Web3 & Blockchain Development

Web3 & Blockchain Development

Overview

Master Web3 development with Solidity smart contracts, DApp architecture, and modern tooling (Hardhat, ethers.js v6). Learn security-first patterns, gas optimization, and AI-enhanced development with ChatWeb3 and Aider + Gemini for Solidity (2024).

When to Use This Skill

• Building decentralized applications (DApps) on Ethereum
• Writing secure smart contracts for DeFi, NFTs, or DAOs
• Creating token contracts (ERC-20, ERC-721, ERC-1155)
• Integrating blockchain functionality into web applications
• Auditing smart contracts for security vulnerabilities
• Optimizing gas costs for efficient contract execution
• Deploying contracts to mainnet or Layer 2 networks

Core Principles

1. Solidity Security Patterns

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/security/Pausable.sol";

// ✅ GOOD: Security-first contract
contract SecureWallet is ReentrancyGuard, Ownable, Pausable {
    mapping(address => uint256) public balances;

    event Deposit(address indexed user, uint256 amount);
    event Withdrawal(address indexed user, uint256 amount);

    // ✅ Use receive() for accepting Ether
    receive() external payable {
        balances[msg.sender] += msg.value;
        emit Deposit(msg.sender, msg.value);
    }

    // ✅ Checks-Effects-Interactions pattern (prevents reentrancy)
    function withdraw(uint256 amount) external nonReentrant whenNotPaused {
        // Checks
        require(amount > 0, "Amount must be greater than 0");
        require(balances[msg.sender] >= amount, "Insufficient balance");

        // Effects (update state BEFORE external call)
        balances[msg.sender] -= amount;

        // Interactions (external call last)
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");

        emit Withdrawal(msg.sender, amount);
    }

    // ✅ Emergency stop mechanism
    function pause() external onlyOwner {
        _pause();
    }

    function unpause() external onlyOwner {
        _unpause();
    }
}

// ❌ BAD: Vulnerable to reentrancy attack
contract VulnerableWallet {
    mapping(address => uint256) public balances;

    function withdraw(uint256 amount) external {
        require(balances[msg.sender] >= amount);

        // ❌ WRONG: External call before state update
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success);

        // Attacker can re-enter withdraw() before this line executes!
        balances[msg.sender] -= amount;
    }
}