Blockchain & Web3 Interview Questions

Question 1

What is blockchain technology?

Accepted Answer

Blockchain is a decentralized, immutable distributed ledger that records transactions across many computers securely. Structure: chain of blocks, each containing transactions, timestamp, and hash of previous block. Decentralized: no central authority, maintained by network of nodes. Immutable: once added, data can't be altered without consensus. Consensus mechanisms: Proof of Work (Bitcoin), Proof of Stake (Ethereum 2.0). Use cases: cryptocurrency, supply chain, identity, DeFi, NFTs. Key properties: transparency, trustlessness, censorship resistance.

Question 2

What are smart contracts?

Accepted Answer

Smart contracts are self-executing programs on blockchain that automatically enforce agreement terms when conditions are met. Written in Solidity (Ethereum), deployed to blockchain. Once deployed, code is immutable and runs exactly as written. Execute automatically when triggered by transactions. Gas: fee paid to execute contract operations. Use cases: token creation (ERC-20, ERC-721), DeFi protocols, DAOs, escrow. Limitations: can't access off-chain data directly (need oracles like Chainlink), gas costs, immutability (can't fix bugs easily). Testing: Hardhat, Foundry.

Question 3

What is the difference between Proof of Work and Proof of Stake?

Accepted Answer

PoW uses computational power to validate transactions (mining); PoS uses staked cryptocurrency as collateral for validation rights. PoW: miners solve complex puzzles, first to solve adds the block and gets reward. Secure but energy-intensive (Bitcoin uses more electricity than some countries). PoS: validators stake tokens, selected based on stake amount + randomness. Slashing: lose stake for malicious behavior. Benefits: 99.95% less energy, faster finality. Ethereum moved from PoW to PoS in 'The Merge' (2022). Other: Delegated PoS, Proof of Authority, Proof of History (Solana).

Question 4

What are ERC-20 and ERC-721 token standards?

Accepted Answer

ERC-20 defines fungible tokens (cryptocurrencies); ERC-721 defines non-fungible tokens (NFTs) with unique properties. ERC-20: standard interface for fungible tokens. Functions: totalSupply, balanceOf, transfer, approve, transferFrom. Examples: USDC, DAI, UNI. ERC-721: each token is unique with tokenId. Functions: ownerOf, safeTransferFrom, tokenURI (metadata). Examples: CryptoPunks, BAYC. ERC-1155: multi-token standard (both fungible and non-fungible in one contract). ERC-4626: tokenized vault standard for DeFi.

Question 5

What is DeFi (Decentralized Finance)?

Accepted Answer

DeFi replaces traditional financial intermediaries with smart contracts for lending, borrowing, trading, and earning yield. Protocols: (1) DEX: Uniswap, SushiSwap (automated market makers). (2) Lending: Aave, Compound (supply/borrow crypto). (3) Stablecoins: DAI (algorithmic), USDC (backed). (4) Yield farming: provide liquidity, earn rewards. (5) Derivatives: Synthetix, dYdX. Risks: smart contract bugs, impermanent loss, oracle manipulation, regulatory uncertainty. Total Value Locked (TVL) measures DeFi adoption. Flash loans: uncollateralized loans within single transaction.

Question 6

What is a 51% attack?

Accepted Answer

A 51% attack occurs when an entity controls majority of network's mining/staking power, enabling double-spending and transaction censorship. In PoW: attacker with >50% hash rate can mine faster than honest chain, create alternative chain, reverse transactions (double-spend). In PoS: need >50% of staked tokens (extremely expensive on Ethereum). Prevention: high network hash rate/stake value, checkpointing, economic penalties (slashing). Smaller chains more vulnerable. Bitcoin's security: would cost billions in hardware + electricity. Not about stealing funds directly — about reversing your own transactions.

Question 7

What are Layer 2 scaling solutions?

Accepted Answer

Layer 2 solutions process transactions off the main chain for speed and cost, then settle on Layer 1 for security. Types: (1) Rollups: bundle transactions off-chain, post compressed data on-chain. Optimistic Rollups (Arbitrum, Optimism): assume valid, fraud proofs challenge. ZK-Rollups (zkSync, StarkNet): cryptographic validity proofs, instant finality. (2) State channels: off-chain transactions between parties (Lightning Network). (3) Sidechains: separate chains with own consensus (Polygon PoS). Benefits: 10-100x cheaper, faster transactions. Tradeoff: some trust assumptions depending on type.

Question 8

How would you audit a smart contract for security?

Accepted Answer

Review code for common vulnerabilities (reentrancy, overflow), use automated tools (Slither, Mythril), and conduct formal verification. Process: (1) Manual code review: check for reentrancy, integer overflow, access control, front-running, oracle manipulation. (2) Automated tools: Slither (static analysis), Mythril (symbolic execution), Echidna (fuzzing). (3) Test coverage: unit tests, integration tests, invariant tests. (4) Formal verification for critical logic. (5) Economic attack modeling. (6) Check against known vulnerability databases. Common fixes: ReentrancyGuard, SafeMath (pre-Solidity 0.8), access modifiers, timelocks. Bug bounty post-launch.

Question 9

Ethereum Foundation: What are the key upgrades in Ethereum's roadmap?

Accepted Answer

Ethereum's roadmap includes: The Merge (PoS), The Surge (sharding), The Scourge, The Verge, The Purge, and The Splurge. The Merge (completed 2022): PoW → PoS, 99.95% energy reduction. The Surge: sharding via danksharding/proto-danksharding (EIP-4844), 100k TPS goal. The Scourge: mitigate MEV centralization risks. The Verge: Verkle trees for lighter nodes, enable statelessness. The Purge: reduce node storage requirements, simplify protocol. The Splurge: miscellaneous improvements. Current focus: proto-danksharding (blobs) for cheaper L2 data, account abstraction (ERC-4337) for better UX.

Question 10

Blockchain & Web3 Development Course

What You'll Learn

Career Opportunities

Course Completion Certificate

Course Modules Overview

Blockchain Fundamentals

Smart Contract Development

DApp Development

DeFi and Tokenomics

Blockchain Security

Advanced Blockchain Topics

DeFi Protocol Development

NFT & Gaming Development

Enterprise Blockchain & Web3

DeFi and Decentralized Finance

DAOs and Governance

Web3 Career and Industry Trends

Course Curriculum

Blockchain Fundamentals

Smart Contract Development

DApp Development

DeFi and Tokenomics

Blockchain Security

Advanced Blockchain Topics

DeFi Protocol Development

NFT & Gaming Development

Enterprise Blockchain & Web3

DeFi and Decentralized Finance

DAOs and Governance

Web3 Career and Industry Trends

Your Learning Path

Beginner

Intermediate

Advanced

Tools & Technologies

Solidity

Hardhat

Web3.js

MetaMask

OpenZeppelin

IPFS

Ready to Start Learning?

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