Pre-requisites

Install Node.js

# add PPA from NodeSource
curl -sL https://deb.nodesource.com/setup_16.8.0 -o nodesource_setup.sh

# call the install script
. nodesource_setup.sh

# install Node.js
sudo apt-get install -y nodejs

# check the version
node -v

The Contract Class

In Hyperledger Fabric, a smart contract needs to inherit all the methods from the contract class which is provided in the fabric-contract-api npm package. These different methods are called in response to transactions that are invoked on the blockchain. The Contract class is declared in the fabric-contract-api npm package:

export class Contract {
    constructor(name?: string);
    static _isContract(): boolean;
    beforeTransaction(ctx: Context): Promise<void>;
    afterTransaction(ctx: Context, result: any): Promise<void>;
    unknownTransaction(ctx: Context): Promise<void>;
    createContext(): Context;
    getName(): string;
}

Considering the above, we can implement a class SimpleContract that extends the Contract class and use context object (ctx) in order to consume APIs to get, update and delete state in the Blockchain ledger:

'use strict';

const { Contract } = require('fabric-contract-api');

class SimpleContract extends Contract {

    async put(ctx, key, value) {
        await ctx.stub.putState(key, Buffer.from(value));
    }

    async get(ctx, key) {
        const value = await ctx.stub.getState(key);
        if (!value || value.length === 0) {
            throw new Error(`The asset ${key} does not exist`);
        }

        return value.toString();
    }

    async del(ctx, key) {
        await ctx.stub.deleteState(key);
    }
    
}

module.exports = SimpleContract;

Let’s save this chaincode in a simpleContract.js file into the simple_chaincode/lib folder.

Node.js Chaincode Structure

As we are using Node.js to write our chaincode we need to also specify the dependencies, engines, start script and main entry point file for the execution in the package.json file. A basic package.json file looks as follows:

{
    "name": "simple_chaincode",
    "version": "1.0.0",
    "main": "index.js",
    "scripts": {
    "start": "fabric-chaincode-node start"
    },
    "engines": {
        "node": ">=12",,
        "npm": ">=5"
    },
    "dependencies": {
        "fabric-contract-api": "^2.2.0",
        "fabric-shim": "^2.2.0"
    }
}

The main entry point of a chaincode package should be set to index.js containing exports of an npm module.

'use strict';
const simpleContract = require('./lib/simpleContract');
module.exports.SimpleContract = simpleContract;
module.exports.contracts = [simpleContract];

It is mandatory to have the contracts element exported in index.js. contracts is an array of classes that are inheritors of the Contract class

The ChaincodeStub Interface

ChaincodeStub Function Groups

Get/Put data into the ledger or private collections

getState
putState
getPrivateData ..

Perform complex queries

getStateByRange
getStateByPartialCompositeKey
getQueryResult ..

Fetch transaction proposal details

getArgs
getCreator
getChannelID ..

Make invocations between chaincodes

invokeChaincode ..

Emit chaincode events

setEvent ..

Simplify chaincode development (helper functions)

getFunctionAndParameters
splitCompositeKey ..

Simple Interaction with Ledger Data: getState, putState, delState

Functions provided by ChaincodeStub interface

putState

putState(key string, value: Uint8Array): Promise <void>

getState

getState(key string): Promise<Uint8Array>

delState