File Upload issue in NestJs

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Summary

The issue encountered is related to file upload in a NestJS application, where the @UploadedFile() decorator returns undefined. This problem occurred when attempting to upload multiple files and then simplified to a single file upload, with the same undefined result.

Root Cause

The root cause of this issue lies in the way the file upload is handled in the controller. The main reasons are:

  • Incorrect usage of the @UploadedFile() decorator
  • Insufficient handling of the multipart/form-data request
  • Lack of proper configuration for the FileInterceptor

Why This Happens in Real Systems

This issue can occur in real systems due to:

  • Misunderstanding of the NestJS framework’s file upload mechanism
  • Inadequate error handling and logging
  • Insufficient testing of the file upload feature
  • Incorrect configuration of the multer middleware

Real-World Impact

The impact of this issue can be significant, including:

  • Failed file uploads: Users may not be able to upload files, leading to a poor user experience
  • Data loss: Files may be lost or corrupted during the upload process
  • Security vulnerabilities: Incorrect handling of file uploads can lead to security vulnerabilities, such as arbitrary file upload

Example or Code

import { Controller, Post, Req, Res, UploadedFile, UseInterceptors } from '@nestjs/common';
import { FileInterceptor } from '@nestjs/platform-express';
import { diskStorage } from 'multer';

@Controller('pdf')
export class PdfController {
  @Post('merge')
  @UseInterceptors(FileInterceptor('file', {
    storage: diskStorage({
      destination: './uploads',
      filename: (req, file, callback) => {
        callback(null, file.originalname);
      }
    })
  }))
  async mergePdfs(@UploadedFile() file: Express.Multer.File) {
    console.log(file);
    // Process the uploaded file
  }
}

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Correctly configuring the FileInterceptor: Ensuring that the FileInterceptor is properly configured to handle the file upload
  • Using the correct decorator: Using the @UploadedFile() decorator correctly to retrieve the uploaded file
  • Implementing proper error handling: Handling errors and exceptions that may occur during the file upload process
  • Testing the file upload feature: Thoroughly testing the file upload feature to ensure it works as expected

Why Juniors Miss It

Juniors may miss this issue due to:

  • Lack of experience: Limited experience with the NestJS framework and file upload mechanisms
  • Insufficient knowledge: Inadequate understanding of the multer middleware and its configuration
  • Inadequate testing: Not thoroughly testing the file upload feature, leading to unexpected errors and issues
  • Poor error handling: Not implementing proper error handling and logging, making it difficult to diagnose and fix issues