Letter.solの解法

sealLetter.js

var fs   = require('fs');
var Web3 = require('web3');

var web3 = new Web3('wss://ropsten.infura.io/ws');

var letterABI     = JSON.parse(fs.readFileSync('./build/contracts/Letter.json')).abi;
var letterAddress = '0xbaDE12c0bd7943A066e77f0466D529D78d2F70dB';
var letter        = new web3.eth.Contract(letterABI, letterAddress);

var address = 'your address';
var privkey = 'your priv key';

sealLetter()

async function sealLetter() {
  web3.eth.defaultAccount = address;
  web3.eth.accounts.wallet.add(privkey);

  var balance;

  // balanceの確認
  balance = await letter.methods.balanceOf(address).call();
  console.log(balance);

  // fallback関数の呼び出し
  await web3.eth.sendTransaction({
    from: 0,
    to: letterAddress,
    value: 1,
    gas: 50000,
  });

  // balanceの確認
  balance = await letter.methods.balanceOf(address).call();
  console.log(balance);

  // seal
  var seal = await letter.methods.seal().send({
    from: address,
    gas: 50000,
  });
  console.log(seal);

  // sealされているかの確認
  var sealed = await letter.methods.isSealed(address).call();
  console.log(sealed);

  web3.currentProvider.connection.close();
}