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# Build a voting dapp

Web3 values such as transparency, trust and decentralization naturally lend themselves to decentralized voting, where removing intermediaries and ensuring transparency can significantly enhance the democratic process.

In this tutorial, we will build a simple voting dapp, and you'll learn how to write, deploy and interact with a smart contract designed for a decentralized voting process.

## Architecture

The voting dapp has three main components:

-   Smart contracts
-   The smart contracts' integration
-   Frontend interface.

The smart contract handles critical functionalities such as:

-   Registering candidates
-   Allowing users to give their votes based on candidates index numbers
-   Enforcing a one-vote-per-address policy
-   Managing the voting period by automatically closing it 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 can interact and cast their votes by inputting the candidate index number. It also can show information about the remaining time period.

To ensure seamless interaction with the Linea blockchain, we'll utilize the MetaMask SDK and 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.

## Set up the environment

Before we start building our 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.

### Initialize the project

Let's create a new project using the [Create Web3 Template CLI](https://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:

```bash
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`:

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

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

```text
? 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**:

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

    _Selected tooling: HardHat_

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

```text
? 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:

```text
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.

### Install dependencies

Navigate into your project directory:

```bash
cd simple-voting-dapp
```

Install all the necessary dependencies:

```bash
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.

### Verify the setup

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

**For the frontend:**

1.  Navigate to the `site` directory:

```bash
cd packages/site
```

2.  Start the Next.js development server:

```bash
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:

```bash
cd packages/blockchain
```

3.  Compile the default smart contracts:

```bash
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:

```jsx
// 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 getAllVotesOfCandidates() 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, enforcing voting rules, and providing functions to retrieve voting results and status. Let's explore the concept of the smart contract a bit more:

![](/img/voting-dapp/voting.png)

**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.

## Deploy the smart contract

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

```tsx
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.

### Compile the contract

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

```bash
npx hardhat compile
```

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

### Set 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:

```bash
# 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.

### Deploy the smart contract

To deploy the smart contract on Linea Sepolia, run the following command from the `blockchain` directory:

```bash
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:

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

Now you can deploy the contract by simply running:

```bash
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

### View frontend

Navigate to the `site` directory in your monorepo:

```bash
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.

### Configure Wagmi and MetaMask SDK

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

```tsx
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.

### Add contract constants

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

```tsx
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

```jsx
"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

```jsx
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

```jsx
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

```jsx
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: "getAllVotesOfCandidates",
}) 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

```jsx
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

```jsx
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

```jsx
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 the [full code](https://github.com/meowyx/simple-voting-dapp).

### Run the development server

```bash
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.

To build on this project, you could add voter registration or vote delegation, enhance security measures, or improve the user interface for better accessibility.

Happy coding!