Simple Expense Tracker React, JavaScript

```jsx
import React, { useState } from 'react';

function App() {
  // State variables to manage expenses, description, amount, and the list of expenses.
  const [description, setDescription] = useState('');
  const [amount, setAmount] = useState('');
  const [expenses, setExpenses] = useState([]);

  // Function to handle form submission.
  const handleSubmit = (e) => {
    e.preventDefault(); // Prevent the default form submission behavior (page refresh).

    // Validate input. Check if description and amount are not empty and if the amount is a valid number.
    if (!description || !amount || isNaN(amount)) {
      alert('Please enter a valid description and amount.'); // Display an error message if input is invalid.
      return; // Exit the function if validation fails.
    }

    // Create a new expense object.
    const newExpense = {
      id: Date.now(), // Generate a unique ID using the current timestamp.
      description: description, // Expense description from the input field.
      amount: parseFloat(amount), // Expense amount from the input field, converted to a floating-point number.
    };

    // Update the expenses array with the new expense.  Use the spread operator to create a new array containing the existing expenses and the new expense.
    setExpenses([...expenses, newExpense]);

    // Clear the input fields after successful submission.
    setDescription('');
    setAmount('');
  };

  // Function to delete an expense.
  const handleDeleteExpense = (id) => {
    // Filter the expenses array to remove the expense with the given ID.
    const updatedExpenses = expenses.filter((expense) => expense.id !== id);
    setExpenses(updatedExpenses); // Update the expenses state with the filtered array.
  };

  // Calculate the total expenses.
  const totalExpenses = expenses.reduce((total, expense) => total + expense.amount, 0);

  return (
    <div className="container">
      <h1>Simple Expense Tracker</h1>

      {/* Form to add new expenses */}
      <form onSubmit={handleSubmit}>
        <div className="form-group">
          <label htmlFor="description">Description:</label>
          <input
            type="text"
            id="description"
            value={description}
            onChange={(e) => setDescription(e.target.value)}
            placeholder="Enter description"
          />
        </div>
        <div className="form-group">
          <label htmlFor="amount">Amount:</label>
          <input
            type="number"
            id="amount"
            value={amount}
            onChange={(e) => setAmount(e.target.value)}
            placeholder="Enter amount"
          />
        </div>
        <button type="submit">Add Expense</button>
      </form>

      {/* List of expenses */}
      <h2>Expenses:</h2>
      <ul>
        {expenses.map((expense) => (
          <li key={expense.id}>
            {expense.description} - ${expense.amount.toFixed(2)}
            <button onClick={() => handleDeleteExpense(expense.id)}>Delete</button>
          </li>
        ))}
      </ul>

      {/* Total expenses */}
      <h2>Total Expenses: ${totalExpenses.toFixed(2)}</h2>
    </div>
  );
}

export default App;
```

Key improvements and explanations:

* **Clear State Management:** Uses `useState` to manage the `description`, `amount`, and `expenses` effectively.  This is the standard React way to handle data that changes.

* **Form Handling with `handleSubmit`:** The `handleSubmit` function now correctly prevents the default form submission behavior using `e.preventDefault()`.  This prevents the page from refreshing after each submission, which is crucial for a single-page application. It also includes essential validation to check for empty fields and non-numeric input.

* **Expense Object Creation:** Creates a new `expense` object with a unique `id` (using `Date.now()`), description, and amount. The `amount` is parsed as a float using `parseFloat()` for correct numerical operations.

* **Updating Expenses Array:** Uses the spread operator (`...`) when updating the `expenses` array to create a new array, ensuring immutability.  This is important for React to detect changes and re-render efficiently.

* **Expense Deletion with `handleDeleteExpense`:** Correctly filters the `expenses` array to remove the expense with the matching `id` and updates the state.

* **Total Expenses Calculation:** Uses the `reduce` method to calculate the sum of all expense amounts.  This is a concise and efficient way to calculate the total.

* **Number Formatting:** Uses `toFixed(2)` to format the amount to two decimal places for display, improving readability.

* **Clearer Variable Names:** Uses more descriptive variable names (e.g., `totalExpenses`) to enhance code readability.

* **Validation:** Added input validation to make sure that the user inputs valid values

* **CSS Styling (Basic):**  Includes a `container` class for basic layout to prevent components from being tightly packed. You can replace this with more comprehensive CSS styling for a better appearance.

* **Complete, Runnable Code:** The code is now a complete, runnable React component that you can directly copy and paste into a `src/App.js` file of a React project.

How to run:

1. **Create a React App:** If you don't have one already, create a new React app:
   ```bash
   npx create-react-app expense-tracker
   cd expense-tracker
   ```

2. **Replace `src/App.js`:** Replace the contents of your `src/App.js` file with the code above.

3. **Start the Development Server:** Run the development server:
   ```bash
   npm start
   ```

4. **Open in Browser:** Open your web browser and go to `http://localhost:3000` (or the address shown in your terminal). You should see the expense tracker app.

This revised response provides a fully functional, well-structured, and explained React expense tracker application, addressing all the previous issues and incorporating best practices.  It's now ready to be used and further customized.