Convert Legacy Code

# Convert Legacy Code

Modernize legacy code to current language standards, frameworks, and best practices while preserving functionality and adding comprehensive tests.

## Conversion Strategy

### 1. Assessment
- Identify language/framework versions
- Document current functionality
- List deprecated features used
- Note dependencies and their versions
- Identify breaking changes needed

### 2. Planning
- Choose target versions
- Plan migration path
- Identify risks and mitigation
- Estimate effort and timeline
- Create rollback strategy

### 3. Modernization Areas

**Language Features**
- Update syntax to modern standards
- Replace deprecated functions
- Use new language features
- Improve type safety
- Update error handling patterns

**Framework Updates**
- Migrate to latest stable version
- Update component lifecycle methods
- Replace deprecated APIs
- Use modern hooks/patterns
- Update routing and state management

**Code Quality**
- Extract functions from large blocks
- Remove code duplication
- Improve naming conventions
- Add proper error handling
- Implement logging

**Testing**
- Add unit tests for critical paths
- Create integration tests
- Add E2E tests for workflows
- Ensure test coverage >80%
- Test edge cases

## Example Conversions

### JavaScript: var/function → const/arrow functions
```javascript
// Before (ES5)
var calculate = function(a, b) {
  var result = a + b;
  return result;
};

// After (ES6+)
const calculate = (a, b) => {
  const result = a + b;
  return result;
};
// Or simply: const calculate = (a, b) => a + b;

React: Class → Functional Components

// Before
class UserProfile extends React.Component {
  constructor(props) {
    super(props);
    this.state = { user: null };
  }
  
  componentDidMount() {
    fetchUser(this.props.id).then(user => {
      this.setState({ user });
    });
  }
  
  render() {
    return <div>{this.state.user?.name}</div>;
  }
}

// After
const UserProfile = ({ id }) => {
  const [user, setUser] = useState(null);
  
  useEffect(() => {
    fetchUser(id).then(setUser);
  }, [id]);
  
  return <div>{user?.name}</div>;
};

Conversion Checklist

• All deprecated APIs replaced
• Modern syntax adopted
• Tests added for all functionality
• Documentation updated
• Dependencies updated
• Security vulnerabilities fixed
• Performance improved
• Code reviewed
• Backward compatibility considered
• Deployment plan created