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has anybody successfully implemented a js library (or other method) to convert data from NS into a proper XLSX format?

This isn't exactly the same, but I have successfully used <http://papaparse.com/> to convert .csv files to JSON for use in a Suitelet

the Suitelet is simply a file chooser for my users to upload a .csv; I take that file, save it to the file cabinet, then load it and parse it with that library. I then send those results to a scheduled script to process records (landed costs on item receipts in this case)