index.js

require('dotenv').config()

const ethers = require('ethers');
const {BigNumber, utils} = ethers

const provider = new ethers.providers.WebSocketProvider(
    `wss://rinkeby.infura.io/ws/v3/${process.env.INFURA_ID}`,
    'rinkeby',
)

const depositWallet = new ethers.Wallet(
    process.env.DEPOSIT_WALLET_PRIVATE_KEY,
    provider,
)

const main = async () => {
    const depositWalletAddress = await depositWallet.getAddress()
    console.log(`Watching for incoming tx to ${depositWalletAddress}...`)

    provider.on('pending', (txHash) => {
        try {
            provider.getTransaction(txHash).then((tx) => {
                if (tx === null) return

                const {from, to, value} = tx

                if (to === depositWalletAddress) {
                    console.log(`Receiving ${utils.formatEther(value)} ETH from ${from}..`)

                    tx.wait(process.env.CONFIRMATIONS_BEFORE_WITHDRAWAL).then(async (_receipt) => {

                        const currentBalance = await depositWallet.getBalance('latest')
                        const gasPrice = await provider.getGasPrice()
                        const gasLimit = 21000
                        const maxGasFee = BigNumber.from(gasLimit).mul(gasPrice)

                        const tx = {
                            to: process.env.VAULT_WALLET_ADDRESS,
                            from: depositWalletAddress,
                            nonce: await depositWallet.getTransactionCount(),
                            value: currentBalance.sub(maxGasFee),
                            chainId: 4,
                            gasPrice: gasPrice,
                            gasLimit: gasLimit
                        }

                        depositWallet.sendTransaction(tx).then((_receipt) => {
                            console.log(
                                `Withdrew ${utils.formatEther(
                                currentBalance.sub(maxGasFee)
                            )} ETH to VAULT ${process.env.VAULT_WALLET_ADDRESS} `
                            )

                        }, (reason) => {
                            console.error("Withdrawal failed", reason)
                        })

                    }, (reason) => {
                        console.error("Receival failed", reason)
                    })
                }
            })
        } catch (e) {
            console.error(e)
        }
    })
}

if (require.main === module) {
    main()
}