I was building a web app and wanted Excel files to live within it and be fully modifiable / saveable, but couldn't find anything open source that fully fit the bill. I ended up building the repo below.

Basically full Excel functionality in our React app including:

• Import .xlsx and csv file, export to .xlsx
• Fully preserves existing Excel formatting
• Real-time calc that's a heavily modified fork of ironcacl
• Formulas and calculations
• Multiple worksheets
• All the formatting options you'd use on day-to-day basis
• Fully open source

https://github.com/pixelsmasher13/gridpath. Please let me know if you end up trying it out.