Web3.js Guide — Connecting to Ethereum from JavaScript

npm install web3

const { Web3 } = require("web3");

// Option 1: Public RPC endpoint (read-only)
const web3 = new Web3(
  "https://eth-mainnet.g.alchemy.com/v2/YOUR_KEY"
);

// Option 2: Browser wallet (MetaMask)
if (window.ethereum) {
  const web3 = new Web3(window.ethereum);
  await window.<a href="/cryptocurrency/ethereum/">Ethereum</a>.enable();
}

// Get ETH balance
const balance = await web3.eth.getBalance(
  "0x742d35Cc6634C0532925a3b844Bc9e7595f2bD18"
);
console.log(`${web3.utils.fromWei(balance, "ether")} ETH`);

// Get latest block
const block = await web3.eth.getBlock("latest");
console.log(`Block #${block.number} — ${block.transactions.length} txs`);

// Get transaction
const tx = await web3.eth.getTransaction(
  "0xabc...123"
);
console.log(tx);

const accounts = await web3.eth.getAccounts();
const tx = await web3.eth.sendTransaction({
  from: accounts[0],
  to: "0x...recipient",
  value: web3.utils.toWei("0.01", "ether"),
  gas: 21000,
});
console.log(`Tx hash: ${tx.transactionHash}`);

const contractABI = [
  // Minimal ERC-20 ABI
  {
    constant: true,
    inputs: [{ name: "_owner", type: "address" }],
    name: "balanceOf",
    outputs: [{ name: "", type: "uint256" }],
    type: "function",
  },
];

const contractAddress = "0xdAC17F958D2ee523a2206206994597C13D831ec7";

const contract = new web3.eth.Contract(
  contractABI,
  contractAddress
);

// Read from contract
const balance = await contract.methods
  .balanceOf("0x...")
  .call();
console.log(`Balance: ${balance}`);

// Write to contract (requires MetaMask)
const result = await contract.methods
  .transfer("0x...recipient", 1000)
  .send({ from: accounts[0] });

// Listen for Transfer events
contract.events
  .Transfer({ filter: { from: "0x..." } })
  .on("data", (event) => {
    console.log("Transfer event:", event.returnValues);
  })
  .on("error", console.error);