Decentralized Finance (DeFi) Loan System Solidity, JavaScript
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Okay, here's a simplified example of a DeFi loan system using Solidity and JavaScript. This is a very basic implementation and doesn't include features like collateralization, interest calculation, or liquidation. It's meant to illustrate the core interaction between a Solidity smart contract for managing loans and a JavaScript frontend for user interaction.
**Solidity Smart Contract (LoanContract.sol):**
```solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract LoanContract {
address public owner;
mapping(address => uint256) public loanBalances; // Mapping to track loan amounts for each address
uint256 public totalLoans;
event LoanIssued(address borrower, uint256 amount);
event LoanRepaid(address borrower, uint256 amount);
constructor() {
owner = msg.sender;
}
modifier onlyOwner() {
require(msg.sender == owner, "Only the owner can call this function.");
_;
}
function issueLoan(address _borrower, uint256 _amount) public onlyOwner {
require(_amount > 0, "Loan amount must be greater than zero.");
loanBalances[_borrower] += _amount;
totalLoans += _amount;
emit LoanIssued(_borrower, _amount);
}
function repayLoan(address _borrower, uint256 _amount) public {
require(_amount > 0, "Repayment amount must be greater than zero.");
require(loanBalances[_borrower] >= _amount, "Repayment amount exceeds outstanding loan.");
loanBalances[_borrower] -= _amount;
totalLoans -= _amount;
// Consider transferring the actual ETH/Tokens here to contract,
// but for simplicity, we only update balances in this example.
emit LoanRepaid(_borrower, _amount);
}
function getLoanBalance(address _borrower) public view returns (uint256) {
return loanBalances[_borrower];
}
function getTotalLoans() public view returns (uint256) {
return totalLoans;
}
// In a real-world scenario, you'd likely have a way to withdraw funds from the contract
// controlled by the owner, handling fees, etc. This is highly simplified.
// function withdrawFunds(uint256 _amount) public onlyOwner {
// // Implementation to withdraw funds to owner's address
// }
}
```
**Explanation of Solidity Code:**
* **`pragma solidity ^0.8.0;`**: Specifies the Solidity compiler version. Use a specific version (e.g., `0.8.9`) for production to avoid unexpected behavior from compiler upgrades.
* **`contract LoanContract { ... }`**: Defines the smart contract named `LoanContract`.
* **`address public owner;`**: Stores the address of the contract owner (the deployer).
* **`mapping(address => uint256) public loanBalances;`**: A mapping that associates each Ethereum address (the borrower) with the amount of their outstanding loan. `public` automatically generates a getter function.
* **`uint256 public totalLoans;`**: Stores the total loans issued.
* **`event LoanIssued(address borrower, uint256 amount);`**: An event that is emitted when a loan is issued, allowing external systems (e.g., a frontend) to track loan issuances.
* **`event LoanRepaid(address borrower, uint256 amount);`**: An event that is emitted when a loan is repaid.
* **`constructor() { ... }`**: The constructor is executed only once when the contract is deployed. It sets the `owner` to the address that deployed the contract.
* **`modifier onlyOwner() { ... }`**: A modifier that restricts access to certain functions, allowing only the contract owner to execute them.
* **`issueLoan(address _borrower, uint256 _amount) public onlyOwner { ... }`**: A function to issue a loan to a specific address. It's restricted to the contract owner. It updates the `loanBalances` mapping and emits the `LoanIssued` event.
* **`repayLoan(address _borrower, uint256 _amount) public { ... }`**: A function that allows a borrower to repay their loan. It updates the `loanBalances` mapping and emits the `LoanRepaid` event. Crucially, this simplified example *doesn't* handle the actual transfer of ETH or tokens from the borrower to the contract. In a real system, you would need to implement that.
* **`getLoanBalance(address _borrower) public view returns (uint256) { ... }`**: A view function (doesn't modify the contract state) that returns the loan balance for a given address.
* **`getTotalLoans() public view returns (uint256)`**: A view function that returns the total loan balance.
