index.js

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

class Transaction {
  constructor(fromAddress, toAddress, amount) {
    this.fromAddress = fromAddress;
    this.toAddress = toAddress;
    this.amount = amount;
  }
}

class Block {
  constructor(index, transactions, previousHash = '') {
    this.index = index;
    this.timestamp = Math.floor(Date.now() / 1000);
    this.transactions = transactions;
    this.previousHash = previousHash;
    this.hash = this.calculateHash();
    this.nonce = 0;
  }

  calculateHash() {
    return SHA256(this.previousHash + this.timestamp + JSON.stringify(this.transactions) + 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);
  }
}


class Blockchain {
  constructor() {
    this.chain = [this.createGenesisBlock()];
    this.difficulty = 2;
    this.pendingTransactions = [];
    this.miningReward = 100;
  }

  createGenesisBlock() {
    return new Block(0, [], "0");
  }

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

  minePendingTransactions(miningRewardAddress) {
    let block = new Block(0, this.pendingTransactions, this.getLatestBlock().hash);
    block.mineBlock(this.difficulty);

    console.log('Block successfully mined!');
    this.chain.push(block);

    this.pendingTransactions = [
      new Transaction(null, miningRewardAddress, this.miningReward)
    ];
  }

  createTransaction(transaction) {
    this.pendingTransactions.push(transaction);
  }

  getBalanceOfAddress(address) {
    let balance = 0;

    for(const block of this.chain) {
      for(const trans of block.transactions) {
        if(trans.fromAddress === address) {
          balance -= trans.amount;
        }

        if(trans.toAddress === address) {
          balance += trans.amount;
        }
      }
    }

    return balance;
  }

  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 myCoin = new Blockchain();
myCoin.createTransaction(new Transaction('address1', 'address2', 100));
myCoin.createTransaction(new Transaction('address2', 'address1', 50));

console.log('Starting the miner...');
myCoin.minePendingTransactions('my-address');

console.log('Balance of my is', myCoin.getBalanceOfAddress('my-address'));

console.log('Starting the miner again...');
myCoin.minePendingTransactions('my-address');

console.log('Balance of my is', myCoin.getBalanceOfAddress('my-address'));