index.js

/* Import dependencies, declare constants */
var fs = require('fs');
var path = require('path');
var XlsxTemplate = require('xlsx-template');

/**
* Your function call
* @param {Object} params Execution parameters
*   Members
*   - {Array} args Arguments passed to function
*   - {Object} kwargs Keyword arguments (key-value pairs) passed to function
*   - {String} remoteAddress The IPv4 or IPv6 address of the caller
*
* @param {Function} callback Execute this to end the function call
*   Arguments
*   - {Error} error The error to show if function fails
*   - {Any} returnValue JSON serializable (or Buffer) return value
*/
module.exports = (params, callback) => {
  // Load an XLSX file into memory
  const filename = path.join(__dirname, 'templates', 'template.xlsx');

  fs.readFile(filename, function(err, data) {
    if (err) {
      console.error(err);
    } else {
      // Create a template
      var template = new XlsxTemplate(data);

      // Replacements take place on first sheet
      var sheetNumber = 1;

      // Set up some placeholder values matching the placeholders in the template
      var values = {
              extractDate: new Date(),
              dates: [ new Date("2013-06-01"), new Date("2013-06-02"), new Date("2013-06-03") ],
              people: [
                  {name: "John Smith", age: 20},
                  {name: "Bob Johnson", age: 22}
              ]
          };
      
      // Perform substitution
      template.substitute(sheetNumber, values);

      // Get binary data
      var data = template.generate();
      // fs.writeFileSync('test.xlsx', data, 'binary'); // <<<<< it's working on local host

      callback(null, data); // <<<<< NOT WORKING
      // callback(null, Buffer.from(data)); // <<<<< NOT WORKING
      // callback(null, Buffer.from(data, 'utf-8)); // <<<<< NOT WORKING
    }
  });
};