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Hardhat Forking

In this article, you’ll learn how to fork smart contracts in Ethereum mainnet using Hardhat.

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Objectives

By the end of this lesson, you should be able to:
  • Use Hardhat Network to create a local fork of mainnet and deploy a contract to it
  • Utilize Hardhat forking features to configure the fork for several use cases

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Overview

Hardhat forking is a powerful feature that allows developers to create a local replica or fork of the Ethereum network or any other EVM-compatible Blockchain. By using this feature, you can develop smart contracts that rely on smart contracts that are already deployed to a particular network. You will create a BalanceReader.sol contract that reads the USDC balance of a particular holder. In order to achieve that, you need to:
  • Create the BalanceReader.sol contract
  • Configure Hardhat to support forking
  • Create a test for the BalanceReader.sol contract
Hardhat forking also has other capabilities like:
  • hardhat_impersonateAccount (useful to impersonate an account and others)
  • hardhat_stopImpersonatingAccount
  • hardhat_setNonce
  • hardhat_setBalance
  • hardhat_setCode
  • hardhat_setStorageAt
Those won’t be covered in this guide, however it’s recommended to explore them a bit more in the following link:

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Creating the Balance Reader contract

The BalanceReader contract is created as follows: 
pragma solidity 0.8.9;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contract BalanceReader {
    function getERC20BalanceOf(address _account, address _tokenAddress)
        external
        view
        returns (uint256)
    {
        // we create an instance only using the interface and the address
        return IERC20(_tokenAddress).balanceOf(_account);
    }
}
You simply pass the address of an account and the address of a token, then you get and return the balance. You will need to install @openzeppelin by running: 
npm install @openzeppelin/contracts
Then, check that everything is working correctly by running: 
npx hardhat compile
You should get: 
Generating typings for: 2 artifacts in dir: typechain-types for target: ethers-v6
Successfully generated 18 typings!
Compiled 2 Solidity files successfully

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Configuring Hardhat to support forking

By default, Hardhat uses a network called hardhat. You must change its default configuration by going to the hardhat.config.ts file and include the following in the network: 
hardhat: {
    forking: {
        url: `https://eth-mainnet.g.alchemy.com/v2/${process.env.ALCHEMY_MAINNET_KEY ?? ""}`,
        enabled: true
    }
},
Be aware that you need to have an ALCHEMY_MAINNET_KEY in your .env file. You can get one directly from Alchemy. Also notice that forking is enabled by specifying enabled: true, however this value can be changed via environment variables.

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Creating a test for the BalanceReader.sol contract

Create a test file in the test folder called BalanceReader.ts and include the following: 
import { Signer } from 'ethers';
import { ethers } from 'hardhat';

import { BalanceReader, BalanceReader__factory } from '../typechain-types';

describe('BalanceReader tests', () => {
  let instance: BalanceReader;
  let accounts: Signer[];

  // Configure the addresses we can to check balances for
  const USDC_MAINNET_ADDRESS = '0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48'; // https://etherscan.io/token/0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48
  const ARBITRUM_ONE_GATEWAY = '0xcEe284F754E854890e311e3280b767F80797180d';
  const USDC_DECIMALS = 6;

  it('gets arbitrum gateway balance', async () => {
    // We get signers as in a normal test
    accounts = await ethers.getSigners();
    const factory = new BalanceReader__factory(accounts[0]);

    // We deploy the contract to our local test environment
    instance = await factory.deploy();

    // Our contract will be able to check the balances of the mainnet deployed contracts and address
    const balance = await instance.getERC20BalanceOf(ARBITRUM_ONE_GATEWAY, USDC_MAINNET_ADDRESS);
    const balanceAsString = ethers.utils.formatUnits(balance, USDC_DECIMALS);

    console.log(
      'The USDC Balance of Arbitrum Gateway is $',
      Number(balanceAsString).toLocaleString(),
    );
  });
});
In this example, the USDC address is used and since USDC is an ERC-20 token, you can explore the token holders of that particular token directly in Etherscan: Hardhat forking Or, visit https://etherscan.io/token/0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48#balances, where you can see, at the time or writing, Arbitrum ONE Gateway is the top token holder. Then, run the following command: 
npx hardhat test ./test/BalanceReader.ts
You should get: 
BalanceReader tests
The USDC Balance of Arbitrum Gateway is $ 1,116,923,836.506
    ✔ gets arbitrum gateway balance (4345ms)

  1 passing (4s)

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Conclusion

In this lesson, you’ve learned how to use hardhat forking capabilities to test smart contracts. You learned how contracts can interact with already-deployed contracts in an easy way.

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See also