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Blockchain / Solidityprogramming~10 mins

Fork testing (mainnet fork) in Blockchain / Solidity - Interactive Code Practice

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Practice - 5 Tasks
Answer the questions below
1fill in blank
easy

Complete the code to import the Hardhat network helpers for mainnet forking.

Blockchain / Solidity
const { [1] } = require('hardhat');
Drag options to blanks, or click blank then click option'
Anetwork
Bethers
Cweb3
Dfs
Attempts:
3 left
💡 Hint
Common Mistakes
Importing 'ethers' instead of 'network'.
Using 'web3' which is not part of Hardhat by default.
2fill in blank
medium

Complete the code to set the mainnet fork URL in the Hardhat config.

Blockchain / Solidity
module.exports = {
  networks: {
    hardhat: {
      forking: {
        url: '[1]'
      }
    }
  }
};
Drag options to blanks, or click blank then click option'
A'https://mainnet.infura.io/v3/YOUR-PROJECT-ID'
B'http://localhost:8545'
C'https://kovan.infura.io/v3/YOUR-PROJECT-ID'
D'https://rinkeby.infura.io/v3/YOUR-PROJECT-ID'
Attempts:
3 left
💡 Hint
Common Mistakes
Using testnet URLs like Rinkeby or Kovan.
Using localhost URL which is not a mainnet fork.
3fill in blank
hard

Fix the error in the code to reset the Hardhat network fork block number.

Blockchain / Solidity
await network.provider.request({
  method: 'hardhat_reset',
  params: [{
    forking: {
      jsonRpcUrl: '[1]',
      blockNumber: 12345678
    }
  }]
});
Drag options to blanks, or click blank then click option'
A'https://rinkeby.infura.io/v3/YOUR-PROJECT-ID'
B'https://kovan.infura.io/v3/YOUR-PROJECT-ID'
C'https://mainnet.infura.io/v3/YOUR-PROJECT-ID'
D'http://localhost:8545'
Attempts:
3 left
💡 Hint
Common Mistakes
Using testnet URLs causes fork reset to fail.
Using localhost URL does not reset the mainnet fork.
4fill in blank
hard

Fill both blanks to create a forked provider and get the block number.

Blockchain / Solidity
const provider = new ethers.providers.JsonRpcProvider([1]);
const blockNumber = await provider.getBlockNumber();
console.log('Forked block number:', [2]);
Drag options to blanks, or click blank then click option'
A'http://localhost:8545'
BblockNumber
C'https://mainnet.infura.io/v3/YOUR-PROJECT-ID'
Dprovider
Attempts:
3 left
💡 Hint
Common Mistakes
Using mainnet URL instead of localhost URL.
Printing the provider object instead of the block number.
5fill in blank
hard

Fill all three blanks to advance the forked network to a specific block and get its timestamp.

Blockchain / Solidity
await network.provider.request({
  method: '[1]',
  params: [{
    blockNumber: [2]
  }]
});
const block = await ethers.provider.getBlock([3]);
console.log('Block timestamp:', block.timestamp);
Drag options to blanks, or click blank then click option'
Ahardhat_mine
Bhardhat_reset
C12345678
Dhardhat_setBlockNumber
Attempts:
3 left
💡 Hint
Common Mistakes
Using wrong method names like 'hardhat_mine'.
Using different block numbers in method and getBlock call.

Practice

(1/5)
1. What is the main purpose of creating a mainnet fork in blockchain development?
easy
A. To safely test code using real blockchain data without affecting the live network
B. To permanently change the main blockchain rules
C. To mine new blocks faster than the main network
D. To create a new cryptocurrency from scratch

Solution

  1. Step 1: Understand what a mainnet fork is

    A mainnet fork is a copy of the live blockchain used for testing.
  2. Step 2: Identify the purpose of testing on a fork

    It allows developers to try code changes safely without risking the real network.
  3. Final Answer:

    To safely test code using real blockchain data without affecting the live network -> Option A
  4. Quick Check:

