Learn & Build a Javascript Blockchain

The best way to understand blockchain is to build one

Dec 28, 2017 · 8 min read

In this Part 1 of the Javascript Blockchain Series, we will cover the block object and blockchain object and discuss its structure and implications.

I’m a firm believer in the idea that the best way to learn a technology is to build it. Naturally, my interest in blockchain technology has led me to try to build a blockchain of my own. As a fellow blockchain enthusiast, I will walk you through the steps of building a basic blockchain with a built-in proof of work system. Along the way, I will discuss the implications of the different features.

What do you need: the latest version of Node.js (I’m using v8.7.0) and npm (Node Package Manager) (I’m using v5.6.0)

Who is this for: anyone who is interested in learning about blockchain and has some basic understanding of Javascript

Where is the repo: the code can be found here.

Step 1: Building Blocks

Create a folder called js-blockchain and create a file called main.js In this guide, we will be using one single file so you can always refer to the source code if you encounter any problems.

Blockchain Structure

Blockchains are built through a combination of linked lists and merkle trees. The linked list structure allows for the chain to continually build on top of itself and is where the name blockchain derives from. A blockchain is literally a chain of blocks linked to one another through the linked list structure. One thing to note however, is that instead of holding a traditional pointer to refer to the previous block, it uses the hash of the previous block to refer to it.

If you are not familiar with what a hash is, here is a primer. Hashes are simply deterministic functions that create specific outputs for each input, and they are usually irreversible meaning it is extremely difficult to derive the input from the output. They are critical to blockchain as they are key to making the chain immutable and to maintaining the integrity of the data.

To not overcomplicate things, we will not implement the merkle tree of transactions in this guide.


Block Object

We’ll create a Block class first which will hold a constructor function, a calculateHash function, and a mineBlock function. We start off with the constructor function which instantiates Block objects and provides it with its properties.

Block Inputs

Each block object takes in a timestamp and block data. The timestamp shows when the block was created. This is useful for instance in Bitcoin(BTC) as BTC is designed to have a difficulty such that the average mining time per block is 10 minutes. Thus, the system can use this timestamp data to rebalance the mining difficulty every month. The block data holds the information stored in the chain. In many currencies such as BTC, this is where transaction data is stored in the form of merkle trees.

Block Data

As you can see, in addition to the 3 data input fields, our Block object also holds an index, a previousHash , a hash, and a nonce. The index communicates where in the chain the block is located. The hash of the previous block is what maintains the integrity of the chain. The block’s hash simply holds the block’s own hash derived from a calculateHash function. The nonce is another important piece of the block that is critical to building in a mining mechanism for our blockchain which we will discuss in part 2 of this series. For now, we will set our nonce variable to 0.

Note, the hash of the previous block is what maintains the integrity of the chain. To check the integrity of the chain we go through the chain calculating each block’s hash and matching it with this previousHash data. If one single piece of the block’s information is tampered with, it will spit out a completely different hash and will be immediately detectable when matched against the previousHash data stored in the block following the tampered block. We will create a checkValid function later that demonstrates this.

Crypto-JS Library

Now we will add in a calculateHash function to our block. For our hash function, we will borrow from crypto-js library and use their SHA256 hash function. The SHA256 hash function was developed by the NSA and is an irreversible hash function. This is actually used in BTC mining as its proof of work algorithm and in BTC address creation thanks to its security.

To install the crypto-js library, go to your terminal and enter your js-blockchain folder then install the library with npm with the following code:

npm install --save crypto-js

You will then see the following output:

npm WARN saveError ENOENT: no such file or directory, open '/Users/spenserhuang/Desktop/js-blockchain/package.json'
npm WARN enoent ENOENT: no such file or directory, open '/Users/spenserhuang/Desktop/js-blockchain/package.json'
npm WARN js-blockchain No description
npm WARN js-blockchain No repository field.
npm WARN js-blockchain No README data
npm WARN js-blockchain No license field.
+ crypto-js@3.1.9-1
updated 1 package in 1.75s

Do not worry about the warning for now as that will not affect us in the context of this project. The error exists because the folder we created is extremely barebones and does not have additional files such as package.json or other node and repo files.

Calculate Hash

Nevertheless, now that you have the library installed, you can create a SHA256 constant and pull the information directly with a require statement. We will use that to create our calculateHash function.

We create our own function called calculateHash that we will use to give each block its own hash. As you can see with the function, the hash takes in every piece of the block object, throws it into a SHA256 function, and converts it into a string. In this project, we convert our SHA256 result into a string as strings are easier to process and match.

The calculateHash function takes in EVERY piece of the block’s data. As a result, if any single piece of data gets tampered with, even if it’s a new decimal point, the block’s hash will immediately be different. This is a great feature in making the blockchain secure. Here is a great guide to learn more about this feature of the blockchain!

