📌[Project 1] TxnScan: Simple Cryptocurrency Transaction Tracking 💱

• The Challenge: Build an API endpoint that accepts a cryptocurrency address (Ethereum, for example). It should retrieve the last 5 transactions for that address from a blockchain explorer API (e.g., Etherscan) and store them in MongoDB. Allow users to query for transactions by address and date range.
• Focus: Exposes knowledge of external APIs, data parsing, and efficient database storage.

1. Core Technologies Used 🛠️

• Express.js: A web framework for Node.js that handles HTTP requests
• Web3.js: Library to interact with Ethereum blockchain
• MongoDB: Database to store transaction data
• Etherscan API: External service to fetch Ethereum transaction data

2. Key Components 🔑

A. Setup and Configuration

const web3 = new Web3(process.env.ETH_RPC_URL);
const ETHERSCAN_API_KEY = process.env.ETHERSCAN_API_KEY;

• RPC URL: Connection point to Ethereum network (using Sepolia testnet)
• Etherscan API: Access to blockchain data without running a full node

B. Main Features 📋

1. Transaction Fetching (/api/transactions/fetch)

   • Fetches last 5 transactions for an Ethereum address
   • Validates Ethereum addresses using express-validator
   • Stores transactions in MongoDB for quick future access

2. Transaction Querying (/api/transactions/:address)

   • Retrieves stored transactions with date filtering
   • Supports pagination (100 transactions per request)
   • Data is pre-processed and stored for faster retrieval

3. Transaction Statistics (/api/transactions/:address/stats)

   • Calculates important metrics:
     • Total number of transactions
     • Total value transferred
     • Average gas used

📌[Project 2] TokenLens: Token Balance Lookup 👀

• The Challenge: Build an API endpoint that accepts a token contract address and a wallet address. It should query the blockchain (using web3.js) to retrieve the balance of the specified token held by the wallet address and return the balance.
• Focus: Understanding of token contracts and token balance retrieval through web3.js.

1. Core Setup & Infrastructure 🏗️

const web3 = new Web3(process.env.ETH_RPC_URL);

• Web3: The gateway to interact with Ethereum blockchain
• RPC URL: Like a phone line to communicate with Ethereum network
• Sepolia: A test network for Ethereum (mentioned in the fallback URL)

2. Smart Contract Interface (ABI) 📘

const tokenABI = [
    // balanceOf, symbol, name, decimals functions
];

• ABI (Application Binary Interface): The "instruction manual" for interacting with smart contracts
• Functions Included:
  • balanceOf: Check token balance
  • symbol: Get token symbol (like "USDT")
  • name: Get token name (like "Tether USD")
  • decimals: Get token precision (usually 18)

3. API Endpoints 🛣️

A. Token Balance Endpoint (/api/token-balance)

app.post('/api/token-balance', validateRequest, async (req, res) => {
    // ... code
});

What it does:

• Takes token and wallet addresses
• Fetches token details and balance
• Returns formatted data

Smart Features:

• Parallel data fetching using Promise.all
• Address normalization for consistency
• Balance conversion from Wei to Ether
• Comprehensive error handling

B. Token Info Endpoint (/api/token-info/:tokenAddress)

app.get('/api/token-info/:tokenAddress', async (req, res) => {
    // ... code
});

Purpose:

• Quick token detail lookup
• Validates token existence
• Returns basic token information