In web3, decentralized applications (dapps) have gained popularity due to their ability to provide secure, transparent, and trustless services. Some of the most popular types of dapps are decentralized marketplaces, which allow users to buy and sell items directly on the blockchain, eliminating the need for intermediaries.

In this tutorial, we will build a simple marketplace dapp on Linea, a Layer 2 zkEVM technology that offers a scalable and cost-efficient environment while maintaining the security of Ethereum.

This guide covers both front-end and smart contract development.

We will be diving into the architecture of the marketplace dapp and its components.

The marketplace will incorporate smart contracts, frontend, and blockchain integration. The smart contract will include features such as listing items, buying items, and transferring ownership of an item.

The frontend serves as the user interface where buyers and sellers can interact with each other using the smart contract. While having a frontend and backend is important, the way the dapp interacts with the Linea blockchain is also crucial. We will be using the Metamask SDK along with Wagmi and Infura RPC providers.

We will start by initializing a monorepo. A monorepo (short for monolithic repository) is a software development strategy where code for multiple projects is stored in a single version control repository.

Let’s create a monorepo with pnpm:

Create a new directory for your monorepo and initialize it:

mkdir web3-marketplace-linea
cd web3-marketplace-linea
pnpm init

Create a pnpm-workspace.yaml file in the root to define your workspace structure:

packages:
  - 'packages/*'

Here’s what our workspace file structure will be like:

packages
├── site         # Frontend built with Next.js, Tailwind CSS, and Shadcn UI
└── blockchain   # Smart contracts using Hardhat

Let’s head over to the blockchain directory and initialize a hardhat project.

cd blockchain
npx hardhat init

You'll be presented with several options:

? What do you want to do? …
❯ Create a JavaScript project
  Create a TypeScript project
  Create a TypeScript project (with Viem)
  Create an empty hardhat.config.js
  Quit

For this tutorial we’ll be using TypeScript project. Hardhat will automatically install the necessary dependencies for you.

After initialization, you'll have a project structure that includes:

• contracts/: Solidity contracts

• ignition/: Ignition deployment modules

• test/: Test files

• hardhat.config.js: Hardhat configuration

Let’s start writing the smart contract:

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

/// @title Marketplace
/// @notice A simple marketplace contract for listing, purchasing, and transferring items
/// @dev This contract manages items, their ownership, and transactions
contract Marketplace {
    /// @notice Structure to represent an item in the marketplace
    /// @dev Each item has a unique ID, name, price, seller, owner, and sale status
    struct Item {
        uint id;
        string name;
        uint price;
        address payable seller;
        address owner;
        bool isSold;
    }

    /// @notice Total number of items listed in the marketplace
    uint public itemCount = 0;

    /// @notice Mapping of item IDs to Item structs
    mapping(uint => Item) public items;

    /// @notice Mapping of owner addresses to arrays of owned item IDs
    mapping(address => uint[]) public ownedItems;

    /// @notice Lists a new item in the marketplace
    /// @param _name The name of the item
    /// @param _price The price of the item in wei
    /// @dev Increments itemCount and adds the new item to the items mapping
    function listItem(string memory _name, uint _price) public {
        require(_price > 0, "Price must be greater than zero");

        itemCount++;
        items[itemCount] = Item(itemCount, _name, _price, payable(msg.sender), msg.sender, false);
        ownedItems[msg.sender].push(itemCount);
    }

    /// @notice Allows a user to purchase an item
    /// @param _id The ID of the item to purchase
    /// @dev Transfers the item's price to the seller and updates ownership
    function purchaseItem(uint _id) public payable {
        Item storage item = items[_id];
        require(_id > 0 && _id <= itemCount, "Item does not exist");
        require(msg.value == item.price, "Incorrect price");
        require(!item.isSold, "Item already sold");
        require(msg.sender != item.seller, "Seller cannot buy their own item");

        item.isSold = true;
        item.seller.transfer(msg.value);

        // Transfer ownership
        _transferOwnership(_id, item.seller, msg.sender);
    }

