SheetJS/sheetjs ? reverse-engineered prompt

Build me a lightweight spreadsheet data toolkit that can read files like Excel and CSV, turn them into plain JavaScript data, and also create new spreadsheets from arrays, tables, or JSON.

I want it to work in a few common environments, like Node and the browser, and to be easy to drop into an app without a lot of setup. It should handle the usual messy spreadsheet stuff well, like multiple sheets, headers, dates, empty cells, and exporting data back out in a format that opens cleanly in common spreadsheet apps.

Please also include a few simple examples or demos that show how to import a file, inspect the data, and write a new workbook. If you need to check current usage details, look up the docs online first.