server_side.js

const sigUtil = require('eth-sig-util')

app.post('/api/message', (req, res) => {
  
  const messageData = req.body.messageData
  const signature = req.body.signature
  
  // recover the public key
  const publicKey = sigUtil.recoverTypedSignature({
    data: messageData,
    sig: signature
  })  
   
  // check if public key is on the approved list
  const approvedPublicKeys = [
    '0xdf396910e694f7De31eF88d0090F2A4333ffcCF3',
    '0x106F681949E222D57A165cD85685E3bD9975b973'
  ]
  if(approvedPublicKeys.indexOf(publicKey) > -1){
    
    // save the vote
     mongoDb.messages.save(messageData)      
  }

})