    /// @notice Internal function to transfer ownership of an item
    /// @param _id The ID of the item
    /// @param _from The current owner's address
    /// @param _to The new owner's address
    /// @dev Updates the item's owner and adjusts the ownedItems mappings
    function _transferOwnership(uint _id, address _from, address _to) internal {
        Item storage item = items[_id];
        item.owner = _to;

        // Remove item from the previous owner's list
        uint[] storage fromItems = ownedItems[_from];
        for (uint i = 0; i < fromItems.length; i++) {
            if (fromItems[i] == _id) {
                fromItems[i] = fromItems[fromItems.length - 1];
                fromItems.pop();
                break;
            }
        }

        // Add item to the new owner's list
        ownedItems[_to].push(_id);
    }

    /// @notice Allows the owner to transfer an item to another address
    /// @param _id The ID of the item to transfer
    /// @param _to The address of the recipient
    /// @dev Calls the internal _transferOwnership function
    function transferItem(uint _id, address _to) public {
        Item storage item = items[_id];
        require(_id > 0 && _id <= itemCount, "Item does not exist");
        require(msg.sender == item.owner, "You do not own this item");

        _transferOwnership(_id, msg.sender, _to);
    }

    /// @notice Retrieves all item IDs owned by a specific address
    /// @param _owner The address of the owner
    /// @return An array of item IDs owned by the specified address
    function getItemsByOwner(address _owner) public view returns (uint[] memory) {
        return ownedItems[_owner];
    }
}

This contract provides a basic framework for a decentralized marketplace where users can list items for sale, purchase items, and transfer ownership of items. It maintains a record of item ownership and ensures that only valid transactions can occur. Let’s explore the concept of the smart contract a bit more:

• Seller:

  • Listing an Item:

    • Sellers can list items for sale by calling the listItem function.

    • This function requires a name and price for the item.

    • The item is added to the marketplace with a unique ID, and the seller is recorded as the owner.

• Buyer:

  • Purchasing an Item:

  • Buyers can purchase items using the purchaseItem function.

  • The function checks that the item exists, the price is correct, and that the item is not already sold.

  • The payment is transferred to the seller, and ownership of the item is updated.

• Transferring Ownership:

  • Ownership Transfer:

    • The _transferOwnership function handles the internal logic for transferring item ownership.

    • It updates the owner in the items mapping and adjusts the ownedItems lists for both the previous and new owners.

  • Manual Transfer:

    • The transferItem function allows current owners to transfer their items to another address.

    • It ensures the sender is the current owner before calling the _transferOwnership function.

• Retrieving Owned Items:

  • Ownership Transfer:

    • The getItemsByOwner function returns a list of item IDs owned by a specific address.

This smart contract facilitates a decentralized marketplace where items can be listed, purchased, and transferred securely, with all transactions and ownership changes recorded on the blockchain.

On the ignition folder let’s create Marketplace.ts to deploy our contract and add the following code:

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

const MarketplaceModule = buildModule("MarketplaceModule", (m) => {
  // Deploy the Marketplace contract
  const marketplace = m.contract("Marketplace");

  // Return the deployed contract instance
  return { marketplace };
});

export default MarketplaceModule;

To compile the contract run the following command:

Before deploying the smart contract, make sure to update the .env file in the packages/blockchain directory with the following values:

# 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

To deploy the smart contract to the Linea testnet, run the following command:

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

This will deploy the marketplace smart contract on Linea Sepolia. You can always write a script to deploy quicker by writing small commands like this on package.json file:

"deploy:testnet": "npx hardhat ignition deploy ignition/modules/Marketplace.ts --network linea-testnet"

After the deployment we’ll get the contract address. Let’s keep that. We will need it soon while integrating with the frontend.

mkdir site
cd site

npx create-next-app@latest .

When prompted, choose the following options:

• TypeScript: Yes

• ESLint: Yes

• Tailwind CSS: Yes

• src/ directory: No (or Yes, if you prefer)

• App Router: Yes

• Import alias: Yes (default @/*)

npx shadcn-ui@latest init

Install the necessary UI components like button, card and input as needed.

We will create a wagmi.config.ts and add the following code:

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(),
  },
});

We will be using wagmi and metamask sdk to connect our wallet and make transactions. Now all that is left is to create a ConnectWallet.tsx UI component.

