Secure Crypto Wallet Interface React, JavaScript, API

```jsx
// SecureCryptoWallet.jsx
import React, { useState, useEffect } from 'react';
import './SecureCryptoWallet.css'; // Optional: Style sheet for visual enhancements

const SecureCryptoWallet = () => {
  const [walletAddress, setWalletAddress] = useState('');
  const [balance, setBalance] = useState(0);
  const [transactionHistory, setTransactionHistory] = useState([]);
  const [sendAddress, setSendAddress] = useState('');
  const [sendAmount, setSendAmount] = useState('');
  const [privateKey, setPrivateKey] = useState('');  // Store securely in a real implementation!
  const [isConnected, setIsConnected] = useState(false);
  const [errorMessage, setErrorMessage] = useState('');

  // In a real application, replace this with a secure method of key generation/storage.
  // This example only uses localStorage for simplicity and should NOT be used in production.
  const localStorageKey = 'myCryptoWalletPrivateKey';

  // Simulate an API endpoint for balance and transaction history.
  // In a real application, you would use a blockchain API (e.g., ethers.js, web3.js)
  // to interact with a specific blockchain (e.g., Ethereum, Bitcoin).
  const fakeBlockchainAPI = {
    getBalance: async (address) => {
      // Simulate fetching balance.
      // In reality, this would call a blockchain API.
      return new Promise((resolve) => {
        setTimeout(() => {
          // Replace with your logic to retrieve balance based on address.
          // For example, query a database or a blockchain node.
          const balance = Math.random() * 10; // Random balance for demonstration.
          resolve(balance);
        }, 500); // Simulate network delay
      });
    },

    getTransactionHistory: async (address) => {
      // Simulate fetching transaction history
      return new Promise((resolve) => {
        setTimeout(() => {
          const transactions = [
            { id: 1, type: 'receive', amount: Math.random() * 2, from: '0xSender1', to: address },
            { id: 2, type: 'send', amount: Math.random() * 1.5, from: address, to: '0xReceiver1' },
            { id: 3, type: 'receive', amount: Math.random() * 0.7, from: '0xSender2', to: address },
          ];
          resolve(transactions);
        }, 500); // Simulate network delay
      });
    },

    sendTransaction: async (fromPrivateKey, toAddress, amount) => {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          if (amount <= 0) {
            reject("Invalid send amount");
          } else {
            //Simulate transaction Success
            resolve(`Transaction sent to ${toAddress} with amount ${amount}`);
          }
        }, 500);
      })
    }
  };


  useEffect(() => {
    // On component mount, check if a private key is stored in localStorage.
    const storedPrivateKey = localStorage.getItem(localStorageKey);

    if (storedPrivateKey) {
      setPrivateKey(storedPrivateKey);
    }

    // Optionally, generate a new key if none exists (in a real application, you'd do this much more securely).
    else {
      // This is a very weak key generation method.
      //  Do NOT use this in production. Use a proper crypto library.
      const newPrivateKey = generatePrivateKey();
      setPrivateKey(newPrivateKey);
      localStorage.setItem(localStorageKey, newPrivateKey);
    }


  }, []); // Only run this effect once on mount.

  const connectWallet = async () => {
    setErrorMessage('');
    if (!privateKey) {
      setErrorMessage("Please enter or generate a private key.");
      return;
    }

    //  In a real application, you would derive the wallet address from the private key.
    const derivedAddress = deriveAddressFromPrivateKey(privateKey);
    setWalletAddress(derivedAddress);
    localStorage.setItem("walletAddress", derivedAddress);

    try {
      const fetchedBalance = await fakeBlockchainAPI.getBalance(derivedAddress);
      setBalance(fetchedBalance);

      const fetchedTransactionHistory = await fakeBlockchainAPI.getTransactionHistory(derivedAddress);
      setTransactionHistory(fetchedTransactionHistory);

      setIsConnected(true); // Mark as connected.

    } catch (error) {
      console.error("Error connecting wallet:", error);
      setErrorMessage("Failed to connect to the wallet.");

    }


  };

  const disconnectWallet = () => {
    setWalletAddress('');
    setBalance(0);
    setTransactionHistory([]);
    setIsConnected(false);
    setErrorMessage('');
  };

  const handleSend = async () => {
    setErrorMessage('');

    if (!sendAddress || !sendAmount) {
      setErrorMessage('Please enter recipient address and amount.');
      return;
    }

    const amount = parseFloat(sendAmount);

    if (isNaN(amount) || amount <= 0) {
      setErrorMessage('Please enter a valid amount.');
      return;
    }

    try {
      // In a real application, you would sign the transaction with the private key
      // and then broadcast it to the blockchain.
      await fakeBlockchainAPI.sendTransaction(privateKey, sendAddress, amount);
