Tutorials and Examples: Building and Deploying a Simple dApp

This tutorial will guide you through creating a simple decentralized application (dApp) on JuChain: a "Greeting" smart contract where users can set and read custom greetings. We'll write the contract in Solidity, deploy it to the JuChain Testnet using Truffle, and build a frontend interface to interact with it via MetaMask. The tutorial showcases JuChain’s development workflow, fast transaction confirmations, and low-cost benefits.

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Prerequisites

• Tools:

  • Node.js (v16 or higher, download: https://nodejs.org/)

  • Truffle Suite (install globally: npm install -g truffle)

  • MetaMask browser extension (install: https://metamask.io/)

• JuChain Testnet:

  • RPC Endpoint: https://testnet-rpc.juchain.org

  • Network ID: 202599

  • Test JU Tokens: Obtain via the JuChain Testnet faucet (assumed: https://faucet-testnet.juchain.org)

• Knowledge: Basic understanding of JavaScript, Solidity, and blockchain concepts.

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Step 1: Set Up the Project

1. Create and Initialize the Project:

   mkdir JuChainGreeting
   cd JuChainGreeting
   truffle init

   This creates folders like contracts/, migrations/, and truffle-config.js.

2. Install Dependencies:

   npm install @openzeppelin/contracts web3 dotenv

   • @openzeppelin/contracts: Secure contract templates.

   • web3: Frontend-blockchain interaction.

   • dotenv: Environment variable management.

3. Create a .env File: In the project root, create .env:

   MNEMONIC="your MetaMask mnemonic"
   RPC_URL="https://testnet-rpc.juchain.org"

   Security Note: Never commit .env to GitHub.

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Step 2: Write the Smart Contract

In the contracts/ folder, create Greeting.sol:

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

contract Greeting {
    string public greeting;

    event GreetingSet(address indexed user, string newGreeting);

    constructor() {
        greeting = "Hello, JuChain!";
    }

    function setGreeting(string memory _greeting) public {
        require(bytes(_greeting).length > 0, "Greeting cannot be empty");
        greeting = _greeting;
        emit GreetingSet(msg.sender, _greeting);
    }

    function getGreeting() public view returns (string memory) {
        return greeting;
    }
}

Explanation:

• greeting: Public variable storing the greeting.

• setGreeting: Sets a new greeting and emits an event.

• getGreeting: Retrieves the current greeting.

• Event: Logs each greeting change for frontend monitoring.

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Step 3: Configure Truffle

Edit truffle-config.js to include the JuChain Testnet:

require("dotenv").config();
const HDWalletProvider = require("@truffle/hdwallet-provider");

const { MNEMONIC, RPC_URL } = process.env;

module.exports = {
  networks: {
    juchain_testnet: {
      provider: () => new HDWalletProvider(MNEMONIC, RPC_URL),
      network_id: 202599, // JuChain Testnet ID
      gas: 5500000,     // Gas limit
      gasPrice: 1000000000, // 1 gwei
      confirmations: 2, // Wait for 2 block confirmations
      timeoutBlocks: 200, // Timeout after 200 blocks
    },
  },
  compilers: {
    solc: {
      version: "0.8.0", // Solidity version
      settings: {
        optimizer: {
          enabled: true,
          runs: 200,
        },
      },
    },
  },
};

Checkpoint:

• Ensure MNEMONIC and RPC_URL in .env are correctly set.

• Test the network connection:

  truffle console --network juchain_testnet
  > web3.eth.getBlockNumber()

  A block number output confirms a successful connection.

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Step 4: Deploy the Contract

1. Create a Deployment Script: In migrations/, create 2_deploy_greeting.js:

   const Greeting = artifacts.require("Greeting");
   
   module.exports = function (deployer) {
     deployer.deploy(Greeting);
   };

2. Deploy to JuChain Testnet:

   truffle migrate --network juchain_testnet

   • Example output:

     2_deploy_greeting.js
     ====================
     Deploying 'Greeting'
     --------------------
     > transaction hash: 0x...
     > contract address: 0x1234567890abcdef1234567890abcdef12345678

   • Note the contract address (e.g., 0x1234...5678) for later use.

3. Verify Deployment: In the Truffle console:

   truffle console --network juchain_testnet
   > let greeting = await Greeting.at("0x1234567890abcdef1234567890abcdef12345678")
   > await greeting.getGreeting()

   Expected output: "Hello, JuChain!".

