Fork testing (mainnet fork) in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Hardhat fork setup code?

Consider this Hardhat config snippet for mainnet forking. What will be the output when running a script that fetches the balance of an address on the forked network?

Blockchain / Solidity

import { ethers } from "hardhat";

async function main() {
  const provider = ethers.provider;
  const balance = await provider.getBalance("0x742d35Cc6634C0532925a3b844Bc454e4438f44e");
  console.log(ethers.formatEther(balance));
}

main();

A Displays the ETH balance of the address on mainnet at the fork block

B Throws an error because the provider is not connected

C Displays zero because the forked network is empty

D Displays the balance in wei without formatting

💡 Hint

Remember that mainnet fork replicates the state at a specific block.

The Hardhat provider connected to a mainnet fork allows reading the state as it was on mainnet at the fork block. So the balance reflects the real mainnet balance at that block.

🧠 Conceptual

intermediate

Which statement best describes mainnet fork testing?

Choose the correct description of mainnet fork testing in blockchain development.

A It forks the mainnet to create a new public blockchain network.

B It deploys contracts directly on the mainnet for live testing.

C It simulates transactions without any real blockchain state.

D It creates a local blockchain that replicates the mainnet state at a specific block for testing.

💡 Hint

Think about how developers test with real mainnet data safely.

Mainnet fork testing creates a local environment that mirrors mainnet state at a block, allowing safe testing with real data without affecting the live network.

🔧 Debug

advanced

Why does this fork test script fail with 'invalid JSON RPC response'?

Given this Hardhat fork config and script, why does the script fail with 'invalid JSON RPC response' error? Config snippet: { forking: { url: "https://mainnet.infura.io/v3/INVALID_KEY", blockNumber: 15000000 } } Script tries to get block number from provider.

Blockchain / Solidity

const blockNumber = await ethers.provider.getBlockNumber();
console.log(blockNumber);

A The script is missing await keyword causing a promise error.

B The blockNumber is set to a future block that does not exist yet.

C The Infura URL contains an invalid API key causing connection failure.

D The provider is not initialized before calling getBlockNumber.

💡 Hint

Check the RPC URL and credentials.

An invalid API key in the RPC URL causes the provider to fail connecting, resulting in an invalid JSON RPC response error.

📝 Syntax

advanced

Which option correctly sets up a mainnet fork in Hardhat config?

Select the correct Hardhat config snippet to enable mainnet forking at block 16000000 using Alchemy RPC.

A { forking: { url: "https://eth-mainnet.alchemyapi.io/v2/your-api-key", blockNumber: 16000000 } }

B { forking: { url: "https://eth-mainnet.alchemyapi.io/v2/your-api-key", blockNumber: "16000000" } }

C { forking: { endpoint: "https://eth-mainnet.alchemyapi.io/v2/your-api-key", block: 16000000 } }

D { fork: { url: "https://eth-mainnet.alchemyapi.io/v2/your-api-key", block: 16000000 } }

💡 Hint

Check property names and syntax carefully.

The correct Hardhat config uses 'forking' with 'url' and 'blockNumber' keys. Option A matches the correct syntax and types.

🚀 Application

expert

How many transactions are recorded after this fork test script runs?

You run a script on a mainnet fork that sends 3 transactions: 1. Transfer 1 ETH from account A to B 2. Deploy a contract 3. Call a function that reverts intentionally After the script finishes, how many transactions remain on the forked chain?

A No transactions are recorded because it's a fork

B All 3 transactions are recorded regardless of revert

C Only the deploy contract transaction is recorded

D 2 transactions are recorded; the reverted one is discarded

💡 Hint

Think about how reverted transactions affect blockchain state.

In a forked environment like Hardhat Network, all transactions—including those that revert—are mined into blocks on the local forked chain. Reverted transactions do not change the state but are still recorded.