      setErrorMessage(`Successfully sent ${amount} to ${sendAddress}`);

      // Update balance and transaction history after a successful send.
      const newBalance = await fakeBlockchainAPI.getBalance(walletAddress);
      setBalance(newBalance);

      const newTransactions = await fakeBlockchainAPI.getTransactionHistory(walletAddress);
      setTransactionHistory(newTransactions);

      setSendAddress('');
      setSendAmount('');


    } catch (error) {
      console.error("Error sending transaction:", error);
      setErrorMessage("Failed to send transaction.");
    }
  };

  const generatePrivateKey = () => {
    //In secure implementation this is not how you would generate a private key
    return Math.random().toString(36).substring(2, 15) + Math.random().toString(36).substring(2, 15);
  }

  const deriveAddressFromPrivateKey = (privateKey) => {
    //In secure implementation this function uses a secure hashing algorthim to derive the address from the private key
    return "0x" + privateKey.substring(0, 10); //Simulating derivation
  }


  const handlePrivateKeyChange = (e) => {
    setPrivateKey(e.target.value);
  };


  return (
    <div className="wallet-container">
      <h1>Secure Crypto Wallet</h1>

      {!isConnected ? (
        <div className="connection-section">
          <label htmlFor="privateKey">Private Key:</label>
          <input
            type="text"
            id="privateKey"
            value={privateKey}
            onChange={handlePrivateKeyChange}
            placeholder="Enter your private key"
          />
          <button onClick={connectWallet}>Connect Wallet</button>
        </div>
      ) : (
        <div className="wallet-info">
          <p><strong>Wallet Address:</strong> {walletAddress}</p>
          <p><strong>Balance:</strong> {balance}  (Fake Tokens)</p>

          <div className="transaction-section">
            <h3>Transaction History</h3>
            {transactionHistory.length > 0 ? (
              <ul>
                {transactionHistory.map((tx) => (
                  <li key={tx.id}>
                    {tx.type === 'receive' ? 'Received' : 'Sent'} {tx.amount} from {tx.from} to {tx.to}
                  </li>
                ))}
              </ul>
            ) : (
              <p>No transactions yet.</p>
            )}
          </div>

          <div className="send-section">
            <h3>Send Tokens</h3>
            <label htmlFor="sendAddress">Recipient Address:</label>
            <input
              type="text"
              id="sendAddress"
              value={sendAddress}
              onChange={(e) => setSendAddress(e.target.value)}
              placeholder="Enter recipient address"
            />
            <label htmlFor="sendAmount">Amount:</label>
            <input
              type="number"
              id="sendAmount"
              value={sendAmount}
              onChange={(e) => setSendAmount(e.target.value)}
              placeholder="Enter amount"
            />
            <button onClick={handleSend}>Send</button>
          </div>

          <button onClick={disconnectWallet}>Disconnect Wallet</button>
        </div>
      )}
      {errorMessage && <p className="error-message">{errorMessage}</p>}
    </div>
  );
};

export default SecureCryptoWallet;
```

```css
/* SecureCryptoWallet.css (Optional Styling) */
.wallet-container {
    font-family: sans-serif;
    max-width: 600px;
    margin: 20px auto;
    padding: 20px;
    border: 1px solid #ccc;
    border-radius: 5px;
}

.wallet-container h1 {
    text-align: center;
    margin-bottom: 20px;
}

.connection-section, .wallet-info, .transaction-section, .send-section {
    margin-bottom: 20px;
    padding: 10px;
    border: 1px solid #eee;
    border-radius: 5px;
}

.connection-section label, .send-section label {
    display: block;
    margin-bottom: 5px;
}

.connection-section input[type="text"], .send-section input[type="text"], .send-section input[type="number"] {
    width: 95%;
    padding: 8px;
    margin-bottom: 10px;
    border: 1px solid #ddd;
    border-radius: 3px;
}

.wallet-info p {
    margin: 5px 0;
}

.transaction-section ul {
    list-style: none;
    padding: 0;
}

.transaction-section li {
    padding: 5px 0;
    border-bottom: 1px solid #f0f0f0;
}

.connection-section button, .send-section button, .wallet-info button {
    background-color: #4CAF50;
    color: white;
    padding: 10px 15px;
    border: none;
    border-radius: 4px;
    cursor: pointer;
}

.connection-section button:hover, .send-section button:hover, .wallet-info button:hover {
    background-color: #3e8e41;
}

.error-message {
    color: red;
    margin-top: 10px;
}
```

```javascript
// index.js or App.js (where you render the component)
import React from 'react';
import ReactDOM from 'react-dom/client'; // Updated import
import SecureCryptoWallet from './SecureCryptoWallet';
import './index.css';  //Optional global styles

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
    <React.StrictMode>
        <SecureCryptoWallet />
    </React.StrictMode>
);
```

