank then click option'

Aimage/jpeg

Bimage/png

Capplication/pdf

Dtext/plain

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'image/jpeg' which allows JPEG files

Using 'application/pdf' which allows PDFs

Using 'text/plain' which allows text files

The FileTypeValidator expects the MIME type string. For PNG files, it is image/png.

4fill in blank

hard

Fill both blanks to set a max file size of 1MB and allow only JPEG files.

NestJS

new ParseFilePipe({ validators: [ new FileTypeValidator({ fileType: '[1]' }), new MaxFileSizeValidator({ maxSize: [2] }) ] })

Drag options to blanks, or click blank then click option'

Aimage/jpeg

B1048576

Cimage/png

D2097152

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'image/png' instead of 'image/jpeg'

Setting maxSize to 2097152 which is 2MB

JPEG files have MIME type image/jpeg. 1MB in bytes is 1048576.

5fill in blank

hard

Fill all three blanks to create a custom validation pipe that checks file size and type.

NestJS

export class CustomFileValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    if (!file) throw new BadRequestException('No file uploaded');
    if (file.mimetype !== '[1]') throw new BadRequestException('Invalid file type');
    if (file.size > [2]) throw new BadRequestException('File too large');
    return file;
  }
}

// Usage: @UploadedFile(new [3]())

Drag options to blanks, or click blank then click option'

Aimage/png

B1048576

CCustomFileValidationPipe

Dimage/jpeg

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'image/png' instead of 'image/jpeg'

Using wrong max size value

Not using the custom pipe class name in the decorator

The custom pipe checks for JPEG files (image/jpeg) and max size 1MB (1048576 bytes). It is used by passing its class name.

Practice

1. What is the main purpose of a FileValidationPipe in NestJS?

easy

A. To upload files to a cloud storage service

B. To convert files into JSON format automatically

C. To check uploaded files for type, size, and presence before processing

D. To compress files before saving them

Solution

1. Step 1: Understand the role of validation pipes in NestJS

   Validation pipes are used to check and validate incoming data before it reaches the controller logic.

2. Step 2: Apply this to file uploads

   A FileValidationPipe specifically checks uploaded files for correct type, size, and presence to prevent bad data.

3. Final Answer:

   To check uploaded files for type, size, and presence before processing -> Option C

4. Quick Check:

   File validation = check file properties [OK]

Hint: Validation pipes check data early to keep apps safe [OK]

Common Mistakes:

• Thinking pipes upload or transform files
• Confusing validation with file storage
• Assuming pipes compress files automatically

2. Which of the following is the correct way to apply a file validation pipe to a file upload in a NestJS controller?

easy

A. @UsePipes(new FileValidationPipe()) uploadFile(@UploadedFile() file: Express.Multer.File) {}

B. @UseGuards(new FileValidationPipe()) uploadFile(@UploadedFile() file: Express.Multer.File) {}

C. @UseFilters(new FileValidationPipe()) uploadFile(@UploadedFile() file: Express.Multer.File) {}

D. @UseInterceptors(new FileValidationPipe()) uploadFile(@UploadedFile() file: Express.Multer.File) {}

Solution

1. Step 1: Identify the correct decorator for validation pipes

   In NestJS, validation pipes are applied using the @UsePipes() decorator.

2. Step 2: Match the usage with file upload

   The FileValidationPipe is a pipe, so it must be passed inside @UsePipes() to validate the uploaded file.

3. Final Answer:

   @UsePipes(new FileValidationPipe()) uploadFile(@UploadedFile() file: Express.Multer.File) {} -> Option A

4. Quick Check:

   Validation pipe = @UsePipes() [OK]

Hint: Use @UsePipes() for validation pipes, not guards or filters [OK]

Common Mistakes:

• Using @UseGuards or @UseFilters instead of @UsePipes
• Applying pipes without instantiating them
• Confusing interceptors with pipes

3. Given this pipe code snippet, what will happen if a user uploads a file larger than 1MB?

export class FileValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    if (!file) throw new BadRequestException('File is required');
    if (file.size > 1_000_000) throw new BadRequestException('File too large');
    return file;
  }
}

medium

A. The pipe ignores the size and processes the file

B. The pipe accepts the file and returns it

C. The pipe throws a NotFoundException

D. The pipe throws a BadRequestException with message 'File too large'

Solution

1. Step 1: Analyze the size check in the transform method

   The pipe checks if file.size is greater than 1,000,000 bytes (1MB).