**JavaScript Frontend (index.html and app.js):**
This example uses Ethers.js to interact with the blockchain. You'll need to install it: `npm install ethers`
**index.html:**
```html
<!DOCTYPE html>
<html>
<head>
<title>DeFi Loan System</title>
<script src="https://cdn.ethers.io/lib/ethers-5.6.umd.min.js" type="application/javascript"></script>
</head>
<body>
<h1>DeFi Loan System</h1>
<p>Connect to MetaMask to interact with the contract.</p>
<button id="connectWallet">Connect Wallet</button>
<div id="accounts">Connected Account: <span id="connectedAccount"></span></div>
<h2>Issue Loan (Owner Only)</h2>
<label for="borrowerAddress">Borrower Address:</label>
<input type="text" id="borrowerAddress"><br><br>
<label for="loanAmount">Loan Amount:</label>
<input type="number" id="loanAmount"><br><br>
<button id="issueLoanButton">Issue Loan</button>
<h2>Repay Loan</h2>
<label for="repaymentAmount">Repayment Amount:</label>
<input type="number" id="repaymentAmount"><br><br>
<button id="repayLoanButton">Repay Loan</button>
<h2>Check Loan Balance</h2>
<label for="checkBalanceAddress">Address:</label>
<input type="text" id="checkBalanceAddress"><br><br>
<button id="checkBalanceButton">Check Balance</button>
<div id="loanBalance"></div>
<h2>Total Loans</h2>
<button id="getTotalLoans">Get Total Loans</button>
<div id="totalLoansAmount"></div>
<script src="app.js"></script>
</body>
</html>
```
**app.js:**
```javascript
const CONTRACT_ADDRESS = "YOUR_CONTRACT_ADDRESS"; // Replace with the deployed contract address
const CONTRACT_ABI = [ // Replace with the ABI of your deployed contract
{
"inputs": [],
"stateMutability": "nonpayable",
"type": "constructor"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"internalType": "address",
"name": "borrower",
"type": "address"
},
{
"indexed": false,
"internalType": "uint256",
"name": "amount",
"type": "uint256"
}
],
"name": "LoanIssued",
"type": "event"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"internalType": "address",
"name": "borrower",
"type": "address"
},
{
"indexed": false,
"internalType": "uint256",
"name": "amount",
"type": "uint256"
}
],
"name": "LoanRepaid",
"type": "event"
},
{
"inputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"name": "loanBalances",
"outputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"name": "owner",
"outputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "_borrower",
"type": "address"
},
{
"internalType": "uint256",
"name": "_amount",
"type": "uint256"
}
],
"name": "issueLoan",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "_borrower",
"type": "address"
},
{
"internalType": "uint256",
"name": "_amount",
"type": "uint256"
}
],
"name": "repayLoan",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "_borrower",
"type": "address"
}
],
"name": "getLoanBalance",
"outputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"name": "getTotalLoans",
"outputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"stateMutability": "view",
"type": "function"
}
];
let provider;
let signer;
let contract;
document.addEventListener("DOMContentLoaded", async () => {
const connectWalletButton = document.getElementById("connectWallet");
const issueLoanButton = document.getElementById("issueLoanButton");
const repayLoanButton = document.getElementById("repayLoanButton");
const checkBalanceButton = document.getElementById("checkBalanceButton");
const getTotalLoansButton = document.getElementById("getTotalLoans");
const connectedAccountSpan = document.getElementById("connectedAccount");
const loanBalanceDiv = document.getElementById("loanBalance");
const totalLoansAmountDiv = document.getElementById("totalLoansAmount");
connectWalletButton.addEventListener("click", connectWallet);
issueLoanButton.addEventListener("click", issueLoan);
repayLoanButton.addEventListener("click", repayLoan);
checkBalanceButton.addEventListener("click", checkLoanBalance);
getTotalLoansButton.addEventListener("click", getTotalLoans);
async function connectWallet() {
if (window.ethereum) {
try {
await window.ethereum.request({ method: "eth_requestAccounts" });
provider = new ethers.providers.Web3Provider(window.ethereum);
