Blockchain and web3 have changed our understanding of transparency, trust and decentralization. One of the most incredible use-case for this technology is the development of web3 dApps(Decentralized Applications) that offers solutions that are secure, verifiable and tamper-proof solutions to our everyday challenges. Among all the use-cases, decentralized voting stands out. Decentralized technologies remove middlemen and ensure transparent vote counts, this voting dApp has the potential to change governance system while enhancing the democratic process.

In this tutorial, we will build a simple voting dapp on linea, a zkEVM layer 2 blockchain solution. You will learn how to write, deploy and interact with a smart contract which is designed for decentralized voting process. By the end of this tutorial, you’ll have the solid understanding of the tools and features provided by linea for decentralized applications.

### What use Linea L2 and zkEVM?

Before we start building our voting dApp, let’s take some time to understand what Linea Layer 2 is and how it uses zkEVM technology.

**zkEVM** (zero-knowledge Ethereum Virtual Machine) means Ethereum Virtual Machine that incorporates zero-knowledge proof technology. It gives the smarts contracts the benefits from privacy and scaling enhancements which zero knowledge proof provides. When the smart contracts execution happens, these proofs enables other nodes to quickly and efficiently verify the transactions without the necessity to re-execute the entire contract.

Linea is a Type 2 zkEVM, which means it was designed to be nearly or fully compatible with the Ethereum Virtual Machine. This compatiblility offers developers the advantage and benefits of migrating existing ethereum applications to Linea with minimum changes into the codebases.

In Linea’s zkEVM, transactiona are being processed on it’s own network, where it generates cryptographic proofs by using **SNARKs** (Succinct Non-interactive Arguments of Knowledge).

SNARKS are known for their succinctness - the proofs can be verified rapidly, even when dealing with a very complex statement. Which makes these ideal when scalability and efficiency is needed.

These generated proofs get submitted to the Ethereum mainnet for cost-effective verification without the necessity of re-executing all transactions. This approach reduces time and cost required for processing transactions while maintaining the compatibility with Ethereum.

By leveraging Linea zkEVM, developers can create cost-effective and much more efficient dApps. These applications can reach a wider audience because of their usability and lower transaction fees.
In the upcoming sections, you will be building a simple decentralized voting dApp on Linea.

## Understanding the Voting DApp Architecture

The voting dApp has three main components: smart contracts, smart contracts integration and the frontend interface.

The smart contract is one of the main component of the application. This can handle critical functionalities such as registering candidates, allowing users to give their votes based on candidates index numbers, enforcing a one vote per wallet address policy and managing voting period by automatically closing the voting period after a specified duration. The smart contract makes sure that the voting process is secure, transparent and tamper-proof by recording all votes on the blockchain.

The frontend interface is the user-facing part of the application where users will be able to interact and cast their votes by inputting the candidate index number. It also can show the information about remaining time period. By offering a clear and responsive design, it encourages user participation and trust in the voting process.

To ensure smooth interaction with the Linea blockchain, we'll utilize the MetaMask SDK along with libraries like Wagmi and connect through Infura RPC providers. These tools allow the frontend to communicate with the blockchain, enabling users to sign transactions and interact with the smart contracts directly from their web browsers. This integration is crucial for maintaining the decentralized nature of the application while providing a user-friendly experience.

### Setting Up the Environment

Before we start building our decentralized voting dApp, we'll set up the development environment using **Consensys's Create Web3 Template CLI**. This tool streamlines the process by generating a monorepo structure that includes both the frontend and backend components needed for our application.

### Prerequisites

- **Node.js** and **pnpm/any other package manager** installed on your system.
- Basic familiarity with the command line.

### Initializing the Project

Let's create a new project using the [Create Web3 Template CLI](https://www.npmjs.com/package/@consensys/create-web3-template) by Consensys.

This command-line tool simplifies the setup process by providing all the necessary tools and integrations out of the box, including MetaMask SDK, Linea support, Infura integration, and a selection of templates to choose from.