    Fork testing = safe real-data testing [OK]
Hint: Mainnet fork = safe playground for real blockchain data [OK]
Common Mistakes:
  • Thinking a fork changes the live blockchain
  • Confusing fork with creating a new coin
  • Assuming fork speeds up mining
2. Which of the following commands is commonly used to create a mainnet fork with Hardhat?
easy
A. npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY
B. npx hardhat compile --fork mainnet
C. npx hardhat deploy --network mainnet
D. npx hardhat test --fork mainnet

Solution

  1. Step 1: Recall Hardhat command for forking

    The command to start a local node forked from mainnet includes node --fork with a mainnet RPC URL.
  2. Step 2: Match the correct syntax

    npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY uses npx hardhat node --fork with a mainnet URL, which is correct.
  3. Final Answer:

    npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY -> Option A
  4. Quick Check:

    Fork command = hardhat node --fork [OK]
Hint: Fork with 'hardhat node --fork' plus mainnet URL [OK]
Common Mistakes:
  • Using compile or deploy commands to fork
  • Missing the --fork flag
  • Not providing a mainnet RPC URL
3. Given this Hardhat script snippet for mainnet fork testing:
const { ethers } = require("hardhat");

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

main();

What will this script output when run on a mainnet fork?
medium
A. The balance of the address on a testnet like Ropsten
B. The Ether balance of the address on the live mainnet
C. An error because the address is invalid
D. Zero, because the fork has no data

Solution

  1. Step 1: Understand mainnet fork data availability

    A mainnet fork copies the live blockchain state, including balances.
  2. Step 2: Analyze the script's balance query

    The script queries the balance of a known mainnet address, so it returns the real Ether amount.
  3. Final Answer:

    The Ether balance of the address on the live mainnet -> Option B
  4. Quick Check:

    Fork has live data = real balance [OK]
Hint: Fork shows real mainnet balances, not zero or errors [OK]
Common Mistakes:
  • Assuming fork has empty state
  • Confusing mainnet fork with testnet
  • Thinking address format is invalid
4. You tried to fork mainnet using Hardhat but got the error: Invalid JSON RPC response. What is the most likely cause?
medium
A. You ran the fork command without internet connection
B. Your Hardhat version does not support forking
C. The address you queried does not exist
D. You forgot to provide a valid mainnet RPC URL with an API key

Solution

  1. Step 1: Understand the error context

    "Invalid JSON RPC response" usually means the node cannot connect to the RPC endpoint.
  2. Step 2: Identify common causes

    Not providing a valid mainnet RPC URL or missing API key causes this error.
  3. Final Answer:

    You forgot to provide a valid mainnet RPC URL with an API key -> Option D
  4. Quick Check:

    Missing RPC URL/API key = JSON RPC error [OK]
Hint: Check RPC URL and API key first for JSON RPC errors [OK]
Common Mistakes:
  • Blaming Hardhat version without checking URL
  • Assuming address errors cause RPC failure
  • Ignoring internet connectivity issues
5. You want to test a smart contract upgrade on a mainnet fork. Which sequence correctly describes the steps to do this safely?
hard
A. Fork mainnet, deploy new contract only, ignore old contract state
B. Deploy new contract on mainnet directly, then fork mainnet to test
C. Fork mainnet locally, deploy old contract, run upgrade scripts, test new contract behavior
D. Test upgrade only on testnet, no need to fork mainnet

Solution

  1. Step 1: Fork mainnet and deploy old contract

    Start by creating a mainnet fork and deploying the current contract version to replicate live state.
  2. Step 2: Run upgrade scripts and test new behavior

    Apply upgrade scripts on the fork and test the new contract to ensure correctness without risk.
  3. Final Answer:

    Fork mainnet locally, deploy old contract, run upgrade scripts, test new contract behavior -> Option C
  4. Quick Check:

    Fork + old contract + upgrade + test = safe upgrade [OK]
Hint: Fork, deploy old, upgrade, then test for safe contract upgrades [OK]
Common Mistakes:
  • Deploying new contract directly on mainnet
  • Ignoring old contract state in tests
  • Skipping mainnet fork and testing only on testnet
Recommended

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10 questions~5 mineasy hard