file-processing

name	file-processing
description	Handle file upload, validation, processing pipelines, and temporary file management in Node.js servers. Use when building file processing APIs with Express and multer, managing temporary files with cleanup, or implementing secure file handling patterns. Triggers include "file upload", "multipart form", "temp file cleanup", "multer config", or any server-side file handling task.

file-processing

Patterns for secure file upload, processing, and cleanup in Node.js Express servers.

When to use

• Configuring multer for file uploads with type and size validation
• Managing temporary files with automatic cleanup
• Building file processing pipelines (upload, process, download)
• Streaming file downloads without blocking
• Implementing secure file storage with non-guessable paths

Setup: multer with validation

import multer from 'multer';
import path from 'node:path';
import crypto from 'node:crypto';

const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    const jobId = crypto.randomUUID();
    const dir = path.join(process.env.TEMP_DIR ?? './tmp', jobId);
    fs.mkdirSync(dir, { recursive: true });
    (req as any).jobId = jobId;
    cb(null, dir);
  },
  filename: (req, file, cb) => {
    cb(null, `input-${Date.now()}.pdf`);
  },
});

const fileFilter = (req: Request, file: Express.Multer.File, cb: multer.FileFilterCallback) => {
  if (file.mimetype !== 'application/pdf') {
    cb(new Error('Only PDF files are accepted'));
    return;
  }
  cb(null, true);
};

export const upload = multer({
  storage,
  fileFilter,
  limits: {
    fileSize: (parseInt(process.env.MAX_UPLOAD_MB ?? '100')) * 1024 * 1024,
  },
});

Temp file cleanup

import fs from 'node:fs';
import path from 'node:path';

const TEMP_DIR = process.env.TEMP_DIR ?? './tmp';
const TTL_MS = (parseInt(process.env.JOB_TTL_MINUTES ?? '60')) * 60 * 1000;

export function scheduleCleanup(): void {
  setInterval(() => {
    const entries = fs.readdirSync(TEMP_DIR, { withFileTypes: true });
    for (const entry of entries) {
      if (!entry.isDirectory()) continue;
      const metaPath = path.join(TEMP_DIR, entry.name, 'meta.json');
      if (!fs.existsSync(metaPath)) continue;
      const meta = JSON.parse(fs.readFileSync(metaPath, 'utf8'));
      if (Date.() -  (meta.).() > ) {
        fs.(path.(, entry.), { :  });
      }
    }
  },  *  * ); 
}

Streaming download

app.get('/api/download/:token', (req, res) => {
  const jobDir = path.join(TEMP_DIR, req.params.token);
  const metaPath = path.join(jobDir, 'meta.json');

  if (!fs.existsSync(metaPath)) {
    res.status(404).json({ error: 'File not found or expired' });
    return;
  }

  const meta = JSON.parse(fs.readFileSync(metaPath, 'utf8'));
  const outputPath = path.join(jobDir, 'output.pdf');

  res.setHeader('Content-Disposition', `attachment; filename="${meta.filename}"`);
  res.setHeader('Content-Type', 'application/pdf');

  const stream = fs.createReadStream(outputPath);
  stream.pipe(res);

  // Delete after streaming
  stream.on('end', () => {
    fs.rmSync(jobDir, { recursive: true });
  });
});

Security checklist

• Validate MIME type AND file extension (not just MIME)
• Name output files by UUID, not user-supplied names
• Set Content-Disposition on downloads (prevent inline execution)
• Limit upload size via multer limits
• Never log file contents; log only metadata (size, page count)
• Clean temp files after download and on TTL expiry