DeFi, Tokens and NFTs Interview Questions for Blockchain

// 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.

Formula: amountOut = (amountIn x 997 x reserveOut) / (reserveIn x 1000 + amountIn x 997). Example: reserves 1,000 ETH and 3,000,000 USDC, swap 10 ETH. amountInWithFee = 10 x 997 = 9,970 numerator = 9,970 x 3,000,000 = 29,910,000,000 denominator = 1,000 x 1000 + 9,970 = 1,009,970 amountOut = 29,615 USDC (approx) The spot price is 3,000, but the trader gets about 2,961.5 per ETH because of fee and price impact.

Health factor = (sum of collateral value x liquidation threshold) / total debt value. Below 1, the position can be liquidated. Example: collateral 10 ETH at 3,000 dollars = 30,000 dollars, liquidation threshold 80 percent, debt 18,000 dollars of stablecoins. HF = (30,000 x 0.80) / 18,000 = 1.33. If ETH falls to 2,250, collateral is 22,500, so HF = 18,000 / 18,000 = 1.0 and liquidation begins. Users raise HF by repaying debt or adding collateral.

import {AggregatorV3Interface} from '@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV3Interface.sol'; AggregatorV3Interface public feed; uint256 constant MAX_DELAY = 1 hours; function getPrice() public view returns (uint256) { (, int256 answer, , uint256 updatedAt, ) = feed.latestRoundData(); require(answer > 0, 'bad price'); require(block.timestamp - updatedAt <= MAX_DELAY, 'stale price'); return uint256(answer); // scale using feed.decimals(), often 8 } Always check staleness, sign and decimals; on L2s also check the sequencer uptime feed.

uint256 public rewardRate; // tokens per second uint256 public rewardPerTokenStored; uint256 public lastUpdate; uint256 public totalStaked; mapping(address => uint256) public paid; mapping(address => uint256) public rewards; mapping(address => uint256) public staked; function rewardPerToken() public view returns (uint256) { if (totalStaked == 0) return rewardPerTokenStored; return rewardPerTokenStored + (block.timestamp - lastUpdate) * rewardRate * 1e18 / totalStaked; } modifier updateReward(address a) { rewardPerTokenStored = rewardPerToken(); lastUpdate = block.timestamp; rewards[a] += staked[a] * (rewardPerTokenStored - paid[a]) / 1e18; paid[a] = rewardPerTokenStored; _; }

ERC-4626 standardises yield vaults: users deposit an asset and receive shares, and share price rises as the vault earns yield. import {ERC4626} from '@openzeppelin/contracts/token/ERC20/extensions/ERC4626.sol'; import {ERC20} from '@openzeppelin/contracts/token/ERC20/ERC20.sol'; import {IERC20} from '@openzeppelin/contracts/token/ERC20/IERC20.sol'; contract YieldVault is ERC4626 { constructor(IERC20 asset_) ERC20('Vault Share', 'vSHR') ERC4626(asset_) {} function totalAssets() public view override returns (uint256) { return IERC20(asset()).balanceOf(address(this)); // plus deployed funds } } OpenZeppelin uses virtual shares to defend against the first-depositor inflation attack.

Formula: IL = 2 x sqrt(r) / (1 + r) - 1, where r is the price ratio change. For r = 2: sqrt(2) = 1.414, so IL = 2 x 1.414 / 3 - 1 = -0.057, about 5.7 percent. Example: deposit 1 ETH plus 3,000 USDC when ETH is 3,000 (6,000 dollars). ETH goes to 6,000. Holding would be 9,000 dollars; the pool position is worth about 8,485. Fees earned must exceed the 515 dollar gap for providing liquidity to beat holding.