File validation pipe in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a file larger than the allowed size is uploaded?

Consider a NestJS file validation pipe that limits file size to 1MB. What is the behavior when a user uploads a file of 2MB?

NestJS

import { Injectable, PipeTransform, BadRequestException } from '@nestjs/common';

@Injectable()
export class FileSizeValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    const maxSize = 1 * 1024 * 1024; // 1MB
    if (file.size > maxSize) {
      throw new BadRequestException('File size exceeds limit');
    }
    return file;
  }
}

A The pipe throws a BadRequestException with message 'File size exceeds limit'.

B The pipe returns null and the upload is ignored.

C The pipe silently accepts the file and returns it without error.

D The pipe logs a warning but allows the file to pass through.

💡 Hint

Think about what happens when the file size condition fails inside the pipe.

The pipe checks the file size and throws a BadRequestException if the file is too large, stopping the request with an error.

📝 Syntax

intermediate

Identify the syntax error in this file validation pipe code

Which option contains a syntax error preventing the file validation pipe from compiling?

NestJS

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class FileTypeValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    const allowedTypes = ['image/png', 'image/jpeg'];
    if (!allowedTypes.includes(file.mimetype)) {
      throw new BadRequestException('Invalid file type');
    }
    return file;
  }
}

A if (!allowedTypes.includes(file.mimetype)) { throw new BadRequestException('Invalid file type'); }

B if (!allowedTypes.includes(file.mimetype)) throw new BadRequestException('Invalid file type');

C if (!allowedTypes.includes file.mimetype) { throw new BadRequestException('Invalid file type'); }

D if (!allowedTypes.includes(file.mimetype)) { throw new BadRequestException('Invalid file type'); } else return file;

💡 Hint

Check the syntax of the includes method call.

Option C is missing parentheses around the argument to includes, causing a syntax error.

❓ state_output

advanced

What is the output when the file validation pipe returns the file object?

Given a file validation pipe that returns the file object after validation, what will be the value passed to the controller handler?

NestJS

import { Injectable, PipeTransform } from '@nestjs/common';

@Injectable()
export class FileValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    // Assume validation passed
    return file;
  }
}

// Controller handler example
async uploadFile(@UploadedFile(new FileValidationPipe()) file: Express.Multer.File) {
  return { filename: file.originalname, size: file.size };
}

A undefined

B { filename: 'uploaded_file.png', size: 2048 } // file info object

C An error is thrown because the pipe does not return a value

D null

💡 Hint

The pipe returns the file object, so the controller receives it.

The pipe returns the validated file object, so the controller gets the file with its properties.

🔧 Debug

advanced

Why does this file validation pipe not catch invalid file types?

This pipe is intended to reject files not of type 'image/png' or 'image/jpeg', but it lets invalid files pass. Why?

NestJS

import { Injectable, PipeTransform, BadRequestException } from '@nestjs/common';

@Injectable()
export class FileTypeValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    const allowedTypes = ['image/png', 'image/jpeg'];
    if (allowedTypes.indexOf(file.mimetype)) {
      return file;
    }
    throw new BadRequestException('Invalid file type');
  }
}

A The file.mimetype property is undefined, so the check fails silently.

B The allowedTypes array is empty, so no types are allowed.

C The pipe does not throw any error because transform is not called.

D indexOf returns -1 for missing items, which is truthy, so the condition is wrong.

💡 Hint

Check how indexOf behaves when the item is not found.

indexOf returns -1 when the item is not found, which is truthy in JavaScript, so the if condition incorrectly passes for invalid types.

🧠 Conceptual

expert

What is the main advantage of using a file validation pipe in NestJS?

Why should you use a dedicated file validation pipe instead of validating files directly inside controller methods?

A It centralizes validation logic, making code cleaner and reusable across multiple routes.

B It automatically compresses files before saving them.

C It prevents the need to import Multer in the controller.

D It allows files to be uploaded without any size or type restrictions.

💡 Hint

Think about code organization and reuse.

Using a pipe keeps validation separate from business logic, making the code easier to maintain and reuse.