Simple blockchain

blockchain.js

function Blockchain() {
  const blocks = this.blocks = []

  const getBlockId = (i) => blocks[i] && blocks[i].id || '00000000'

  function hash(str) {
    const hash = Array(8).fill(0);
    for (let i = 0; i < str.length; i++)
      hash[i % 8] ^= str.charCodeAt(i)
    return hash.map((e) => e % 10).join('')
  }

  this.push = (str) => blocks.push({id: hash(getBlockId(blocks.length - 1) + str), data: str})

  this.check = () => blocks.every((b, i) => b.id === hash(getBlockId(i - 1) + b.data))
}

const b = new Blockchain
b.push('test')
b.push('test')
b.push('testing long string')
console.log(b.blocks, `valid=${b.check()}`)
// Corrupting blochchain
b.blocks[1].data = 'TEST'
b.blocks[1].id = '82086666'
console.log(b.blocks, `valid=${b.check()}`)

// $ node blockchain.js
// [ { id: '85788888', data: 'test' },
//   { id: '60866666', data: 'test' },
//   { id: '14677370', data: 'testing long string' } ] 'valid=true'
// [ { id: '85788888', data: 'test' },
//   { id: '82086666', data: 'TEST' },
//   { id: '14677370', data: 'testing long string' } ] 'valid=false'