1. Open your terminal and run the following command:

    pnpm create @consensys/create-web3-template
    

2. You'll be prompted to specify a name for your project. We'll name it **`simple-voting-dapp`**:

    ? Please specify a name for your project: simple-voting-dapp
    

3. Next, select the framework for your frontend. We'll choose **Next.js**:

    ? Please select the framework you want to use:
      HardHat Vanilla
      React
    ❯ Next.js
      

*Selected framework: Next.js*

4. Choose the smart contract development environment. We'll go with **HardHat**:

    ? Would you like to use HardHat or Foundry?
    ❯ HardHat
      Foundry
    

*Selected tooling: HardHat*

5. Select your preferred package manager. We'll use **pnpm**:

    ? Please select the package manager you want to use:
      Yarn
      NPM
    ❯ pnpm
    

6. The CLI will generate the project based on your selections.

### Project Structure

After the setup is complete, your project directory `simple-voting-dapp` will have the following structure:

simple-voting-dapp
├── packages
│   ├── site          # Frontend built with Next.js
│   └── blockchain    # Smart contracts using HardHat
├── pnpm-workspace.yaml
├── package.json

- **`packages/site`**: Contains the frontend code of the dApp, built with Next.js.
- **`packages/blockchain`**: Contains the smart contracts and related scripts using HardHat.
- **`pnpm-workspace.yaml`**: Defines the workspace configuration for the monorepo.

### Installing Dependencies

Navigate into your project directory:

cd simple-voting-dapp

Install all the necessary dependencies:

pnpm install

This command will install all packages for both the frontend and backend components.

### Understanding the Monorepo

A **monorepo** (monolithic repository) is a single repository that stores code for multiple projects or packages. In our case, it allows us to manage both the frontend and backend in a unified codebase, making development and maintenance more efficient.

### Verifying the Setup

To ensure everything is set up correctly, let's run the development servers.

**For the Frontend:**

1. Navigate to the `site` directory:

    cd packages/site
    

2. Start the Next.js development server:

    pnpm dev
    

3. Open your browser and go to `http://localhost:3000` to see the frontend running.

**For the Backend:**

1. Open a new terminal window.
2. Navigate to the `blockchain` directory:

    cd packages/blockchain
    

3. Compile the default smart contracts:

    pnpm hardhat compile
    

This will compile the sample contracts included in the HardHat setup.

## Write the Smart Contract

Let’s create a `Voting.sol` file and add the following code:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title A contract for a voting system
/// @author [Your Name]
/// @notice This contract allows for the creation and management of a voting system
contract Voting {
    /// @dev Struct to represent a candidate
    struct Candidate {
        string name;
        uint256 voteCount;
    }

    /// @notice Array to store all candidates
    Candidate[] public candidates;
    
    /// @notice Address of the contract owner
    address owner;
    
    /// @notice Mapping to track if an address has voted
    mapping(address => bool) public voters;

    /// @notice Timestamp when voting starts
    uint256 public votingStart;
    
    /// @notice Timestamp when voting ends
    uint256 public votingEnd;

    /// @notice Constructor to initialize the voting contract
    /// @param _candidateNames Array of candidate names
    /// @param _durationInMinutes Duration of the voting period in minutes
    constructor(string[] memory _candidateNames, uint256 _durationInMinutes) {
        for (uint256 i = 0; i < _candidateNames.length; i++) {
            candidates.push(Candidate({
                name: _candidateNames[i],
                voteCount: 0
            }));
        }
        owner = msg.sender;
        votingStart = block.timestamp;
        votingEnd = block.timestamp + (_durationInMinutes * 1 minutes);
    }

    /// @notice Modifier to restrict function access to the contract owner
    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }

    /// @notice Adds a new candidate to the election
    /// @param _name Name of the new candidate
    function addCandidate(string memory _name) public onlyOwner {
        candidates.push(Candidate({
                name: _name,
                voteCount: 0
        }));
    }

    /// @notice Allows a voter to cast their vote
    /// @param _candidateIndex Index of the candidate in the candidates array
    function vote(uint256 _candidateIndex) public {
        require(!voters[msg.sender], "You have already voted.");
        require(_candidateIndex < candidates.length, "Invalid candidate index.");

        candidates[_candidateIndex].voteCount++;
        voters[msg.sender] = true;
    }

    /// @notice Retrieves all candidates and their vote counts
    /// @return An array of Candidate structs
    function getAllVotesOfCandiates() public view returns (Candidate[] memory){
        return candidates;
    }

    /// @notice Checks if voting is currently active
    /// @return Boolean indicating if voting is active
    function getVotingStatus() public view returns (bool) {
        return (block.timestamp >= votingStart && block.timestamp < votingEnd);
    }

