Blockchain Complete Guide — How It Works, Why It Matters, When to Use It

const crypto = require('crypto');

// Hash function demonstration
function sha256(data) {
  return crypto.createHash('sha256').update(data).digest('hex');
}

// Same input → same hash (deterministic)
console.log(sha256('Hello blockchain'));
// → 'd3fbb8d87de659c89978f3b5c6e06b9b45b5b1f7bace4e5b3d6f8e3b9e9c3a8'

// Tiny change → completely different hash (avalanche effect)
console.log(sha256('Hello blockchain!'));
// → '7f9c2ba4e88f827d616045507605853ed73b8093f6efbc88eb1a6eacfa66ef26'

// Block structure
const block = {
  index: 3,
  timestamp: Date.now(),
  transactions: [
    { from: 'Alice', to: 'Bob', amount: 1.5 },
    { from: 'Charlie', to: 'Diana', amount: 0.5 },
  ],
  previousHash: '000abc123def456...',
  nonce: 0,
};

// Block hash includes everything — change anything and hash changes
block.hash = sha256(JSON.stringify(block));

console.log('Block hash:', block.hash);

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleStorage {
    uint256 private storedValue;
    address public owner;

    constructor() {
        owner = msg.sender;  // whoever deploys = owner
    }

    // Only owner can set value
    function setValue(uint256 _value) public {
        require(msg.sender == owner, "Not the owner");
        storedValue = _value;
    }

    // Anyone can read
    function getValue() public view returns (uint256) {
        return storedValue;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// Minimal ERC-20 token implementation
contract SimpleToken {
    string public name = "MyToken";
    string public symbol = "MTK";
    uint8 public decimals = 18;
    uint256 public totalSupply;

    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;

    event Transfer(address indexed from, address indexed to, uint256 value);

    constructor(uint256 _initialSupply) {
        totalSupply = _initialSupply * 10 ** uint256(decimals);
        balanceOf[msg.sender] = totalSupply;
    }

    function transfer(address _to, uint256 _value) public returns (bool) {
        require(balanceOf[msg.sender] >= _value, "Insufficient balance");
        balanceOf[msg.sender] -= _value;
        balanceOf[_to] += _value;
        emit Transfer(msg.sender, _to, _value);
        return true;
    }
}

import { ethers } from 'ethers';

// Connect to Ethereum via a public RPC provider
const provider = new ethers.JsonRpcProvider(
  'https://mainnet.infura.io/v3/YOUR_PROJECT_ID'
);

// Read the latest block
const block = await provider.getBlock('latest');
console.log('Block number:', block.number);
console.log('Block hash:', block.hash);
console.log('Transactions:', block.transactions.length);

// Check an account balance
const balance = await provider.getBalance('0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045');
console.log('Balance (ETH):', ethers.formatEther(balance));

// Send a transaction (requires a wallet/signer)
const wallet = new ethers.Wallet('YOUR_PRIVATE_KEY', provider);

const tx = await wallet.sendTransaction({
  to: '0xRecipientAddress',
  value: ethers.parseEther('0.01'), // 0.01 ETH
});

console.log('Transaction hash:', tx.hash);
await tx.wait(); // Wait for confirmation
console.log('Confirmed!');