index.js

const functions = require('firebase-functions');
const { google } = require('googleapis');
const sheets = google.sheets('v4')
const spreadsheetId = 'your_spread_sheet_id'
const serviceAccount = require('./serviceAccount.json')

// Required for authentication
const jwtClient = new google.auth.JWT({
  email: serviceAccount.client_email,
  key: serviceAccount.private_key,
  scopes: [ 'https://www.googleapis.com/auth/spreadsheets' ],  // read and write sheets
})
const jwtAuthPromise = jwtClient.authorize()

// functions for updating sheet through database collection
exports.updateSheet = functions.database.ref('/your_collection_name').onUpdate(async change => {
  const data = change.after.val()
  let feedbacks = [];
  Object.keys(data).forEach((key) => {
    feedbacks.push([data[key].questionId, data[key].feedback]);
  });
  
    await jwtAuthPromise
    await sheets.spreadsheets.values.update({
      auth: jwtClient,
      spreadsheetId: spreadsheetId,
      range: `A1:B${feedbacks.length}`,  // update this range of cells
      valueInputOption: 'RAW',
      requestBody: {
        values: feedbacks,
        majorDimension: 'ROWS'
      }
    }, {}).then((sheetRes) => {
      console.log('Susscess', sheetRes);
    }).catch((err) => {
      console.log('Error', err);
    })
});