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Blockchain demo

A simple typescript implementation of a Bitcoin-like distributed blockchain consensus algortihm.

The aim is to have a working code base to make demoes and experiments with it.

It has a node with many of the bitcoin concepts implemented, and some more (branches, merches, and so on).

The code woks both on server (NodeJS) and client (browser) environments so it is fit for doing demoes.

A demo application is available at It is a chat (web) application using the underlying blockchain as the data storage.


This is perfect for showing simple code on how the bitcoin works, so this can be used as a support for demoes.

This program maintains a distributed database accross active nodes, interconnected by the network.

Since this is written in typescript, blockchain nodes can either run on NodeJS or in the browser.

Traditionnal blockchain implemented concepts :

  • block chain (data is a list of blocks),

  • block validity,

  • proof of work (proof of stake can also be implementation as well as other algorithms,

Concepts not in Bitcoin (as far as I know)

  • branches : like with git, the blockchain structure supports branches. There is a master branch called master, then any branch can be built on it !

  • block data is in Ordered-JSON.

Build and run the demo application

Those instructions will allow you to launch the demo application which is a chat application storing its data on a shared blockchain. This is a web application where every connected browser is a full blockchain node. All nodes have their own version of the blockchain data and update it when they receive new blocks they consider valid. They connect each other by relying on a meeting server (included in the project, but separated from it) and have no a priori confidence at all in any of the connected peers. It is also possible to connect to other nodes by specifying a Http WebSocket endpoint. The UI allows to create a node, connect to the meeting server, receive blockchain notifications and see blocks data and update the blockchain by mining new blocks with adjustable difficulty.

As a prerequisite, for the ui, you need to install angular’s cli :

yarn global add @angular/cli

First you need to build and link projects together.

cd blockchain-js-core
yarn install
yarn build
yarn link
cd ..
cd blockchain-js-ui
yarn link blockchain-js-core
yarn install
yarn build
cd ..

Then to build the three projects (blockchain-js-core and blockchain-js-ui), simply call :


To run the demo projects :

cd blockchain-js-ui/dist
# if you had not installed http-server before :
yarn global add http-server
http-server -p 8080 ./

You can then go with several browsers to http://localhost:8080/ and you should see the demo application, which is a blockchain distributed amongst all connected nodes in which the client add messages by mining some blocks and transmitting them to peer nodes.

Development mode

cd blockchain-js-ui
ng serve

Then open some browsers on http://localhost:4200

Run a NodeJS blockchain node

To run a blockchain node on NodeJS, go to the blockchain-js-core directory and run node dist/blockchain-node.js. This launches a blockchain node on NodeJs. It listens by default on the 9091 TCP port.

Most of the API goes through the web socket at http://localhost:9091/api.

There are also some available REST calls you can find in the blockchain-node.ts file.

Connection between nodes

On the browser, click on the "Add peer" button. This will connect to the NodeJs blockchain node.

If you click on the mine button, you will min a block on the browser. This block gets ingested by the other nodes adn resolves as the new branch head if it embeds a stronger proof-of-work than previously known.


Look first at the index.ts, test.ts and blockchain-node.ts. Those are the executable files.

The main data structure is in block.ts.

Node API is in node-api.ts.

Implementation of a node is in node-impl.ts.

The algorithm for transfering data between nodes is in node-transfer.ts.

Other files are more technical (tools, networking, …​)

What does the block store for the users ?

A list of JSON data

How are blocks identified

A SHA256 hashing function over the Ordered-JSON representation of the block.


The idea is to have only one valid JSON representation of the data.

In JSON one can write one same object in different equivalent forms. For instance :

    "a": 5,
    "b": 6


    "b": 6,
    "a": 5

represent the same data.

This poses a problem because it means that depending on how the data has been JSON serialized, it will get to different SHA256 hashs.

So the solution to this is to have a stricter JSON representation where data has ony one representation. The only difference with normal JSON is that in Ordered-JSON, the fields object data type are serialized by lexical order.

This means that between the two JSON representations above, only the first is a valid Ordered-JSON representation.


A generic and simple blockchain distributed consensus implementation in typescript




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