main.js

const SHA256 = require('crypto-js/sha256');

class Block { 
  constructor(index, timestamp, data, previousHash = '') {
    this.index = index; 
    this.timestamp = timestamp;
    this.data = data; 
    this.previousHash = previousHash;
    this.hash = this.calculateHash(); 
    this.nonce = 0; 
  } 

  calculateHash() { 
    return SHA256(this.index + this.previousHash + this.timestamp + JSON.stringify(this.data) + this.nonce).toString();
  }  

  mineBlock(difficulty) {
    while (this.hash.substring(0, difficulty) !== Array(difficulty + 1).join("0")) {  
      this.nonce++;
      this.hash = this.calculateHash(); 
    }

    console.log("Block mined: " + this.hash);
  }
}  // <-- You were missing this curly brace to close the Block class

class Blockchain { 
  constructor() {
    this.chain = [this.createGenesisBlock()]; 
    this.difficulty = 4; 
  }
    
  createGenesisBlock() { 
    return new Block(0, "01/01/2020", "Genesis Block", "0"); 
  }

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

  addBlock(newBlock) { 
    newBlock.previousHash = this.getLatestBlock().hash; 
    newBlock.mineBlock(this.difficulty); 
    this.chain.push(newBlock); 
  }

  isChainValid() {
    for (let i = 1; i < this.chain.length; i++) {
      const currentBlock = this.chain[i];
      const previousBlock = this.chain[i - 1];
      
      if (currentBlock.hash !== currentBlock.calculateHash()) {
        return false;
      }
      if (currentBlock.previousHash !== previousBlock.hash) {
        return false;
      }
    }
    return true;
  }
}

let samikaCoin = new Blockchain();

console.log('Mining block 1...'); 
samikaCoin.addBlock(new Block(1, "10/07/2023", { amount: 4 }));

console.log('Mining block 2...'); 
samikaCoin.addBlock(new Block(2, "12/07/2023", { amount: 10 }));