Client-Side Multi-Download Technology Demonstrator

💡 What does this do?

This application demonstrates client-side file download capabilities:

📋 Fetches a file list from a remote server (https://testfiles.xxip.de/index.txt)
✅ Offers a user-friendly interface for selecting files (could be changed to automatically download a server-side generated file list)
📦 When downloading:

🏷️ Automatically renames files with prefixes (single & multiple files)
🗜️ Bundles multiple selected files into ZIP archives
⚡ Uses modern streaming APIs for memory efficiency
🚀 No backend required – all logic runs in your browser!

📦 Repository: GitHub

🔬 Demonstrated Technologies ▼ Click to expand

This project demonstrates advanced client-side browser capabilities:

Stream Processing: Memory-efficient handling of large files using ReadableStream API
On-the-fly ZIP Compression: Real-time file packing with fflate library
File Renaming: Automatic "singlefile_" prefix for single files, "batchload_" prefix for multiple files in ZIP
Download Streaming: Direct-to-disk writing via StreamSaver.js or File System Access API
Single File Optimization: Bypasses ZIP creation for single file downloads (direct streaming)
Intelligent Fallback: Automatic method selection: File System API → StreamSaver → Blob
User Control: Manual download method selection and force fallback option
Page Protection: Prevents accidental closure during active downloads

💡 Fallback Strategy Discussion: The current blob fallback method (used when streaming fails) has high memory usage and may fail with large files. An alternative approach is to skip ZIP creation entirely in fallback scenarios and instead download each file individually with direct browser downloads. This would eliminate memory constraints and improve reliability for large batch downloads, though users would receive multiple separate files instead of a single ZIP archive. Important CORS limitation: Due to browser security policies, cross-origin downloads cannot be renamed via the download attribute—files will retain their original names instead of receiving sequential prefixes like batchload_001_filename.ext. To enable file renaming, files would need to be loaded into memory (blob), which defeats the purpose of this low-memory fallback strategy. The current implementation prioritizes reliability over naming consistency for cross-origin batch downloads.

⚠️ Not Yet Implemented: Page-independent service worker for background downloads (would allow page navigation during download)

⚙️ Download Options

Use File System Access API (Ask where to save)

When enabled and available, shows a dialog to choose where to save the file. When disabled or unavailable, starts streaming download directly.

Blob Fallback Strategy

If streaming methods fail or are unavailable, the system will use blob fallback. Choose your preferred strategy:

Download files individually (no ZIP) - Safest option

Downloads each file separately with prefix fallback_1_, fallback_2_, etc. Avoids memory issues completely. Recommended for large batches.

Automatic: Individual if > 101 MiB, otherwise ZIP

Smart fallback: Uses individual downloads for large batches (>101 MiB total), creates ZIP via blob for smaller batches. Balances convenience and safety.

Always create ZIP via blob - High memory usage!

⚠️ Forces ZIP creation via blob method. May cause browser freeze or crash with large files!

Force Blob Fallback (Debug/Testing)

Forces the use of blob method instead of streaming APIs. Only use for testing or compatibility issues. The blob fallback strategy selected above will be applied.