Mini Blockchain Project — Part 1: Basic Block & Chain Structure

Let's think about this for a second

In this project we'll take the block, hashing, and immutability concepts you learned in the Basic chapter and actually wire them together in real code. In real blockchains like Bitcoin and Ethereum, each block's hash gets stored in the next block's previousHash field, linking them into a chain. To really understand this mechanism, we'll compute a SHA256 hash directly with Node.js's built-in crypto module and write our own Block class. This part covers building the Block class and getting the Blockchain class's genesis block started — transactions and mining come in the next part.

Let's build it

Open a new project folder and create a blockchain.js file. Require the crypto module as a built-in (no npm install needed). Give the Block class an index, timestamp, data, previousHash, and hash field, and inside the calculateHash() method, hash index+previousHash+timestamp+JSON.stringify(data) with SHA256. In the Blockchain class, initialize the chain array with createGenesisBlock() and add a getLatestBlock() method. Finally, print the genesis block with console.log(JSON.stringify(myChain, null, 2)).

• Have you written the Block class with the index, previousHash, data, and hash fields?
• Is calculateHash() correctly returning a SHA256 output?
• Have you confirmed that chain[0] on the Blockchain class is really the genesis block?

Code Example

const crypto = require('crypto');

class Block {
  constructor(index, timestamp, data, previousHash = '') {
    this.index = index;
    this.timestamp = timestamp;
    this.data = data;
    this.previousHash = previousHash;
    this.hash = this.calculateHash();
  }

  calculateHash() {
    return crypto
      .createHash('sha256')
      .update(this.index + this.previousHash + this.timestamp + JSON.stringify(this.data))
      .digest('hex');
  }
}

class Blockchain {
  constructor() {
    this.chain = [this.createGenesisBlock()];
  }

  createGenesisBlock() {
    return new Block(0, Date.now(), 'Genesis Block', '0');
  }

  getLatestBlock() {
    return this.chain[this.chain.length - 1];
  }
}

const myChain = new Blockchain();
console.log(JSON.stringify(myChain, null, 2));

Running node blockchain.js will print the chain array to the terminal, showing one genesis block with its hash value in hex string format, formatted as JSON.