Step 2: Building the Blockchain

Now that we have the individual block structure created, we need to create a structure, a Blockchain class object, to link them together and to build the basic functionality that comes along with normal blockchains. To keep things as simple as possible, our blockchain will only hold a constructor function, a createGenesis function, a latestBlock function, an addBlock function, and a checkValid function.

Blockchain Object

Our Blockchain class needs a few key functionalities, namely the ability to be instantiated, to start, to access the latest block’s information, and to extend itself by adding a new block to it.

Constructor Function

First, we need a constructor function to get our blockchain instantiated. This will be a simple function that simply creates a blockchain object with achain property in a list structure. This is why our Block objects held indices. The index property of our blocks are used to find its position in our chain list.

Create Genesis Function

Second, our blockchain needs a createGenesis function that creates the very first block of our chain. In blockchain convention, the very first block of any chain is also known as the “Genesis block,” thus the name createGenesis. With this block, we usually manually input its information. As it’s the first block in the chain, it has an index value of 0. For its timestamp and data fields, we can just put in today’s date and “Genesis block” or anything you like. As for the previousHash field, we can simply put “0” for now as there is no hash preceding the Genesis block.

Get Latest Block Function

Third, our chain needs a latestBlock function. This is used for get the information about the most recent block. This functionality will be used for implementing our addBlock function.

Add New Block Function

Fourth, to continuously extend our block, we need an addBlock function. Blockchains have such a functionality as they need to get longer and longer as more information gets inputted into it. Thus, in the case of BTC for instance, a new block is created approx. every 10 minutes and each of these new blocks will hold all the transaction info that occurred within that 10 min time frame.

Implementing a addBlock function is simple. Our function takes in a Block object as an input which already possesses a timestamp, and data. Then, our addBlock function will use the latestBlock function to give our new block an index and a previousHash field. After that, we give our new block its own hash using the calculateHash function we wrote earlier. Finally, we push this new block onto our chain and now our blockchain has one new block.

Check Validity Function

The final function we will implement is the checkValid function. This is used to check the integrity of the blockchain and to detect whether or not anything has been tampered with.

As stated earlier, hashes are critical for detecting changes in our blockchain as even the smallest change in the object will result in an entirely different hash. Thus, for our checkValid function, we will use a for loop to go through the blockchain and try to match its hashes to detect changes.

There are two parts of our loop, the first is matching currentBlock.hash with currentBlock.calculateHash() and the other is matching currentBlock.previousHash with previousBlock.hash. The former is used to check whether or currentBlock’s info has been tampered with without updating currentBlock.hash The latter is used to check whether or not a previousBlock has been tampered with.

You may have the question of whether or not you can put in the if condition of

if (currentBlock.previousHash !== previousBlock.calculateHash()) {
return false;

This theoretically seems like it should work. However, previousBlock is actually a slightly different object than the actual block object from the chain even though it basically holds all the same info as the actual block. As a result, they produce different hashes. For instance, during my test run, the actual block’s hash is 0c27196c36691e915fb2a2f83e63867ec5042abfda4832b02383a6ab40aa075c
while previousBlock’s hash is e6a312951e9e4ed6e4b2ef049246b282f71144599939ff6a5b97b0c53c941295
This is how sensitive hashes are.

Putting it All Together

Now, your overall program should look like this:

You can test out your blockchain by adding in the following lines in the end

Then, go to your terminal and run

node main.js

You should now see the following output:

"chain": [
"index": 0,
"timestamp": 0,
"data": "01/01/2017",
"previousHash": "Genesis block",
"hash": "8163cbd8feafd38a96cd193f2b44940473c22b21ddbb7445bd99ee310dac28ae",
"nonce": 0
"index": 1,
"timestamp": "12/25/2017",
"data": {
"amount": 5
"previousHash": "8163cbd8feafd38a96cd193f2b44940473c22b21ddbb7445bd99ee310dac28ae",
"hash": "744ce201216f78bba5b87e371579898b97e473ac644d6a13ddda9cdbe05100f6",
"nonce": 0
"index": 2,
"timestamp": "12/26/2017",
"data": {
"amount": 10
"previousHash": "744ce201216f78bba5b87e371579898b97e473ac644d6a13ddda9cdbe05100f6",
"hash": "9e3cf69ee7a3f3f651b33500ea3f32ccf1a13590115c6739dda74920d54702c8",
"nonce": 0
Is blockchain valid? true

Summing Up

Now you have a version 1 of a functioning blockchain! However, there is one small security issue that exists with this blockchain. I encourage you to build a blockchain like this and test it out. Try to identify where the security flaw is by trying to tamper with the data.

Hint: this security flaw is directly tied to the implementation of the mining function that we will cover in Part 2 of this tutorial.

Happy coding and exploring!

Spenser Huang

Written by

rookie — www.spenserhuang.com

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