2. Step 2: Understand the exception thrown on size violation

   If the file is too large, it throws BadRequestException('File too large'), stopping the request.

3. Final Answer:

   The pipe throws a BadRequestException with message 'File too large' -> Option D

4. Quick Check:

   File size > 1MB = BadRequestException [OK]

Hint: Check exception messages in transform() for validation errors [OK]

Common Mistakes:

• Assuming the pipe accepts large files
• Confusing BadRequestException with NotFoundException
• Ignoring the file presence check

4. Identify the error in this file validation pipe code:

export class FileValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    if (!file) throw new BadRequestException('File is required');
    if (file.mimetype !== 'image/png' || file.mimetype !== 'image/jpeg') {
      throw new BadRequestException('Invalid file type');
    }
    return file;
  }
}

medium

A. The mimetype check always throws because of incorrect logical operator

B. The file presence check is missing

C. The pipe does not return the file

D. The exception type is incorrect

Solution

1. Step 1: Examine the mimetype condition

   The condition uses || (OR) between two checks: file.mimetype !== 'image/png' OR file.mimetype !== 'image/jpeg'.

2. Step 2: Understand why this always triggers

   Because a file cannot be both 'image/png' and 'image/jpeg' at the same time, one of these will always be true, causing the exception every time.

3. Final Answer:

   The mimetype check always throws because of incorrect logical operator -> Option A

4. Quick Check:

   Use AND (&&) to check mimetype correctly [OK]

Hint: Use && to check if mimetype is neither png nor jpeg [OK]

Common Mistakes:

• Using || instead of && in mimetype checks
• Forgetting to return the file after validation
• Throwing wrong exception types

5. You want to create a file validation pipe that only accepts PDF files smaller than 2MB and throws a clear error if validation fails. Which of the following code snippets correctly implements this?

hard

A. export class PdfValidationPipe implements PipeTransform {
transform(file: Express.Multer.File) {
if (!file) throw new BadRequestException('File is required');
if (file.mimetype === 'application/pdf') throw new BadRequestException('Only PDFs allowed');
if (file.size > 2_000_000) throw new BadRequestException('File too large');
return file;
}
}

B. export class PdfValidationPipe implements PipeTransform {
transform(file: Express.Multer.File) {
if (!file) throw new BadRequestException('File is required');
if (file.mimetype !== 'application/pdf') throw new BadRequestException('Only PDFs allowed');
if (file.size >= 2_000_000) throw new BadRequestException('File too large');
return file;
}
}

C. export class PdfValidationPipe implements PipeTransform {
transform(file: Express.Multer.File) {
if (!file) throw new BadRequestException('File is required');
if (file.mimetype !== 'application/pdf' && file.size > 2_000_000) throw new BadRequestException('Invalid file');
return file;
}
}

D. export class PdfValidationPipe implements PipeTransform {
transform(file: Express.Multer.File) {
if (!file) throw new BadRequestException('File is required');
if (file.mimetype !== 'application/pdf' || file.size > 2_000_000) throw new BadRequestException('Invalid file');
return file;
}
}

Solution

1. Step 1: Verify correct logic for presence and mimetype

   Check if (!file) throw, then if (file.mimetype !== 'application/pdf') throw 'Only PDFs allowed'. Uses !== to reject non-PDFs.

2. Step 2: Separate size check for smaller than 2MB

   if (file.size >= 2_000_000) throw 'File too large'. Uses >= to reject 2MB or larger.

3. Step 3: Eliminate distractors

   B throws if PDF; C's && accepts non-PDF if small; D's > accepts exactly 2MB.

4. Final Answer:

   export class PdfValidationPipe implements PipeTransform { transform(file: Express.Multer.File) { if (!file) throw new BadRequestException('File is required'); if (file.mimetype !== 'application/pdf') throw new BadRequestException('Only PDFs allowed'); if (file.size >= 2_000_000) throw new BadRequestException('File too large'); return file; } } -> Option B

5. Quick Check:

   !== mimetype, >= size, separate ifs [OK]

Hint: Check each condition separately with !== and clear exceptions [OK]

Common Mistakes:

• Using === instead of !== for mimetype check
• Combining conditions incorrectly with && or ||
• Not throwing specific error messages