In this series of blog posts, I will be writing about the key concepts of blockchain with javascript. Blockchain is a database that record all the transactions. Think it as a ledger or a book and each page is a block. Each block contains the data of transactions, timestamp which is like page number, last block&#39;s hash value or page number, difficulty, nonce value and its own hash value. First, we start with defining these properties in a constructor. <code> class Block {</code><code> constructor({ lastBlockHash, hash, timestamp, data, nonce, difficulty }) {</code><code> this.timestamp = timestamp;</code><code> this.lastBlockHash = lastBlockHash;</code><code> this.hash = hash;</code><code> this.data = data;</code><code> this.nonce = nonce;</code><code> this.difficulty = difficulty;</code><code> } </code><code> }</code><code> </code>if there are many arguments to pass, it is better to wrap them up with an object so we do not need to remember the order of arguments. Each block is referencing the previous block with <code>lastBlockHash. </code>This makes blocks linked to each other. Just like a page in a book, pages come in order. But what is the <code>lastBlockHash</code> of the first block? First block of a blockchain is called genesis-block. In &quot;&#39;constants.js&quot; file we can manually define properties for genesis-block. <code>export const GENESIS_DATA = {</code><code> timestamp: 1,</code><code> lastBlockHash: &quot;-----&quot;,</code><code> hash: &quot;hash-one&quot;,</code><code> difficulty: INITIAL_DIFFICULTY,</code><code> nonce: 0, // we will get this</code><code> data: [],</code><code> };</code> Since javascript is a functional programming language, we add a new method to the class to create the genesis block. <code> import { GENESIS_DATA} from &quot;./constants.js&quot;;</code><code></code> <code> class Block {</code><code> constructor({ lastBlockHash, hash, timestamp, data, nonce, difficulty }) {</code><code> this.timestamp = timestamp;</code><code> this.lastBlockHash = lastBlockHash;</code><code> this.hash = hash;</code><code> this.data = data;</code><code> this.nonce = nonce;</code><code> this.difficulty = difficulty;</code><code> }</code><code> static getGenesisBlock() {</code><code> return new Block(GENESIS_DATA);</code><code> }</code><code> }</code><code></code> Next step, define a method to mine the block. Mining the block is the creation of a new block. Since it is very expensive operation, it is called mining. Each block is created by the blockchain class, we will explore blockchain next, blockchain will add the block to the blockchain if the new block is valid. <code>static mineBlock({ lastBlock, data }) {</code><code> let timestamp, binaryFormOfHash, hash;</code><code> const lastBlockHash = lastBlock.hash;</code><code> let { difficulty } = lastBlock;</code><code> let nonce = 0;</code><code> do {</code><code> nonce++;</code><code> timestamp = Date.now();</code><code> difficulty = Block.adjustDifficulty({</code><code> originalBlock: lastBlock,</code><code> timestamp,</code><code> });</code><code> hash = cryptoHash(timestamp, nonce, lastBlockHash, data, difficulty);</code><code> binaryFormOfHash = hexToBinary(hash);</code><code> } while (</code><code> binaryFormOfHash.substring(0, difficulty) !== &quot;0&quot;.repeat(difficulty)</code><code> );</code><code> // creating a new block</code><code> return new this({</code><code> timestamp,</code><code> lastBlockHash,</code><code> data,</code><code> difficulty,</code><code> hash,</code><code> nonce,</code><code> });</code><code> }</code> This will be called by Blockchain class which holds the chain of blocks as an array. This is how we will have access to last block inside Blockhain: <code>lastBlock: this.chain[this.chain.length - 1]</code>, Second argument &quot;data&quot; represents the array of valid transactions which will be passed by the Transactions class later on. Now let&#39;s talk about nonce and difficulty. Nonce is short for &quot;number used once&quot;. It is just a simple number but probably the most important input of the equation. After mining a block, bitcoin algorithm automatically adjust so that it takes approximately 10 minutes to create a new blog. This difficulty is a number that represents how many 0&#39;s should hash value start with. For example, if difficulty=4, first 4 characters should be 0. <code> 0000110111001100101011010110010010100011110011001111110100000110</code><code> 0101100010111110111110110011110100001101010010100011110111001010</code><code> 0010000011101010110000110101010001110001100110000110011111001110</code><code> 1111000010000111001110101100000111000110101000001111111110011010</code> this would be valid hash and whoever finds this first would collect the mining rewards. I used node&#39;s &quot;crypto&quot; module for hashing. <code>export const cryptoHash = (...args) =&gt; {</code><code> // this is an instance of Hash class</code><code> const