blockchain.js

const hash = require("crypto-js/sha256");

/**
 * Block class implements a typical block in the chain, has all the methods calculateHash and mineBlock and a constructor method to
 * initialize the properties of the block
 */
class Block {
	/**
	 * [constructor method to initialize the properties of the block]
	 * @param  {[Number]} index        [the position of the block in the chain]
	 * @param  {[String]} timestamp    [timestamp in the format DD/MM/YYYY]
	 * @param  {[Object]} data         [data the block stores/holds]
	 * @param  {[String]} previoushash [the hash of the previous block, 0 if its the first block (Genesis block)]
	 * @param  {[String]} hash         [the hash of the current block]
	 * @param  {{Number}} nounce       [the nounce is used to track the iteration involved in mining a new block in the mineBlock method]
	 */
	constructor(index, timestamp, data, previoushash = " "){
		this.index = index;
		this.timestamp = timestamp;
		this.data = data;
		this.previoushash = previoushash;
		this.hash = this.calculateHash();
		this.nounce = 0;
	}

	/**
	 * [calculateHash method to calculate the hash of the block from the index, timestamp, data, previousHash and nounce properties]
	 * @return {[String]} [returns the hash value in string format]
	 */
	calculateHash(){
		return hash(this.index + this.timeStamp + JSON.stringify(this.data).toString() + this.previoushash + this.nounce).toString();
	}

	/**
	 * [mineBlock method to mine a block, it ensures the computed hash 
	 * of the block starts with a number of zeroes that 
	 * is equal to the difficulty parameter. The nounce variable is used to track
	 * he iteration]
	 * @param  {[Number]} difficulty [the number of leading zeroes in the computed hash of the block]
	 */
	mineBlock(difficulty){
		while(this.hash.substring(0, difficulty) !== Array(difficulty + 1).join("0")){
			this.nounce++;
			this.hash = this.calculateHash();
		}
	}
}

/**
 * BlockChain class implements the blockchain and implements the following methods: createGenesisBlock, getLatestBlock, addBlock, isChainValid; 
 * and a constructor method to initialize the chain and the difficulty level associated with mining a block in this particular chain
 */
class Blockchain {

	/**
	 * [constructor method to initialize the chain array and difficulty level. The chain array is initialized with the 
	 * first block in the chain which is returned from the createGenesis method call.]
	 */
	constructor(difficultyLevel){
		this.chain = [this.createGenesisBlock()];
		this.difficulty = difficultyLevel;
	}

	/**
	 * [createGenesisBlock method to create the first block in a chain setting the previousHash to 0]
	 * @return {[Block]} [a new block]
	 */
	createGenesisBlock(){
		return new Block(0, "14/01/2018", "Genesis Block", "0");
	}

	/**
	 * [getLatestBlock method to retrieve the last block that was added to the chain]
	 * @return {[Block]} [the last block that was added to the chain]
	 */
	getLatestBlock(){
		return this.chain[this.chain.length -1];
	}

	/**
	 * [addBlock method to add a new block to the chain. It sets the previousHash properties, calls the mineBlock method of the Block Object,
	 * and pushes the new block to the end of the chain]
	 */
	addBlock(newblock){
		newblock.previoushash = this.getLatestBlock().hash;
		newblock.mineBlock(this.difficulty);
		this.chain.push(newblock);
	}

	/**
	 * [isChainValid method to verify the integrity or validity of the blockchain by performing the following checks:
	 * -- checks the validity of the hash value of the block
	 * -- checks if the previousHash property of the current block is ]
	 * @return {Boolean} [whether the chain is valid or not: True of False]
	 */
	isChainValid(){
		for(let i = 1; i < this.chain.length; i++){
			const currentBlock = this.chain[i];
			const prevBlock = this.chain[i-1];

			if(currentBlock.hash !== currentBlock.calculateHash()){
				return false;
			}
			if(currentBlock.previoushash !== prevBlock.hash){
				return  false;
			}

		}
		return true;
	}
}
/**
 * How to run this code:
 *
 * 1.	Install the module (preferably globally) with this command in your commanline
 * 		
 * 			npm install -g crypto-js
 * 	 		yarn add global crypto-js
 *
 *	or any package manager of your choice.
 * 2.	To run the code below simply run this file with the command below
 * 			node main.js
 * 			
 * 
 */

let blc = new Blockchain(4); // create a new blockchain -  an instance of the BlockChain class
blc.addBlock(new Block(1, "12/2/33",{amount: 56})); //add new blocks to the chain
blc.addBlock(new Block(2, "13/2/33",{amount: 34}));
blc.addBlock(new Block(3, "14/2/33",{amount: 20}));
console.log(blc.isChainValid()); //checks and print the validity of the chain
console.log(JSON.stringify(blc, null, 4))