Really simple demonstration of how block chain works.

bitCoin.js

// you need to first run `npm i --save crypto-js`
const SHA256 = require('crypto-js/sha256');

class Block{
  
  constructor(index, data, previsousHash='', date = new Date()){
    // takes in the following params; index: number, timestamp: string/object, data: object and previousHash: string
    this.index = index,
    this.timestamp = this.formatTimeStamp(),
    this.data = data,
    this.previsousHash = this.calculateHash(),
    this.date = date
  }

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

  getTime(){
    let date = new Date();
    return {
        hours: date.getHours(),
        minutes: date.getMinutes(),
        seconds: date.getSeconds(),
        milliseconds: date.getMilliseconds()
      }
  }

  formatTimeStamp(){
    let date = new Date();
    let day = date.getDay();
    let month = date.getMonth();
    let year = date.getFullYear();
    return {
      day: day,
      month: month,
      time: this.getTime(),
      year: year
    }
  }
}

class BlockChain{

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

  createGenesisBlock(){
    return new Block(0, 'Start 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(){
    // loops through all the chains doing some checks
    for(let i = 1; i < this.chain.length; i++){
      let currBlock = this.chain[i];
      let prevBlock = this.chain[i - 1];
      
      // it currentHash doesn't match the calculateHash, chain is invalid
      if(currBlock.hash !== currBlock.calculateHash()){
          return false;
      }

      // compare previousHash from currentblock doesn't match that from previous block hash chain is invalid
      if(currBlock.previousHash !== prevBlock.hash){
          return false;
      }
    }
    // else chain is valid
    return true;
  }
}

let bitCoin = new BlockChain();
bitCoin.addBlock(new Block(1, {amount: 4}));
bitCoin.addBlock(new Block(2, {amount: 10}));

console.log('First time validation before inteferance: ' + bitCoin.isChainValid());

bitCoin.chain[1].data = {amount: 10};
bitCoin.chain[1].hash = bitCoin.chain[1].calculateHash();

console.log('After inteferance: ' + bitCoin.isChainValid()); //this returns false

// console.log(JSON.stringify(bitCoin, null, 4)); //uncomment this to see the full bitCoin object