Bitcoin APIs & Development

Example: Fetch Current Fee Estimates

// JavaScript / Node.js
const response = await fetch(
  'https://mempool.space/api/v1/fees/recommended'
);
const fees = await response.json();
// { fastestFee: 12, halfHourFee: 8, hourFee: 4, economyFee: 2 }
console.log(`Next block fee: ${fees.fastestFee} sat/vB`);

Example: Query Address Balance

# Python
import requests

address = 'bc1qxy2kgdygjrsqtzq2n0yrf2493p83kkfjhx0wlh'
url = f'https://mempool.space/api/address/{address}'
data = requests.get(url).json()

funded = data['chain_stats']['funded_txo_sum']
spent = data['chain_stats']['spent_txo_sum']
balance_sats = funded - spent
print(f'Balance: {balance_sats} sats ({balance_sats/1e8} BTC)')

Example: Get Block Info via Bitcoin Core RPC

# Using curl with Bitcoin Core RPC
curl --user myuser:mypassword \
  --data-binary '{"jsonrpc":"1.0","method":"getblockcount"}' \
  -H 'Content-Type: text/plain;' \
  http://127.0.0.1:8332/

# Response: {"result":890123,"error":null,"id":null}

Example: Fetch Bitcoin Price in Multiple Currencies

// JavaScript - CoinGecko API (no key required)
const response = await fetch(
  'https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=usd,eur,gbp'
);
const data = await response.json();
// { bitcoin: { usd: 87500, eur: 80200, gbp: 69800 } }
console.log(`BTC/USD: $${data.bitcoin.usd.toLocaleString()}`);

Example: Query Transaction Details

# Python - Blockchain.info API
import requests

# Get latest Bitcoin exchange rates
rates = requests.get('https://blockchain.info/ticker').json()
usd_price = rates['USD']['last']
print(f'BTC/USD: ${usd_price:,.2f}')

# Get address transaction count
addr = 'bc1qxy2kgdygjrsqtzq2n0yrf2493p83kkfjhx0wlh'
info = requests.get(f'https://blockchain.info/rawaddr/{addr}?limit=0').json()
print(f'Total transactions: {info["n_tx"]}')

Example: Combined Price and Fee Tracker (TypeScript)

interface BitcoinData {
  price: { usd: number; eur: number };
  fees: { fastest: number; halfHour: number; hour: number };
  blockHeight: number;
}

async function getBitcoinData(): Promise<BitcoinData> {
  const [priceRes, feeRes, blockRes] = await Promise.all([
    fetch('https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=usd,eur'),
    fetch('https://mempool.space/api/v1/fees/recommended'),
    fetch('https://mempool.space/api/blocks/tip/height'),
  ]);

  const price = await priceRes.json();
  const fees = await feeRes.json();
  const blockHeight = await blockRes.json();

  return {
    price: { usd: price.bitcoin.usd, eur: price.bitcoin.eur },
    fees: { fastest: fees.fastestFee, halfHour: fees.halfHourFee, hour: fees.hourFee },
    blockHeight,
  };
}

// Usage
const data = await getBitcoinData();
console.log(`BTC: $${data.price.usd.toLocaleString()}`);
console.log(`Next-block fee: ${data.fees.fastest} sat/vB`);
console.log(`Block height: ${data.blockHeight.toLocaleString()}`);

Example: Mempool.space WebSocket (Real-Time Blocks)

// Browser or Node.js with ws package
const ws = new WebSocket('wss://mempool.space/api/v1/ws');

ws.onopen = () => {
  // Subscribe to new block notifications
  ws.send(JSON.stringify({ action: 'want', data: ['blocks'] }));
};

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  if (data.block) {
    console.log(`New block: #${data.block.height}`);
    console.log(`Transactions: ${data.block.tx_count}`);
    console.log(`Size: ${(data.block.size / 1e6).toFixed(2)} MB`);
  }
};

Example: Kraken WebSocket (Real-Time Price Feed)

const ws = new WebSocket('wss://ws.kraken.com');

ws.onopen = () => {
  ws.send(JSON.stringify({
    event: 'subscribe',
    pair: ['XBT/USD'],
    subscription: { name: 'ticker' },
  }));
};

ws.onmessage = (event) => {
  const msg = JSON.parse(event.data);
  if (Array.isArray(msg) && msg[2] === 'ticker') {
    const ticker = msg[1];
    console.log(`BTC/USD: $${parseFloat(ticker.c[0]).toLocaleString()}`);
  }
};