docs.sheetjs.com/docz/static/azure/index.js

/* sheetjs (C) SheetJS -- https://sheetjs.com */
const { Blob } = require('buffer');
const { app } = require('@azure/functions');
const XLSX = require('xlsx');

app.http('SheetJSAzure', {
  methods: ['GET', 'POST'],
  authLevel: 'anonymous',
  handler: async (req, context) => {
    if (req.method == "POST") {
      /* grab the file at form key `upload` */
      const formData = await req.formData();
      const f = formData.get("upload");

      if (!f || !(f instanceof Blob)) {
        return { status: 400, body: "Must submit a file for processing!" };
      } else {
        /* parse file */
        const ab = await f.arrayBuffer();
        const wb = XLSX.read(ab);

        /* generate CSV from first sheet */
        const csv = XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]);
        return { status: 200, body: csv };
      }

    } else if (req.method == "GET") {
      var ws = XLSX.utils.aoa_to_sheet(["SheetJS".split(""), [5, 4, 3, 3, 7, 9, 5]]);
      var wb = XLSX.utils.book_new(); XLSX.utils.book_append_sheet(wb, ws, "Data");
      var buf = XLSX.write(wb, { type: "buffer", bookType: "xlsx" });
      return {
        status: 200,
        headers: { "Content-Disposition": `attachment; filename="SheetJSAzure.xlsx";` },
        body: buf
      };
    } else return { status: 500, body: `Unsupported method ${req.method}` };
  }
});