前言
区块链太复杂，那我们就讲点简单的。用JS来构建你自己的区块链系统，寥寥几行代码就可以说明区块链的底层数据结构、POW挖矿思想和交易过程等。当然了，真实的场景远远远比这复杂。本文的目的仅限于让大家初步了解、初步认识区块链。
文章内容主要参考视频：Building a blockchain with Javascript (https://www.youtube.com/playlist?list=PLzvRQMJ9HDiTqZmbtFisdXFxul5k0F-Q4)
感谢原作者，本文在原视频基础上做了修改补充，并加入了个人理解。
认识区块链
区块链顾名思义是由区块连接而成的链，因此最基本的数据结构是Block。每个Block都含有timestamp、data、hash、previousHash等信息。其中data用来存储数据，previousHash是前一个区块的hash值。示意如下：
hash是对区块信息的摘要存储，hash的好处是任意长度的信息经过hash都可以映射成固定长度的字符串，如可用sha256：
calculateHash() { ???return SHA256(this.previousHash + this.timestamp + JSON.stringify(this.data)).toString();}Block的数据结构
Block的最基本数据结构如下：
class Block { ???constructor(timestamp, data, previousHash = ‘‘) { ???????this.timestamp = timestamp; ???????this.data = data; ???????this.previousHash = previousHash; ???????//对hash的计算必须放在最后，保证所有数据赋值正确后再计算 ???????this.hash = this.calculateHash(); ????} ???calculateHash() { ???????return SHA256(this.previousHash + this.timestamp + JSON.stringify(this.data)).toString(); ???}}BlockChain的数据结构
多个Block链接而成BlockChain，显然可用用数组或链表来表示，如：
class BlockChain { ???constructor() { ???????this.chain = []; ???}}创世区块
正所谓万物始于一，区块链的第一个区块总是需要人为来手动创建，这个区块的previousHash为空，如：
createGenesisBlock() { ???return new Block("2018-11-11 00:00:00", "Genesis block of simple chain", "");}区块链的构造方法也应改为：
class BlockChain { ???constructor() { ???????this.chain = [this.createGenesisBlock()]; ???}}添加区块
每新加一个区块，必须保证与原有区块链连接起来，即：
class BlockChain { ???getLatestBlock() { ???????return this.chain[this.chain.length - 1]; ???} ???????addBlock(newBlock) { ???????//新区块的前一个hash值是现有区块链的最后一个区块的hash值； ???????newBlock.previousHash = this.getLatestBlock().hash; ???????//重新计算新区块的hash值（因为指定了previousHash)； ???????newBlock.hash = newBlock.calculateHash(); ????????//把新区块加入到链中； ???????this.chain.push(newBlock); ????} ???...}校验区块链
区块链数据结构的核心是保证前后链接、无法篡改，但是如果有人真的篡改了某个区块，我们该如何校验发现呢？最笨也是最自然是想法就是遍历所有情况，逐一校验，如：
isChainValid() { ???//遍历所有区块 ???for (let i = 1; i < this.chain.length; i++) { ???????const currentBlock = this.chain[i]; ???????const previousBlock = this.chain[i - 1]; ???????//重新计算当前区块的hash值，若发现hash值对不上，说明该区块有数据被篡改，hash值未重新计算 ???????if (currentBlock.hash !== currentBlock.calculateHash()) { ???????????console.error("hash not equal: " + JSON.stringify(currentBlock)); ???????????return false; ???????} ???????//判断当前区块的previousHash是否真的等于前一个区块的hash，若不等，说明前一个区块被篡改，虽然hash值被重新计算正确，但是后续区块的hash值并未重新计算，导致整个链断裂 ???????if (currentBlock.previousHash !== previousBlock.calculateHash) { ???????????console.error("previous hash not right: " + JSON.stringify(currentBlock)); ???????????return false; ???????} ???} ???return true;}Just run it
跑起来看看，即：
let simpleChain = new BlockChain();simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));console.log(JSON.stringify(simpleChain, null, 4));console.log("is the chain valid? " + simpleChain.isChainValid());结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js { ???"chain": [ ???????{ ???????????"timestamp": "2018-11-11 00:00:00", ???????????"data": "Genesis block of simple chain", ???????????"previousHash": "", ???????????"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:01", ???????????"data": { ???????????????"amount": 10 ???????????}, ???????????"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89", ???????????"hash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:02", ???????????"data": { ???????????????"amount": 20 ???????????}, ???????????"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529", ???????????"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34" ???????} ???]}is the chain valid? true注意看其中的previousHash与hash，确实是当前区块的previousHash指向前一个区块的hash。
篡改下试试
