Using local private key with Web3.js

Question

How can I interact with smart contracts and send transactions with Web3.js by having a local private key? The private key is either hardcoded or comes from an environment (.env) file?

This is needed for Node.js and server-side interaction or batch jobs with Ethereum/Polygon/Binance Smart Chain smart contracts.

You may encounter e.g. the error

Error: The method eth_sendTransaction does not exist/is not available

Answer 1

Ethereum node providers like Infura, QuikNode and others require you to sign outgoing transactions locally before you broadcast them through their node.

Web3.js does not have this function built-in. You need to use @truffle/hdwallet-provider package as a middleware for your Ethereum provider.

Example in TypeScript:

 const Web3 = require('web3');
 const HDWalletProvider = require("@truffle/hdwallet-provider");
 import { abi } from "../../build/contracts/AnythingTruffleCompiled.json";
 
 //
 // Project secrets are hardcoded here
 // - do not do this in real life
 //

 // No 0x prefix
const myPrivateKeyHex = "123123123";
const infuraProjectId = "123123123";
 
const provider = new Web3.providers.HttpProvider(`https://mainnet.infura.io/v3/${infuraProjectId}`);

// Create web3.js middleware that signs transactions locally
const localKeyProvider = new HDWalletProvider({
  privateKeys: [myPrivateKeyHex],
  providerOrUrl: provider,
});
const web3 = new Web3(localKeyProvider);

const myAccount = web3.eth.accounts.privateKeyToAccount(myPrivateKeyHex);

// Interact with existing, already deployed, smart contract on Ethereum mainnet
const address = '0x123123123123123123';
const myContract = new web3.eth.Contract(abi as any, address);

// Some example calls how to read data from the smart contract
const currentDuration = await myContract.methods.stakingTime().call();
const currentAmount = await myContract.methods.stakingAmount().call();

console.log('Transaction signer account is', myAccount.address, ', smart contract is', address);

console.log('Starting transaction now');
// Approve this balance to be used for the token swap
const receipt = await myContract.methods.myMethod(1, 2).send({ from: myAccount.address });
console.log('TX receipt', receipt);

You need to also avoid to commit your private key to any Github repository. A dotenv package is a low entry solution for secrets management.