XLSX.js is a JavaScript library for converting the data in base64 XLSX files into JavaScript objects - and back! Please note that this library is licensed under the Microsoft Office Extensible File License - a license NOT approved by the OSI. While this license is based off of the MS-PL, which is OSI-approved, there are significant differences.
There's a new Library called
https://github.com/sutoiku/formula.js
that implements most of Microsoft Excel formula functions. Maybe you can use this with xlsx.js generated JavaScript objects.