Secure File Upload to Cloud Storage via Pre-Signed URLs

// --- Node.js Backend Snippet (Example using AWS SDK for S3) ---
const AWS = require('aws-sdk');
const express = require('express');
const app = express();
const port = 3001;

// Configure AWS S3 (ensure AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION are set as env vars)
const s3 = new AWS.S3({ signatureVersion: 'v4' });
const BUCKET_NAME = 'your-unique-s3-bucket-name'; // Replace with your S3 bucket name

app.get('/generate-upload-url', (req, res) => {
  const filename = req.query.filename;
  const filetype = req.query.filetype;

  if (!filename || !filetype) {
    return res.status(400).send('Filename and filetype are required.');
  }

  const s3Params = {
    Bucket: BUCKET_NAME,
    Key: `uploads/${Date.now()}-${filename}`, // Path in S3
    Expires: 60, // URL expires in 60 seconds
    ContentType: filetype,
    ACL: 'public-read' // Or private, depending on your use case
  };

  s3.getSignedUrl('putObject', s3Params, (err, url) => {
    if (err) {
      console.error('Error generating S3 pre-signed URL:', err);
      return res.status(500).send('Error generating upload URL.');
    }
    res.send({ uploadUrl: url, fileKey: s3Params.Key });
  });
});

app.listen(port, () => {
  console.log(`Backend for pre-signed URLs running on http://localhost:${port}`);
});

// --- Frontend JavaScript Snippet (React example, but transferable) ---
// <input type="file" id="fileInput" />
// <button onclick="uploadFile()">Upload</button>

async function uploadFile() {
  const fileInput = document.getElementById('fileInput');
  const file = fileInput.files[0];

  if (!file) {
    alert('Please select a file first.');
    return;
  }

  try {
    // 1. Get a pre-signed URL from your backend
    const response = await fetch(`http://localhost:3001/generate-upload-url?filename=${file.name}&filetype=${file.type}`);
    if (!response.ok) {
      throw new Error('Failed to get pre-signed URL.');
    }
    const { uploadUrl, fileKey } = await response.json();

    // 2. Upload the file directly to S3 using the pre-signed URL
    const uploadResponse = await fetch(uploadUrl, {
      method: 'PUT',
      headers: {
        'Content-Type': file.type,
      },
      body: file,
    });

    if (!uploadResponse.ok) {
      throw new Error('Failed to upload file to S3.');
    }

    console.log('File uploaded successfully to S3!', fileKey);
    alert('File uploaded successfully!');
    // You might want to send fileKey back to your backend to record the upload
  } catch (error) {
    console.error('Upload error:', error);
    alert(`Upload failed: ${error.message}`);
  }
}