The package.json description says it all: "Library to enable client-side code to stream potentially large files into a zipped download"

• Client-side generation of ZIP files from supplied single-file download URLs
• Support for ZIP64 (ZIP files bigger than 4GB)
• Everything is streamed, no data has to be held in RAM
• No compression is applied, so the CPU load is tiny

1. Install npm package: npm install downzip
2. Make sure the service worker is copied to your output directory. For example, setup copy-webpack-plugin to put the service worker in your output directory:

// Add copy rule for downzip service worker
new CopyPlugin([
    {
        from: 'node_modules/downzip/dist/downzip-sw.js',
        to: '.'
    }
])

import DownZip from 'downzip'

// Setup downzip object
const downZip = new DownZip()
await downZip.register()

// Initialize download
const downloadId = "aaaabbbb"
const zipFileName = "downzip-file"
const files = [
    {
        name: 'picture1.jpg', 
        downloadUrl: 'https://your-download-url.com/picture1.jpg',
        size: 1234      // In bytes
    }, 
    {
        name: 'picture2.jpg', 
        downloadUrl: 'https://your-download-url.com/picture2.jpg',
        size: 4567      // In bytes
    }
]
const downloadUrl = await downZip.downzip(
    downloadId,
    zipFileName,
    files
)

// Start download when user clicks the link
<a href={downloadUrl}>Click to start downloading!</a>

Can pass mapScriptUrl function to the register method. That function gets used by service-worker-loader. docs

    const mapScriptUrl = scriptUrl => scriptUrl.replace('localhost', "127.0.0.1")

   // Setup downzip object
    const downZip = new DownZip()
    await downZip.register({ mapScriptUrl })

All improvements are welcome, but the main things that need to be improved at the moment are:

• Automated Testing
• Only send the keep-alive service worker message when there is a download queued up / in progress
• Find an easier way to install this package / service worker in projects

Please submit pull requests, I'm more than happy to merge in improvements!