Sign a message using Web3 and send it to the server

client_side.js

import Web3 from 'web3'

// Setup a message with instructions for the server.
const messageData = [
  {
    name: ‘Contents’,
    type: ‘string’,
    value: 'extra! extra! read all about it!'
  }
]

// create instance of Web3 from the web3 object provided by the browser
const web3 = new Web3(window.web3.currentProvider)

// get the user's account
web3.eth.getAccounts().then(accounts => {
  
  const userAccount = accounts[0]
  
  // Sign the message.  
  // This will trigger a popup where the user can review the 
  // message. If they confirm the popup, the message will be signed using 
  // their private key and the callback will be called.
  
  web3.currentProvider.sendAsync({
    method: ‘eth_signTypedData’,
    params: [messageData, userAccount],
    from: userAccount, 
  }, function (err, result) {

    // result.result is the signature
    console.log('Signature:', result.result)
  
    // send the message and signature to server as normal http request
    return fetch('/api/vote', {
        method: "POST",
        headers: {
          "Content-Type": "application/json"
        },
        body: JSON.stringify({
          messageData: messageData,
          signature: result.result
        })
      }).then(response => {
        console.log('sent to server.')
      })
  
  })
})