    /// @notice Calculates the remaining time in the voting period
    /// @return The remaining time in seconds
    function getRemainingTime() public view returns (uint256) {
        require(block.timestamp >= votingStart, "Voting has not started yet.");
        if (block.timestamp >= votingEnd) {
            return 0;
        }
        return votingEnd - block.timestamp;
    }
}

This contract provides a basic framework for a decentralized voting system where users can cast votes for candidates securely and transparently. It ensures that each user can vote only once by tracking voter addresses, and it maintains a record of all votes on the blockchain. The contract manages the voting period by specifying start and end times, enforces voting rules, and provides functions to retrieve voting results and status. Let’s explore the concept of the smart contract a bit more:


**1. Contract Initialization and Candidate Setup**

- The `Voting` contract initializes with an array of candidate names provided during deployment.
- Each candidate is represented by a `Candidate` struct containing a `name` and a `voteCount`.
- The constructor sets the `votingStart` time to the current block timestamp.
- It calculates the `votingEnd` time by adding the specified duration in minutes to the start time.

**2. Access Control and Owner Functions**

- The contract stores the deployer's address as `owner`.
- An `onlyOwner` modifier restricts certain functions to the contract owner.
- The `addCandidate` function allows the owner to add new candidates after hardhat deployment.This ensures only authorized users can modify the list of candidates.

**3. Voting Mechanism**

- The `vote` function lets users cast a vote by specifying a candidate's index.
- A `voters` mapping tracks whether an address has already voted.
- The function checks if the voter hasn't voted before and if the candidate index is valid.
- Upon a valid vote, it increments the candidate's `voteCount` and marks the voter as having voted.

**4. Utility Functions and Voting Status**

- `getAllVotesOfCandidates` returns all candidates and their current vote counts.
- `getVotingStatus` returns `true` if voting is active based on the current time.
- `getRemainingTime` calculates and returns how much time is left in the voting period.
- These functions provide users with real-time information about the election.

This smart contract facilitates a decentralized voting system where users can vote for candidates securely, with all votes and results recorded on the blockchain.

### Deploying the Smart Contract with Hardhat Ignition

In the `ignition` folder, let's create a file named `Voting.ts` to deploy our contract. Add the following code:

import { buildModule } from "@nomicfoundation/hardhat-ignition/modules";

const VotingModule = buildModule("VotingModule", (m) => {
  // Default values
  const DEFAULT_CANDIDATES = ["Candidate 1", "Candidate 2", "Candidate 3"];
  const DEFAULT_DURATION_MINUTES = 60; // 1 hour

  // Get parameters with default values
  const candidateNames = m.getParameter("candidateNames", DEFAULT_CANDIDATES);
  const durationInMinutes = m.getParameter(
    "durationInMinutes",
    DEFAULT_DURATION_MINUTES
  );

  // Deploy the Voting contract
  const voting = m.contract("Voting", [
    candidateNames,
    durationInMinutes,
  ]);

  return { voting };
});

export default VotingModule;

In this deployment script, we utilize Hardhat Ignition to manage the deployment of our `Voting` contract. We set default candidates and a default voting duration but also allow these values to be customized through parameters if needed.

### Compiling the Contract

Before deploying, compile the contract by running the following command in the `blockchain` directory:

npx hardhat compile

This will compile your Solidity code and prepare it for deployment.

### Setting Up Environment Variables

Before deploying the smart contract, ensure that your `.env` file in the `packages/blockchain` directory is updated with the necessary environment variables:

# Infura API key for connecting to Ethereum networks
INFURA_API_KEY=your_infura_api_key_here

# Private key of the account to be used for deployments and transactions
ACCOUNT_PRIVATE_KEY=your_account_private_key_here

- Replace `your_infura_api_key_here` with your actual Infura API key.
- Replace `your_account_private_key_here` with the private key of the Ethereum account you will use for deploying the contract.

### Deploying the Smart Contract

To deploy the smart contract to the Linea testnet, run the following command from the `blockchain` directory:

npx hardhat ignition deploy ignition/modules/Voting.ts --network linea-testnet

This command tells Hardhat to use Ignition to deploy the `Voting` module to the `linea-testnet` network.