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Step 5: Create the Frontend Interface

1. Initialize the Frontend Folder:

   mkdir public && cd public
   touch index.html

2. Write index.html: Add the following to public/index.html:

   <!DOCTYPE html>
   <html lang="en">
   <head>
     <meta charset="UTF-8">
     <title>JuChain Greeting dApp</title>
     <style>
       body { font-family: Arial, sans-serif; text-align: center; margin-top: 50px; }
       input { padding: 5px; margin: 10px; width: 200px; }
       button { padding: 10px 20px; margin: 5px; }
       #status { color: green; }
     </style>
   </head>
   <body>
     <h1>JuChain Greeting dApp</h1>
     <p>Current Greeting: <span id="greeting">Loading...</span></p>
     <input id="newGreeting" type="text" placeholder="Enter new greeting" />
     <br />
     <button onclick="setGreeting()">Set Greeting</button>
     <p id="status"></p>
   
     <script src="https://cdn.jsdelivr.net/npm/web3@1.5.3/dist/web3.min.js"></script>
     <script>
       const contractAddress = "0x1234567890abcdef1234567890abcdef12345678"; // Replace with your contract address
       const abi = [
         {
           "inputs": [],
           "stateMutability": "nonpayable",
           "type": "constructor"
         },
         {
           "anonymous": false,
           "inputs": [
             {"indexed": true, "internalType": "address", "name": "user", "type": "address"},
             {"indexed": false, "internalType": "string", "name": "newGreeting", "type": "string"}
           ],
           "name": "GreetingSet",
           "type": "event"
         },
         {
           "inputs": [],
           "name": "getGreeting",
           "outputs": [{"internalType": "string", "name": "", "type": "string"}],
           "stateMutability": "view",
           "type": "function"
         },
         {
           "inputs": [{"internalType": "string", "name": "_greeting", "type": "string"}],
           "name": "setGreeting",
           "outputs": [],
           "stateMutability": "nonpayable",
           "type": "function"
         },
         {
           "inputs": [],
           "name": "greeting",
           "outputs": [{"internalType": "string", "name": "", "type": "string"}],
           "stateMutability": "view",
           "type": "function"
         }
       ];
       let web3, contract;
   
       async function init() {
         if (window.ethereum) {
           web3 = new Web3(window.ethereum);
           try {
             await window.ethereum.request({ method: "eth_requestAccounts" });
             contract = new web3.eth.Contract(abi, contractAddress);
             updateGreeting();
           } catch (error) {
             console.error("User denied account access", error);
             document.getElementById("status").innerText = "Please connect MetaMask";
           }
         } else {
           alert("Please install MetaMask!");
         }
       }
   
       async function updateGreeting() {
         const greeting = await contract.methods.getGreeting().call();
         document.getElementById("greeting").innerText = greeting;
       }
   
       async function setGreeting() {
         const accounts = await web3.eth.getAccounts();
         const newGreeting = document.getElementById("newGreeting").value;
         if (!newGreeting) {
           document.getElementById("status").innerText = "Greeting cannot be empty!";
           return;
         }
         document.getElementById("status").innerText = "Setting greeting...";
         try {
           await contract.methods.setGreeting(newGreeting).send({ from: accounts[0] });
           document.getElementById("status").innerText = "Greeting updated successfully!";
           updateGreeting();
         } catch (error) {
           document.getElementById("status").innerText = "Error: " + error.message;
         }
       }
   
       window.onload = init;
     </script>
   </body>
   </html>

3. Obtain the ABI:

   • Copy the "abi" field from build/contracts/Greeting.json and replace the abi variable in index.html.

   • Update contractAddress with the address from Step 4.

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Step 6: Run and Test

1. Start a Local Server:

   npx http-server ./public

   • Access http://localhost:8080 by default.

2. Connect MetaMask:

   • Add the JuChain Testnet to MetaMask:

     • Network Name: JuChain Testnet

     • RPC URL: https://testnet-rpc.juchain.org

     • Chain ID: 202599

     • Currency Symbol: JU

   • Ensure your account has test JU tokens (via the faucet).

3. Test the dApp:

   • Open http://localhost:8080 in your browser.

   • Verify the initial greeting displays as "Hello, JuChain!".

   • Enter a new greeting (e.g., "Welcome to JuChain!") and click “Set Greeting”.

   • Confirm the status updates to “Greeting updated successfully!” and the new greeting appears.

   • Check MetaMask: Transactions should confirm in 3-6 seconds with minimal fees (~0.001 JU).

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Outcome

You’ve successfully built and deployed a simple “Greeting” dApp:

• Smart Contract: Deployed on the JuChain Testnet, supporting greeting updates and retrieval.

• Frontend Interface: Interacts with the contract via MetaMask, displaying results intuitively.

• JuChain Advantages:

  • Fast Confirmation: Transactions complete in seconds, enhancing user experience.

  • Low Cost: Setting a greeting incurs minimal fees, ideal for frequent interactions.

  • EVM Compatibility: Leverages familiar Ethereum tools (Truffle, Web3.js) for seamless development.

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Notes

• Network Status: Ensure the JuChain Testnet RPC is operational; contact JuChain support if issues arise.

• Gas Fees: Testnet costs are low, but estimate mainnet expenses before production deployment.

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