signer = provider.getSigner();
contract = new ethers.Contract(CONTRACT_ADDRESS, CONTRACT_ABI, signer);
const accounts = await provider.listAccounts();
connectedAccountSpan.textContent = accounts[0];
console.log("Connected to MetaMask");
} catch (error) {
console.error("Error connecting to MetaMask:", error);
}
} else {
console.log("MetaMask not detected. Please install MetaMask.");
}
}
async function issueLoan() {
const borrowerAddress = document.getElementById("borrowerAddress").value;
const loanAmount = document.getElementById("loanAmount").value;
try {
const transaction = await contract.issueLoan(borrowerAddress, loanAmount);
await transaction.wait();
console.log("Loan issued successfully!");
} catch (error) {
console.error("Error issuing loan:", error);
}
}
async function repayLoan() {
const repaymentAmount = document.getElementById("repaymentAmount").value;
try {
const transaction = await contract.repayLoan(signer.getAddress(), repaymentAmount);
await transaction.wait();
console.log("Loan repaid successfully!");
} catch (error) {
console.error("Error repaying loan:", error);
}
}
async function checkLoanBalance() {
const checkBalanceAddress = document.getElementById("checkBalanceAddress").value;
try {
const balance = await contract.getLoanBalance(checkBalanceAddress);
loanBalanceDiv.textContent = `Loan Balance: ${balance.toString()}`;
} catch (error) {
console.error("Error getting loan balance:", error);
}
}
async function getTotalLoans() {
try {
const total = await contract.getTotalLoans();
totalLoansAmountDiv.textContent = `Total Loans: ${total.toString()}`;
} catch (error) {
console.error("Error getting total loans:", error);
}
}
});
```
**Explanation of JavaScript Code:**
1. **Setup:**
* Includes the Ethers.js library from a CDN. For a production environment, it's better to bundle it with your application.
* `CONTRACT_ADDRESS`: **Important:** You must replace this placeholder with the actual address of your deployed smart contract on the blockchain.
* `CONTRACT_ABI`: **Important:** Replace this with the ABI (Application Binary Interface) of your compiled Solidity contract. The ABI describes the contract's functions and data structures in a format that JavaScript can understand. You can get the ABI from the Solidity compiler output (e.g., when using Remix).
* Declares variables to hold the provider, signer (MetaMask account), and contract instance.
2. **`DOMContentLoaded` Event Listener:**
* This ensures that the JavaScript code runs after the HTML document has been fully loaded.
* It gets references to all the HTML elements you'll be interacting with (buttons, input fields, divs for displaying data).
* It attaches event listeners to the buttons, so that when a button is clicked, the corresponding function is called.
3. **`connectWallet()` Function:**
* Checks if MetaMask (or another compatible Ethereum provider) is installed in the browser.
* Uses `window.ethereum.request({ method: "eth_requestAccounts" })` to prompt the user to connect their MetaMask account to your application. This is a crucial step for security and user consent.
* Creates an Ethers.js `Web3Provider` using the `window.ethereum` provider.
* Gets the `signer` (the user's account) using `provider.getSigner()`. The signer is needed to send transactions to the blockchain.
* Creates an Ethers.js `Contract` instance, which represents your deployed smart contract. It takes the contract address, the contract ABI, and the signer as arguments. This allows you to call functions on the contract.
* Displays the connected account address in the `connectedAccountSpan` element.
4. **`issueLoan()` Function:**
* Gets the borrower address and loan amount from the input fields.
* Calls the `issueLoan()` function on the smart contract using `contract.issueLoan(borrowerAddress, loanAmount)`. This creates a transaction that will be sent to the blockchain.
* `await transaction.wait()`: This is very important. It waits for the transaction to be mined (included in a block) on the blockchain. Until the transaction is mined, the contract state will not be updated.