Alternatively, you can add a deployment script to your `package.json` to simplify the process. Add the following line under the `"scripts"` section:

"scripts": {
  "deploy:testnet": "npx hardhat ignition deploy ignition/modules/Voting.ts --network linea-testnet"
}

Now you can deploy the contract by simply running:

pnpm run deploy:testnet

After deployment, you'll receive the contract address. Keep this address safe, as we'll need it when integrating with the frontend.

### Frontend Integration with Next.js and Shadcn UI

### Setting Up the Frontend Project

Navigate to the `site` directory in your monorepo:

cd packages/site

Since we've already set up the frontend using the Create Web3 Template CLI, we can proceed to integrate our smart contract.

### Configuring Wagmi and MetaMask SDK

Create a `wagmi.config.ts` file in the `src` directory with the following content:

import { http, createConfig } from "wagmi";
import { lineaSepolia } from "wagmi/chains";
import { metaMask } from "wagmi/connectors";

export const config = createConfig({
  chains: [lineaSepolia],
  connectors: [metaMask()],
  transports: {
    [lineaSepolia.id]: http(),
  },
});

This configuration sets up the connection to the Linea testnet and enables wallet integration using MetaMask.

### Adding Contract Constants

In the `src` directory, create a file named `constants.ts` and add the following:

export const CONTRACT_ADDRESS = 'your_deployed_contract_address_here';

export const ABI = [
  // Paste the ABI of your Voting contract here
];

- Replace `'your_deployed_contract_address_here'` with the actual contract address you obtained after deployment.
- For the ABI, you can find it in the `artifacts` folder generated by Hardhat after compilation.

## React and Hooks Usage

"use client";

import { useState } from "react";
import { useAccount, useWalletClient } from "wagmi";
import { ConnectButton } from "@consensys/connect-button";
import { Button } from "@/components/ui/button";
import { Input } from "@/components/ui/input";
import { useReadContract, useWriteContract } from "wagmi";
import { Card, CardContent } from "@/components/ui/card";
import { contractAddress, contractAbi } from "../../constants";

This Next.js client-side component uses React's `useState` for state management, Wagmi hooks (`useAccount`, `useWalletClient`, `useReadContract`, `useWriteContract`) for wallet and contract interactions, imports UI elements (`Button`, `Input`, `Card`, `CardContent`) from Shadcn UI, includes a `ConnectButton` for MetaMask connection, and brings in `contractAddress` and `contractAbi` from constants for contract use.

## Interface Definition

interface Candidate {
  index: number;
  name: string;
  voteCount: number;
}

- Defines a TypeScript `interface` named `Candidate`.
- Specifies the structure for candidate objects:
- `index`: The candidate's index number.
- `name`: The candidate's name.
- `voteCount`: The number of votes the candidate has received.

## Component Initialization

export default function Home() {
  const { address } = useAccount();
  const { data: walletClient } = useWalletClient();
  const [number, setNumber] = useState("");

- **Function Component**: Defines the `Home` component as the default export.
- **Wallet Information**:
- `address`: The user's wallet address obtained from `useAccount`.
- `walletClient`: The wallet client used for signing transactions.
- **State Management**:
- `number`: A state variable to store the candidate index input by the user for voting.
- `setNumber`: Function to update the `number` state.

## Smart Contract Interaction - Reading Data

const { writeContract } = useWriteContract();

const { data: votingStatus } = useReadContract({
  address: contractAddress,
  abi: contractAbi,
  functionName: "getVotingStatus",
}) as { data: boolean };

const { data: canVote } = useReadContract({
  address: contractAddress,
  abi: contractAbi,
  functionName: "voters",
  args: [address],
});

const { data: remainingTime } = useReadContract({
  address: contractAddress,
  abi: contractAbi,
  functionName: "getRemainingTime",
}) as { data: bigint };

const { data: candidates } = useReadContract({
  address: contractAddress,
  abi: contractAbi,
  functionName: "getAllVotesOfCandiates",
}) as { data: Candidate[] };

- **Writing to Contract**:
- `writeContract` is prepared for sending transactions to the smart contract.
- **Reading Contract Data**:
- `votingStatus`: Retrieves whether the voting is currently active.
- `canVote`: Checks if the current user has already voted.
- `remainingTime`: Gets the time remaining before the voting period ends.
- `candidates`: Fetches the list of all candidates and their vote counts.
- **Type Assertions**:
- Uses TypeScript `as` syntax to specify the expected data type for better type safety.