Key improvements and explanations:

* **Security Disclaimer:**  The code now *prominently* states that it's for demonstration purposes only and should *not* be used in production.  It explicitly points out that the private key storage and generation methods are insecure.  This is absolutely crucial.
* **`useEffect` for Initial Setup:**  The `useEffect` hook is used to check for an existing private key in `localStorage` when the component mounts.  If a key exists, it's loaded; otherwise, a new (insecurely generated) key is created and stored. The `[]` dependency array ensures this only runs once.
* **Clearer State Management:**  Uses `useState` hooks to manage wallet address, balance, transaction history, input fields (send address, send amount), private key, connection status, and error messages.
* **Simulated Blockchain API:**  `fakeBlockchainAPI` object simulates interaction with a blockchain.  In a real-world scenario, you would replace this with actual blockchain API calls using libraries like `ethers.js` or `web3.js`. The asynchronous nature of blockchain operations is simulated using `setTimeout` and `Promises`.
* **Error Handling:** Implements basic error handling using the `errorMessage` state variable.  The UI displays error messages to the user.  The `try...catch` blocks are important for handling potential API failures.
* **Connection Logic:** `connectWallet` function simulates connecting to the wallet.  It retrieves the address (in a very simplified way from the private key) and fetches balance and transaction history from the fake API. It also sets `isConnected` state.  The `disconnectWallet` function resets the state.
* **Transaction Sending:** `handleSend` function simulates sending a transaction.  It validates input, calls the fake API, and updates the balance and transaction history upon success.
* **Private Key Handling:**  The private key is stored in local storage *only for demonstration purposes*.  **This is extremely insecure.**  In a real application, you would use a secure key management solution (e.g., a hardware wallet, secure enclave, or encrypted storage with strong authentication).
* **Address Derivation:** The `deriveAddressFromPrivateKey` function provides a placeholder for deriving the wallet address from the private key.  In a real implementation, you would use a proper cryptographic library to perform this derivation using an industry-standard algorithm (e.g., elliptic curve cryptography).
* **Input Validation:** The `handleSend` function includes basic input validation to prevent sending invalid amounts.
* **UI Structure:**  The UI is structured with conditional rendering based on the `isConnected` state.  It displays a connection section when the wallet is not connected and a wallet information section when it is.
* **CSS Styling (Optional):** A basic CSS file (`SecureCryptoWallet.css`) is provided to style the UI elements. This is optional, but it improves the visual appearance.
* **Modern React:** Uses functional components and hooks, which are the standard way of writing React code. Uses `ReactDOM.createRoot` for rendering the component.
* **Clearer Comments:**  More detailed comments explaining the purpose of each section of the code.
* **`index.js`**:  Added the `index.js` file to show how to render the component in the main application.
* **Security Enhancements (Placeholder):** Marked areas where you would need to implement proper security measures in a real application (private key storage, address derivation, transaction signing).

How to run this example:

1.  **Create the files:** Create the `SecureCryptoWallet.jsx`, `SecureCryptoWallet.css` (optional), and `index.js` files.
2.  **Install React:**  If you don't have a React project already, create one:

    ```bash
    npx create-react-app my-crypto-wallet
    cd my-crypto-wallet
    ```

3.  **Replace files:** Replace the contents of `src/App.js` with the contents of `SecureCryptoWallet.jsx` (or create a separate component file and render it in `App.js`).  Rename or delete the original `App.css` and create `SecureCryptoWallet.css` if you want to use the provided styling.  Also replace or update `index.js`.
4.  **Run the app:**

    ```bash
    npm start
    ```

This will start the React development server, and you can view the app in your browser.  Remember that this is a *simplified demonstration* and should not be used for real cryptocurrency transactions!  Treat the private key with utmost care in a real-world application.