都说区块链不可篡改，是真的吗？让我们篡改第2个区块试试，如：
let simpleChain = new BlockChain();simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));console.log("is the chain valid? " + simpleChain.isChainValid());//将第2个区块的数据，由10改为15simpleChain.chain[1].data = {amount: 15};console.log("is the chain still valid? " + simpleChain.isChainValid());console.log(JSON.stringify(simpleChain, null, 4));结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js is the chain valid? truehash not equal: {"timestamp":"2018-11-11 00:00:01","data":{"amount":15},"previousHash":"fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89","hash":"150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529"}is the chain still valid? false{ ???"chain": [ ???????{ ???????????"timestamp": "2018-11-11 00:00:00", ???????????"data": "Genesis block of simple chain", ???????????"previousHash": "", ???????????"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:01", ???????????"data": { ???????????????"amount": 15 ???????????}, ???????????"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89", ???????????"hash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:02", ???????????"data": { ???????????????"amount": 20 ???????????}, ???????????"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529", ???????????"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34" ???????} ???]}显然，篡改了数据之后，hash值并未重新计算，导致该区块的hash值对不上。
再篡改下试试
那么，如果我们聪明点，篡改后把hash值也重新计算会如何？
let simpleChain = new BlockChain();simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));console.log("is the chain valid? " + simpleChain.isChainValid());//篡改后重新计算hash值simpleChain.chain[1].data = {amount: 15};simpleChain.chain[1].hash = simpleChain.chain[1].calculateHash();console.log("is the chain still valid? " + simpleChain.isChainValid());console.log(JSON.stringify(simpleChain, null, 4));结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js is the chain valid? trueprevious hash not right: {"timestamp":"2018-11-11 00:00:02","data":{"amount":20},"previousHash":"150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529","hash":"274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34"}is the chain still valid? false{ ???"chain": [ ???????{ ???????????"timestamp": "2018-11-11 00:00:00", ???????????"data": "Genesis block of simple chain", ???????????"previousHash": "", ???????????"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:01", ???????????"data": { ???????????????"amount": 15 ???????????}, ???????????"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89", ???????????"hash": "74d139274fb692495b7c805dd5822faa0c5b5e6058b6beef96e87e18ab83a6b1" ???????}, ???????{ ???????????"timestamp": "2018-11-11 00:00:02", ???????????"data": { ???????????????"amount": 20 ???????????}, ???????????"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529", ???????????"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34" ???????} ???]}显然，第3个区块的previousHash并未指向第2个区块的hash。
是真的无法篡改吗
其实并不是，如果我们再聪明一点，把后续区块的hash值也重新计算一下，不就OK了吗？ 确实如此，如：
let simpleChain = new BlockChain();simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));console.log("is the chain valid? " + simpleChain.isChainValid());//篡改第2个区块simpleChain.chain[1].data = {amount: 15};simpleChain.chain[1].hash = simpleChain.chain[1].calculateHash();//并把第3个区块也重新计算simpleChain.chain[2].previousHash = simpleChain.chain[1].hash;simpleChain.chain[2].hash = simpleChain.chain[2].calculateHash();console.log("is the chain still valid? " + simpleChain.isChainValid());console.log(JSON.stringify(simpleChain, null, 4原文链接
本文为云栖社区原创内容，未经允许不得转载。
初识区块链——用JS构建你自己的区块链
原文地址：https://www.cnblogs.com/yunqishequ/p/10063447.html