Voting Backend (Express + Web3.js)

A lightweight Node.js/Express backend that interacts with the Voting.sol smart contract deployed via Hardhat. It provides REST APIs for fetching candidates, adding candidates (owner-only), casting votes, and retrieving the winner. It is the backend service for the voting_hardhat project here, and also a complete explanation of the whole project with tests and live demo are here.

This backend communicates with the blockchain using Web3.js and the contract ABI.

---

🚀 Features

• Connects to an Ethereum RPC provider (Hardhat, Ganache, Anvil, etc.)

• Reads and writes from/to the deployed Voting smart contract

• Owner-protected candidate creation

• Endpoints for:

  • Fetching candidates
  • Adding candidates (owner only)
  • Casting votes
  • Getting election winner

• Logging & centralized error handling

• Clean route/controller structure

---

📂 Project Structure

voting_backend/
│
├── server.js
├── abi.js
├── .env
├── .env.example
├── package.json
│
└── src/
    ├── config/
    │   └── web3.js
    ├── controllers/
    │   └── voting.controller.js
    ├── middlewares/
    │   └── ownerAuth.js
    ├── routes/
    │   └── voting.routes.js
    └── utils/
        └── errorHandler.js

---

🔧 Installation & Setup

1. Clone the repository

git clone <your_repo_url>
cd voting_backend

2. Install dependencies

npm install

3. Configure environment variables

Create a .env file:

cp .env.example .env

Edit values inside .env:

PROVIDER_URL=http://127.0.0.1:8545
CONTRACT_ADDRESS=0x...
OWNER_PRIVATE_KEY=0x...
PORT=4000

Ensure the contract is deployed and CONTRACT_ADDRESS matches your Hardhat deployment.

---

▶️ Running the Server

node server.js

The API will start on:

http://localhost:4000

---

📡 API Endpoints (Base: /api/v1)

🟦 1. Check API Status

GET /

Response: { "message": "Voting API v1 is live" }

---

🟩 2. Get All Candidates

GET /candidates

Response:
{
  "candidates": [
    { "name": "Alice", "voteCount": 0 },
    { "name": "Bob", "voteCount": 3 }
  ]
}

---

🟧 3. Add Candidate (Owner Only)

POST /candidates

Required JSON body:

{
  "candidate_name": "Charlie",
  "privatekey": "<OWNER_PRIVATE_KEY>"
}

Response:

{
  "message": "Candidate added",
  "txHash": "0x..."
}

---

🟨 4. Cast Vote

POST /vote

Required JSON body:

{
  "candidate_index": 1,
  "address": "0xYourWalletAddress"
}

Response:

{
  "message": "Vote cast",
  "txHash": "0x..."
}

---

🟪 5. Get Winner

GET /winner

Response:
{
  "winner": "Alice"
}

---

🔐 Owner Authentication

The backend uses a simple middleware:

ownerAuth → verifies req.body.privatekey === process.env.OWNER_PRIVATE_KEY

Only the contract owner can add new candidates.

---

🔌 Web3.js Integration

The file src/config/web3.js:

• Initializes Web3 provider
• Loads ABI
• Creates contract instance
• Loads owner wallet into Web3

const contract = new web3.eth.Contract(voting_abi, process.env.CONTRACT_ADDRESS);

---

🛠 Error Handling

All errors are handled centrally by:

src/utils/errorHandler.js

Ensures clean, consistent 500 responses.

---

📝 Notes

• This backend assumes your Voting smart contract is deployed and running.
• Uses Web3.js v4, which requires ESM imports (type: module).
• Ensure Hardhat node is running before calling the API.

---

👤 Author

Dhruv Charne (Dhruv4ne)

---

📜 License

MIT