Easy Blockchain Interview Questions

Solidity: bytes32 h1 = keccak256(abi.encodePacked('hello')); bytes32 h2 = keccak256(abi.encode(addr, amount)); ethers v6: import { keccak256, toUtf8Bytes, solidityPackedKeccak256 } from 'ethers'; const h = keccak256(toUtf8Bytes('hello')); const p = solidityPackedKeccak256(['address','uint256'], [addr, amount]); Prefer abi.encode over abi.encodePacked when hashing several dynamic values, because packed encoding can cause collisions.

ethers v6 provides getFeeData, and sendTransaction fills in fields automatically, but you can set them explicitly. import { JsonRpcProvider, Wallet, parseEther } from 'ethers'; const provider = new JsonRpcProvider(process.env.RPC_URL); const wallet = new Wallet(process.env.PRIVATE_KEY, provider); const fee = await provider.getFeeData(); const tx = await wallet.sendTransaction({ to: '0xRecipient...', value: parseEther('0.01'), maxFeePerGas: fee.maxFeePerGas, maxPriorityFeePerGas: fee.maxPriorityFeePerGas }); const receipt = await tx.wait(); console.log(receipt.status);

Inherit the audited ERC20 implementation and mint the initial supply in the constructor. // SPDX-License-Identifier: MIT pragma solidity ^0.8.24; import {ERC20} from '@openzeppelin/contracts/token/ERC20/ERC20.sol'; import {Ownable} from '@openzeppelin/contracts/access/Ownable.sol'; contract CampusToken is ERC20, Ownable { constructor() ERC20('Campus Token', 'CAMP') Ownable(msg.sender) { _mint(msg.sender, 1_000_000 * 10 ** decimals()); } function mint(address to, uint256 amount) external onlyOwner { _mint(to, amount); } }

Install it with pip and run it from the project root; it detects Foundry automatically. pip install slither-analyzer slither . # Only medium and high impact issues slither . --exclude-low --exclude-informational # Exclude dependencies slither . --filter-paths 'lib|test' # Print a summary of contracts and functions slither . --print human-summary Triage each finding: fix real bugs and record why false positives are safe.

// SPDX-License-Identifier: MIT pragma solidity ^0.8.24; import {ERC721} from '@openzeppelin/contracts/token/ERC721/ERC721.sol'; import {Ownable} from '@openzeppelin/contracts/access/Ownable.sol'; contract CertificateNFT is ERC721, Ownable { uint256 public nextId; string private baseURI; constructor(string memory uri) ERC721('Course Cert', 'CERT') Ownable(msg.sender) { baseURI = uri; } function mint(address to) external onlyOwner { _safeMint(to, nextId++); } function _baseURI() internal view override returns (string memory) { return baseURI; } }

import { Contract, parseUnits } from 'ethers'; const abi = [ 'function approve(address spender, uint256 amount) returns (bool)', 'function decimals() view returns (uint8)', 'function allowance(address owner, address spender) view returns (uint256)' ]; const token = new Contract(TOKEN_ADDRESS, abi, signer); const dec = await token.decimals(); const amount = parseUnits('25', dec); const tx = await token.approve(SPENDER, amount); await tx.wait(); console.log(await token.allowance(signer.address, SPENDER)); The spender contract then calls transferFrom to pull exactly that amount.

tokenURI(id) returns a URI, usually ipfs://<CID>/<id>.json, pointing to JSON that marketplaces read: { "name": "Course Certificate #12", "description": "Completed Blockchain Developer course", "image": "ipfs://bafy.../12.png", "attributes": [ { "trait_type": "Grade", "value": "A" }, { "trait_type": "Year", "value": 2026 } ] } Store images and JSON on IPFS or Arweave rather than a normal server, so metadata cannot silently change or disappear.

Browser wallets inject an EIP-1193 provider at window.ethereum. Wrap it with BrowserProvider and request accounts. import { BrowserProvider } from 'ethers'; async function connect() { if (!window.ethereum) throw new Error('Install a wallet'); const provider = new BrowserProvider(window.ethereum); const signer = await provider.getSigner(); // prompts the user const address = await signer.getAddress(); const { chainId } = await provider.getNetwork(); return { signer, address, chainId }; } In production, use EIP-6963 discovery or a connector library so users with several wallets can choose.

Create a Contract with address, ABI and a provider for reads or a signer for writes. import { Contract, parseEther } from 'ethers'; const abi = [ 'function count() view returns (uint256)', 'function increment() payable' ]; const readOnly = new Contract(ADDR, abi, provider); console.log(await readOnly.count()); // free eth_call const writable = new Contract(ADDR, abi, signer); const tx = await writable.increment({ value: parseEther('0.001') }); const receipt = await tx.wait(); console.log(receipt.hash, receipt.status);

curl -L https://foundry.paradigm.xyz | bash && foundryup forge init my-dapp # new project with src, test, script forge install OpenZeppelin/openzeppelin-contracts forge build # compile forge test -vvv # run tests with traces forge coverage # coverage report forge snapshot # gas snapshot anvil # local node cast call ADDR 'count()(uint256)' --rpc-url $RPC Foundry tests are written in Solidity and run fast; Hardhat is the JavaScript or TypeScript alternative with a large plugin ecosystem.

Verification uploads source and compiler settings so the explorer can recompile and match the bytecode, letting users read the code. # Foundry, using an explorer API key forge verify-contract 0xDeployedAddr src/Counter.sol:Counter \ --chain sepolia --etherscan-api-key $KEY --watch # With constructor arguments --constructor-args $(cast abi-encode 'constructor(address)' 0xOwner) # Hardhat npx hardhat verify --network sepolia 0xDeployedAddr 0xOwner Compiler version, optimiser runs and via-IR settings must match exactly.