hash = crypto.createHash(&quot;sha256&quot;);</code><code> //creates hash value. args should be string</code><code> hash.update(</code><code> args</code><code> .map((arg) =&gt; JSON.stringify(arg))</code><code> .sort()</code><code> .join(&quot; &quot;)</code><code> );</code><code> //digest means the result of the hash. 4 bits=1hex</code><code> return hash.digest(&quot;hex&quot;);</code><code> };</code> This is how I call it inside <code>mineBlock</code>: <code>hash = cryptoHash(timestamp, nonce, lastBlockHash, data, difficulty)</code> Hashing is the digital fingerprint of given arguments. In our example, we pass &quot;timestamp&quot;, &quot;nonce&quot;, &quot;lastBlockHash&quot;, &quot;data&quot; and &quot;difficulty&quot; to cryptoHash function and it converts those arguments into 0&#39;s and 1&#39;s. Since &quot;sha256&quot; is used in algorithm, total of 0&#39;s and 1&#39;s will be 256 and it will be called 256 bits hash value. Hashing is a 1 way conversion, that means given a hash value, you cannot find which data has passed to the function. Another key important thing in hashing, hashing algorithm would always create same hash value for the given data. If you call <code>cryptoHash(&quot;hash-me&quot;) </code>for million times, each time you will get the same hash value. After a short introduction of hashing, let&#39;s get back to our &quot;nonce&quot;. Among those 5 arguments that we are passing to &quot;cryptoHash()&quot;, &quot;timestamp&quot;, &quot;difficulty&quot;, &quot;lastBlockHash&quot; and &quot;data&quot; are constant values. So this <code> hash = cryptoHash(timestamp,lastBlockHash, data, difficulty)</code> would always create the same value. But our goal is to find a hash value that will meet this criteria: <code> binaryFormOfHash.substring(0, difficulty) !== &quot;0&quot;.repeat(difficulty)</code><code></code><code> </code>So we need to create a mechanism that will generate a new hash value till we meet the above condition. That&#39;s why we start from &quot;nonce=0&quot; and we simply increment it by 1 till we meet the condition. Lastly we add a method that adjust the difficulty based on mine rate. <code> static adjustDifficulty({ lastBlock, timestamp }) {</code><code> const { difficulty } = lastBlock;</code><code> // we do not want difficulty goes below 1</code><code> if (difficulty &lt; 1) return 1;</code><code></code><code> const newBlockCreationTime = timestamp - lastBlock.timestamp;</code><code> if (newBlockCreationTime &gt; 10*60) {</code><code> return difficulty - 1;</code><code> } else {</code><code> return difficulty + 1;</code><code> }</code><code> }</code> This is our complete Block class: <code> import hexToBinary from &quot;hex-to-binary&quot;;</code><code> import { GENESIS_DATA, MINE_RATE } from &quot;./constant&quot;;</code><code> import { cryptoHash } from &quot;./crypto-hash&quot;</code><code></code><code> class Block {</code><code> constructor({ lastBlockHash, hash, timestamp, data, nonce, difficulty }) {</code><code> this.timestamp = timestamp;</code><code> this.lastBlockHash = lastBlockHash;</code><code> this.hash = hash;</code><code> this.data = data;</code><code> this.nonce = nonce;</code><code> this.difficulty = difficulty;</code><code> }</code><code> static getGenesisBlock() {</code><code> return new Block(GENESIS_DATA);</code><code> }</code><code></code><code> static mineBlock({ lastBlock, data }) {</code><code> let timestamp, binaryFormOfHash, hash;</code><code> // const timestamp = Date.now();</code><code> const lastBlockHash = lastBlock.hash;</code><code> let { difficulty } = lastBlock;</code><code> let nonce = 0;</code><code> do {</code><code> nonce++;</code><code> timestamp = Date.now();</code><code> difficulty = Block.adjustDifficulty({</code><code> originalBlock: lastBlock,</code><code> timestamp,</code><code> });</code><code> hash = cryptoHash(timestamp, nonce, lastBlockHash, data, difficulty);</code><code> binaryFormOfHash = hexToBinary(hash);</code><code> } while (</code><code> binaryFormOfHash.substring(0, difficulty) !== &quot;0&quot;.repeat(difficulty)</code><code> );</code><code> // creating a new block</code><code> return new this({</code><code> timestamp,</code><code> lastBlockHash,</code><code> data,</code><code> difficulty,</code><code> hash,</code><code> nonce,</code><code> });</code><code> }</code><code> </code><code> static adjustDifficulty({lastBlock, timestamp }) {</code><code> const { difficulty } =lastBlock;</code><code> // we do not want difficulty goes below 1</code><code> if (difficulty &lt; 1) return 1;</code><code> const newBlockCreationTime = timestamp -lastBlock.timestamp;</code><code> if (newBlockCreationTime &gt; 10*60) {</code><code> return difficulty - 1;</code><code> } else {</code><code> return difficulty + 1;</code><code> }</code><code> }</code><code> }</code>

Coding Bitcoin in Javascript part-1

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