The first logical step is to decide the block structure. To keep things as simple as possible we include only the most necessary: index, timestamp, data, hash and previous hash.

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

The block needs to be hashed to keep the integrity of the data. A SHA-256 is taken over the content of the block.

var calculateHash = (index, previousHash, timestamp, data) => { return CryptoJS.SHA256(index + previousHash + timestamp + data).toString(); };

To generate a block we must know the hash of the previous block and create the rest of the required content (= index, hash, data and timestamp). Block data is something that is provided by the end-user.

var generateNextBlock = (blockData) => { var previousBlock = getLatestBlock(); var nextIndex = previousBlock.index + 1; var nextTimestamp = new Date().getTime() / 1000; var nextHash = calculateHash(nextIndex, previousBlock.hash, nextTimestamp, blockData); return new Block(nextIndex, previousBlock.hash, nextTimestamp, blockData, nextHash); };

A in-memory Javascript array is used to store the blockchain. The first block of the blockchain is always a so-called “genesis-block”, which is hard coded.

var getGenesisBlock = () => { return new Block(0, "0", 1465154705, "my genesis block!!", "816534932c2b7154836da6afc367695e6337db8a921823784c14378abed4f7d7"); };

At any given time we must be able to validate if a block or a chain of blocks are valid in terms of integrity. This is true especially when we receive new blocks from other nodes and must decide whether to accept them or not.

var blockchain = [getGenesisBlock()]; var isValidNewBlock = (newBlock, previousBlock) => { if (previousBlock.index + 1 !== newBlock.index) { console.log('invalid index'); return false; } else if (previousBlock.hash !== newBlock.previousHash) { console.log('invalid previoushash'); return false; } else if (calculateHashForBlock(newBlock) !== newBlock.hash) { console.log('invalid hash: ' + calculateHashForBlock(newBlock) + ' ' + newBlock.hash); return false; } return true; };

There should always be only one explicit set of blocks in the chain at a given time. In case of conflicts (e.g. two nodes both generate block number 72) we choose the chain that has the longest number of blocks.

var replaceChain = (newBlocks) => { if (isValidChain(newBlocks) && newBlocks.length > blockchain.length) { console.log('Received blockchain is valid. Replacing current blockchain with received blockchain'); blockchain = newBlocks; broadcast(responseLatestMsg()); } else { console.log('Received blockchain invalid'); } };

The user must be able to control the node in some way. This is done by setting up a HTTP server.

var initHttpServer = () => { var app = express(); app.use(bodyParser.json()); app.get('/blocks', (req, res) => res.send(JSON.stringify(blockchain))); app.post('/mineBlock', (req, res) => { var newBlock = generateNextBlock(req.body.data); addBlock(newBlock); broadcast(responseLatestMsg()); console.log('block added: ' + JSON.stringify(newBlock)); res.send(); }); app.get('/peers', (req, res) => { res.send(sockets.map(s => s._socket.remoteAddress + ':' + s._socket.remotePort)); }); app.post('/addPeer', (req, res) => { connectToPeers([req.body.peer]); res.send(); }); app.listen(http_port, () => console.log('Listening http on port: ' + http_port)); };

It should be noted that the node actually exposes two web servers: One for the user to control the node (HTTP server) and one for the peer-to-peer communication between the nodes.(Websocket HTTP server)