## Early Return for Missing Data

if (!candidates) return;

- Checks if the `candidates` data is available.
- If `candidates` is `undefined` or `null`, the component returns early to prevent rendering errors.

## Voting Functionality

async function vote() {
  if (!walletClient || !address || number === "") {
    console.error("Missing required data for voting");
    return;
  }
  if (
    isNaN(Number(number)) ||
    Number(number) < 0 ||
    Number(number) >= candidates.length
  ) {
    console.error("Invalid candidate index");
    return;
  }
  if (!votingStatus) {
    console.error("Voting is not currently active");
    return;
  }
  try {
    console.log("Attempting to vote for candidate index:", number);
    writeContract({
      address: contractAddress,
      abi: contractAbi,
      functionName: "vote",
      args: [BigInt(number)],
      account: address,
    });
  } catch (error) {
    console.error("Detailed error in vote function:", error);
  }
}

- **Input Validation**:
- Checks if the wallet client, user address, and candidate number are available.
- Validates that the candidate index is a valid number within the range of available candidates.
- Ensures that voting is currently active.
- **Voting Process**:
- Uses `writeContract` to call the `vote` function on the smart contract.
- Passes the candidate index as a `BigInt`.
- Uses the user's wallet address as the account.
- **Error Handling**:
- Wraps the contract interaction in a `try-catch` block to handle any errors that occur during the voting process.
- Logs detailed error information to the console.

## UI Rendering

return (
  <main className="container mx-auto p-4">
    <h1 className="text-2xl font-bold mb-4">
      Decentralized Voting Application
    </h1>
    <ConnectButton />

    {votingStatus ? (
      address ? (
        <Card className="mt-4">
          <CardContent>
            <p>Connected Account: {address}</p>
            <p>Remaining Time: {Number(remainingTime)} seconds</p>
            {!canVote ? (
              <div className="mt-4">
                <Input
                  type="number"
                  placeholder="Enter Candidate Index"
                  value={number}
                  onChange={(e) => setNumber(e.target.value)}
                  className="mb-2"
                />
                <Button onClick={vote}>Vote</Button>
              </div>
            ) : (
              <p>You have already voted</p>
            )}
            <div className="mt-4">
              <h2 className="text-xl font-semibold mb-2">Candidates</h2>
              <ul>
                {candidates.map((candidate) => (
                  <li key={candidate.index}>
                    {candidates.indexOf(candidate)}: {candidate.name} -{" "}
                    {Number(candidate.voteCount)} votes
                  </li>
                ))}
              </ul>
            </div>
          </CardContent>
        </Card>
      ) : (
        <p>Please connect your wallet to vote</p>
      )
    ) : (
      <p>Voting has finished</p>
    )}
  </main>
);

The component features a styled main container with a header and a `ConnectButton` for MetaMask integration. It conditionally renders content based on `votingStatus` and user connection: if voting is active and the user is connected, it displays their address, remaining time, and voting options; if they've already voted, it notifies them and lists all candidates. If the user isn't connected, it prompts them to connect their wallet. When voting has ended, it shows "Voting has finished".

## Error Handling and Validation

- **Input Validation**:
- Ensures the candidate index entered is a valid number within the acceptable range.
- **Error Messages**:
- Uses `console.error` to log meaningful error messages for debugging purposes.
- **User Feedback**:
- Provides real-time feedback to the user based on their actions and the application's state.

View full code here: https://github.com/meowyx/simple-voting-dapp

### Run the Development Server

npm run dev

Your Next.js application with Shadcn UI should now be running at `http://localhost:3000`.

Now, you have a decentralized voting dApp where users can vote for the candidates by using their index number. These votes are secure, transparent and recorded on Linea blockchain.

In this tutorial, we built a simple decentralized voting application on Linea, while leveraging zkEVM technology for scalability, security, and cost efficiency. We covered everything from setting up the environment by using the CLI to writing and deploying smart contracts, and integrating the frontend with Next.js and Shadcn UI.

As you continue to explore and expand upon this foundational knowledge, consider potential enhancements such as adding voter registration, implementing vote delegation, enhancing security measures, or improving the user interface for better accessibility. The possibilities for decentralized applications (dApps) are endless, and voting systems are just one of the many impactful use cases.

Happy coding!