In the site/src directory let’s create a file called constants.ts and add the following:

export const CONTRACT_ADDRESS = //paste deployed contract here
export const ABI = 
  // Paste the ABI here

Remember the deployed contract that we’ve saved earlier, we’ll add it here. Head over to blockchain/ignition module and find the artifacts folder to find the ABI.

Let’s break down the code app.tsx and explain its key aspects in more detail:

import { useState, useEffect } from "react";
import { useAccount, useWalletClient } from "wagmi";

• The component uses React's useState for local state management and useEffect for side effects.

• It also uses custom hooks from wagmi (useAccount and useWalletClient) for blockchain wallet integration.

const [items, setItems] = useState<any[]>([]);
const [ownedItems, setOwnedItems] = useState<any[]>([]);
const [newItemName, setNewItemName] = useState("");
const [newItemPrice, setNewItemPrice] = useState("");

• Multiple state variables are defined to manage the component's data.

• items and ownedItems are arrays to store marketplace items.

• newItemName and newItemPrice are for form inputs when listing a new item.

useEffect(() => {
  loadItems();
  loadOwnedItems();
}, []);

• This effect runs once when the component mounts.

• It calls loadItems() and loadOwnedItems() to populate the state with data from the blockchain.

const loadItems = async () => {
  try {
    const itemCount = await client.readContract({
      address: CONTRACT_ADDRESS,
      abi: ABI_STRING_ARRAY,
      functionName: "itemCount",
    });
    // ... (fetching individual items)
  } catch (error) {
    console.error("Error loading items:", error);
  }
};

• This function reads data from the smart contract using client.readContract.

• It first gets the total item count, then fetches details for each item.

const listItem = async () => {
  try {
    if (!walletClient) return;
    const { request } = await client.simulateContract({
      // ... contract call details
    });
    await walletClient.writeContract(request);
    loadItems();
  } catch (error) {
    console.error("Error listing item:", error);
  }
};

• This function writes data to the blockchain (listing a new item).

• It first simulates the contract call, then uses walletClient.writeContract to execute the transaction.

<Card className="p-4 sm:p-6" key={index}>
  <li key={item.id} className=" p-4">
    <p><strong>Name:</strong> {item.name}</p>
    {/* ... other item details */}
    <Button
      variant="outline"
      onClick={() => purchaseItem(item.id, item.price)}
      className="border-2 border-green-500 text-green-500 hover:bg-green-500 hover:text-white py-2 px-4 rounded  duration-200 hover:shadow-xl"
    >
      Purchase
    </Button>
  </li>
</Card>

• The component uses custom UI components like Card and Button.

• Tailwind CSS classes are used for styling (className props).

• Conditional rendering is used to show/hide the purchase button based on item status and ownership.

<Input
  type="text"
  placeholder="Item Name"
  value={newItemName}
  onChange={(e) => setNewItemName(e.target.value)}
  className="border p-2 flex-1"
/>

• Controlled inputs are used for the form fields.

• The value and onChange props connect the input to the component's state.

try {
  // ... contract interaction
} catch (error) {
  console.error("Error loading items:", error);
}

• Try-catch blocks are used throughout the code to handle potential errors in asynchronous operations, especially during blockchain interactions.

The app.tsx can become quite large so we can also refactor the code insite/src/app/components/ directory.

/components
  - ListItem.tsx
  - AvailableItems.tsx
  - OwnedItems.tsx
/hooks
  - useItems.ts

View the full code here.

Also view the refactored code here.

This component demonstrates advanced React patterns, integration with blockchain technology, and modern UI practices. It showcases how to build a decentralized application frontend that interacts with a smart contract while providing a user-friendly interface.

npm run dev

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

Now we should be able to list and item, buy, and transfer ownership. See the built live website here.

In this guide, we built a simple decentralized marketplace dapp on Linea, leveraging zkEVM technology for scalability and cost efficiency. We covered everything from setting up the environment to writing and deploying smart contracts, and integrating the frontend with Next.js and ShadcnUI.

As we continue to explore and expand on this foundational knowledge, consider the potential for further enhancements, such as integrating additional features or optimizing the user experience, bidding on items, and auctions.