step3.js

const fs = require('fs');
const crypto = require('crypto');

const secret = 'windranger';
let blocks, block, hash, content;

if (fs.existsSync('./blocks.txt')) {
  blocks = JSON.parse(fs.readFileSync('./blocks.txt'));
  const blocksAmount = blocks.length;
  for (let i = 0; i < blocksAmount; i++) {
    const potentialHash = i > 0?
        crypto
          .createHmac('sha256', secret)
          .update(i+'_'+blocks[i]['content']+'_'+blocks[i-1]['block_hash'])
          .digest('hex'):
        crypto
          .createHmac('sha256', secret)
          .update('0_'+blocks[0]['content'])
          .digest('hex');
    if (blocks[i]['block_hash'] !== potentialHash) {
      throw new Error('Hash is not satisfying');
    }
  }
  content = makeid();
  hash = crypto.createHmac('sha256', secret)
                     .update(blocksAmount+'_'+content+'_'+blocks[blocksAmount-1]['block_hash'])
                     .digest('hex');

  block = {
    block_number: blocksAmount,
    block_hash: hash,
    content
  }
} else {
  blocks = [];
  content = makeid();
  hash = crypto.createHmac('sha256', secret)
                     .update('0_'+content)
                     .digest('hex');
  block = {
    block_number: 0,
    block_hash: hash,
    content
  }
}
blocks.push(block);
fs.writeFileSync('./blocks.txt', JSON.stringify(blocks));

function makeid() {
  let text = "";
  let possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

  for (let i = 0; i < 5; i++) {
    text += possible.charAt(Math.floor(Math.random() * possible.length));
  }

  return text;
}