* Logs a success message to the console. You'd typically update the UI to provide feedback to the user.
* Includes error handling to catch any exceptions that occur during the transaction.
5. **`repayLoan()` Function:**
* Gets the repayment amount from the input field.
* Calls the `repayLoan()` function on the smart contract. Note, in this example, it uses the connected account address as the borrower, and the repayment amount as the argument.
* Waits for the transaction to be mined.
* Logs a success message or an error message.
6. **`checkLoanBalance()` Function:**
* Gets the address to check from the input field.
* Calls the `getLoanBalance()` function on the smart contract.
* Updates the `loanBalanceDiv` element to display the loan balance.
7. **`getTotalLoans()` Function:**
* Calls the `getTotalLoans()` function on the smart contract.
* Updates the `totalLoansAmountDiv` element to display the total loans.
**How to Run This Example:**
1. **Set up a Development Environment:**
* Install Node.js and npm (Node Package Manager).
* Install MetaMask or a similar Ethereum wallet browser extension.
2. **Deploy the Solidity Contract:**
* Use Remix IDE (remix.ethereum.org) or Hardhat/Truffle to compile and deploy the `LoanContract.sol` contract to a test network like Ganache or a public testnet like Sepolia.
* **Important:** Note the deployed contract address and the ABI. You'll need to put them in the `app.js` file.
3. **Create the Frontend:**
* Create an `index.html` file and an `app.js` file in the same directory.
* Copy the code provided above into those files.
4. **Install Dependencies:**
* Open a terminal in the directory where you created the files and run: `npm install ethers`
5. **Update `app.js`:**
* Replace `"YOUR_CONTRACT_ADDRESS"` in `app.js` with the address of your deployed contract.
* Replace the placeholder for `CONTRACT_ABI` with the ABI of your deployed contract. The ABI is a JSON array that describes the contract's functions and variables. You can usually get the ABI from the Remix IDE or your deployment tool.
6. **Serve the Frontend:**
* You can use a simple HTTP server to serve the `index.html` file. For example, you can use `npx serve` (you might need to install `serve` globally: `npm install -g serve`).
* Open your browser and navigate to the address where the server is running (usually `http://localhost:3000` or similar).
7. **Interact with the Contract:**
* Connect your MetaMask wallet. Make sure you're connected to the correct network (the one you deployed the contract to).
* Use the form to issue loans, repay loans, and check balances.
**Important Considerations and Improvements:**
* **Security:** This is a very basic example and is *not* secure for production use. It lacks proper input validation, access control, and protection against common smart contract vulnerabilities. A real-world DeFi loan system would require extensive security audits and formal verification.
* **Collateralization:** Real DeFi loans are typically collateralized. Borrowers must deposit collateral (e.g., ETH, other tokens) to secure the loan. The contract needs to track the collateral and liquidate it if the borrower defaults.
* **Interest Rates:** This example doesn't include interest. You would need to implement a mechanism for calculating and accruing interest.
* **Liquidation:** You need to implement logic for liquidating the collateral of borrowers who fail to repay their loans.
* **Decentralized Governance:** More advanced DeFi systems use decentralized governance (e.g., DAOs) to manage parameters like interest rates, collateral ratios, and supported assets.
* **Token Transfers:** The `repayLoan` function needs to handle the actual transfer of ETH or tokens from the borrower to the contract. You can use the `transfer` or `transferFrom` functions for this, but you need to handle potential errors (e.g., the borrower doesn't have enough tokens).
* **Error Handling:** The JavaScript code should include more robust error handling and display informative messages to the user.
* **User Interface:** The HTML is very basic. A real application would have a more user-friendly and visually appealing interface.
* **Gas Optimization:** Solidity code should be optimized for gas efficiency to reduce transaction costs.
This example provides a starting point for understanding the basics of DeFi loan systems. To build a real-world system, you'll need to address the security, functionality, and scalability considerations mentioned above. Remember to thoroughly test and audit your code before deploying it to a live network.
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