Build Your Own Blockchain with JavaScript

main.js

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

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


class Blockchain{
    constructor() {
        this.chain = [this.createGenesisBlock()];
        this.difficulty = 1;
    }

    createGenesisBlock() {
        return new Block(0, '01/01/2018', 'Genesis block', '0');
    }

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

    getBlockByIndex(index) {
        if (index >= this.chain.length) {
            return 'Error: Index out of range! Could not find block.'
        }
        return this.chain[index];
    }

    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 carChain = new Blockchain();
console.log('Mining block 1...');
carChain.addBlock(new Block(1, '01/01/2018', { owner: 'Ben', license: '1-45A-UD9' }));

console.log('Retrieve block 1: ', JSON.stringify(carChain.getBlockByIndex(1), null, 4))

console.log('\n\nMining block 2...');
carChain.addBlock(new Block(2, '01/01/2018', { owner: 'Alice', license: '3-6EY-KL0' }));

console.log('\nTry to change blockchain data: give myself a new car which actually belongs to Ben.')
carChain.chain[1].data = { owner: 'Michiel', license: '1-45A-UD9' };
console.log('Regenerating hash for changed block and adding it back...\n')
carChain.chain[1].hash = carChain.chain[1].calculateHash();

console.log('Blockchain valid? ' + carChain.isChainValid());

console.log('Print My Blockchain\n');
console.log(JSON.stringify(carChain, null, 4));

package.json

{
  "name": "build-your-own-blockchain",
  "main": "main.js",
  "scripts": {
    "start": "node main.js"
  },
  "author": "Michiel Mulders",
  "dependencies": {
    "crypto-js": "^3.1.9-1"
  }
}

Great post, thank you for sharing.
One question: what is the importance of mining, why do we need to have zeros at the beginning of the hash?

Hi @mrharel , thanks!
We don't need to have zeros. For example ethereum uses '0x' as prefix and uses a much harder algorithm.
You can hard code whatever you want to use as a prefix.
Mining allows us to have public validation of the chain.

Side note: I'm more a fan of systems that use a Tangle / DAG / Hashgraph where mining is 'eliminated'.
The transaction proposer has to mine some other transactions first before his transaction will be verified.
Interesting read!