server.js

const express = require('express')
const ForwarderAbi = require('./src/abi/Forwarder.json')
const { Forwarder } = require('./src/deploy.json')
const ethers = require('ethers')
require('dotenv').config()


const app = express()
app.use(express.json());
app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*"); // update to match the domain you will make the request from
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
  });

app.post('/relayTransaction', async (req, res) => {

    // setup to verify incoming signature and request
    const types = {
        ForwardRequest: [
            { name: 'from', type: 'address' },
            { name: 'to', type: 'address' },
            { name: 'value', type: 'uint256' },
            { name: 'gas', type: 'uint256' },
            { name: 'nonce', type: 'uint256' },
            { name: 'data', type: 'bytes' },
          ]
        }

    const domain = {
            name: 'Forwarder',
            version: '0.0.1',
            chainId: 31337,
            verifyingContract: Forwarder,
            }

    const { request, signature } = req.body
    const verifiedAddress = ethers.utils.verifyTypedData(domain, types, request, signature)
    // Verify, that the message and the transaction are from the original signer else return error
    if(request.from !== verifiedAddress) {
        return res.status(400).send({
            message: 'The Transaction could not get verified.'
        })
    }
    // create Wallet from private key and connect to Hardhat local network
    const wallet = new ethers.Wallet(process.env.PRIVATE_KEY)
    const provider = ethers.getDefaultProvider('http://localhost:8545')
    const connectedWallet = wallet.connect(provider)

    const gasLimit = (parseInt(request.gas) + 50000).toString()
    // send transaction to forwarder contract
    const forwarderContract = new ethers.Contract(Forwarder, ForwarderAbi, connectedWallet)
    const contractTx = await forwarderContract.executeDelegate(request, signature, { gasLimit })
    const transactionReceipt = await contractTx.wait()

    return res.json(transactionReceipt)
})

app.listen(4000, () => console.log('listening on port 4000!'))