A Javascript Block class for a blockchain

block.js

var sha256 = require("sha256");
var method = Block.prototype;

function Block(data, previousHash) {
	if(typeof previousHash === "undefined") previousHash = "genesis_block";
	this._previousHash = previousHash;
	this._data = JSON.stringify({ data: data });
	this._nonce = 0;
	this._timeStamp = new Date().getTime();		
	this._hash = this.computeHash(); // constructor    
}

method.mineBlock = function(difficulty) {
	var targetDifficulty = Array(difficulty).join("0");
	// the objective is creating a hash containing a string of '0's based on the difficulty
	while(this._hash.indexOf(targetDifficulty) < 0) {
		this._nonce++;   // the nounce must change in order to get a different sha1 hash
		this._hash = this.computeHash();			
	}
	// the new hash must be something like this (containing N zeroes)
	// 0000000a2d032b6d98651082082b47ace27621cbaf2aac654e58ad0dfbb2c697
	console.log("BLOCK MINED ==> " + this._hash);
}

method.getData = function() {
	var e = JSON.parse(this._data);
	return e.data;
}

method.getHash = function() {
	return this._hash;
}

method.getPreviousHash = function() {
	return this._previousHash;
}

method.computeHash = function() {
	return sha256(
  		  	this.previousHash 
  			+ this._data
		  	+ this._timeStamp
		  	+ this._nonce
  	);
};

module.exports = Block;