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();
    }

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

    }
}

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

    }

    createGenesisBlock(){
        return new Block(0, "01/01/2017", "Genesis block", "0");
    }

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

    addBlock(newBlock) {
        newBlock.previousHash = this.getLatestBlock().hash;
        newBlock.hash = newBlock.calculateHash();
        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 sCoin = new Blockchain();
sCoin.addBlock(new Block(1, "17/03/2022", {amount: 4}));
sCoin.addBlock(new Block(2, "20/03/2022", {amount: 10}));
console.log('Is blockchain valid? ' + sCoin.isChainValid());

sCoin.chain[1].data = {amount:100}; 
console.log('Is blockchain valid? ' + sCoin.isChainValid());
//console.log(JSON.stringify(sCoin, null, 4));