quicknotes/.cursor/rules/error-handling.mdc

---
description: Standards for consistent error handling patterns across frontend and backend
globs: **/*.{ts,tsx,js,jsx,go}
---
<rule>
filters:
  - type: file_extension
    pattern: "\\.(ts|tsx|js|jsx|go)$"
  - type: content
    pattern: "(?s)try|catch|error|err|throw|panic|recover"

actions:
  - type: suggest
    message: |
      # Error Handling Standards

      ## Frontend (TypeScript/JavaScript)
      
      1. Always use typed error handling:
         ```typescript
         try {
           // Operation that might fail
         } catch (error) {
           if (error instanceof ApiError) {
             // Handle API errors
           } else if (error instanceof ValidationError) {
             // Handle validation errors
           } else {
             // Handle unexpected errors
             console.error('Unexpected error:', error);
           }
         }
         ```
         
      2. Define custom error classes:
         ```typescript
         class ApplicationError extends Error {
           constructor(message: string) {
             super(message);
             this.name = 'ApplicationError';
           }
         }
         
         class ApiError extends ApplicationError {
           statusCode: number;
           
           constructor(message: string, statusCode: number) {
             super(message);
             this.name = 'ApiError';
             this.statusCode = statusCode;
           }
         }
         ```
         
      3. For async functions, always use try/catch with async/await:
         ```typescript
         async function fetchData() {
           try {
             const response = await api.get('/endpoint');
             return response.data;
           } catch (error) {
             handleError(error);
             throw error; // Re-throw if needed
           }
         }
         ```
         
      4. For React components, implement error boundaries:
         ```tsx
         import { ErrorBoundary } from 'react-error-boundary';
         
         function ErrorFallback({ error, resetErrorBoundary }) {
           return (
             <div role="alert">
               <p>Something went wrong:</p>
               <pre>{error.message}</pre>
               <button onClick={resetErrorBoundary}>Try again</button>
             </div>
           );
         }
         
         function MyComponent() {
           return (
             <ErrorBoundary FallbackComponent={ErrorFallback}>
               <ComponentThatMightError />
             </ErrorBoundary>
           );
         }
         ```

      ## Backend (Go)
      
      1. Return errors rather than using panic:
         ```go
         func ProcessData(data []byte) (Result, error) {
           if len(data) == 0 {
             return Result{}, errors.New("empty data provided")
           }
           // Process data
           return result, nil
         }
         ```
         
      2. Use error wrapping for context:
         ```go
         import "fmt"
         
         func FetchUserData(userID string) ([]byte, error) {
           data, err := database.Query(userID)
           if err != nil {
             return nil, fmt.Errorf("fetching user data: %w", err)
           }
           return data, nil
         }
         ```
         
      3. Use custom error types for specific cases:
         ```go
         type NotFoundError struct {
           Resource string
           ID       string
         }
         
         func (e NotFoundError) Error() string {
           return fmt.Sprintf("%s with ID %s not found", e.Resource, e.ID)
         }
         
         // Usage
         if data == nil {
           return NotFoundError{Resource: "User", ID: userID}
         }
         ```
         
      4. Check errors immediately:
         ```go
         resp, err := http.Get(url)
         if err != nil {
           return nil, err
         }
         defer resp.Body.Close()
         ```

      ## General Principles
      
      1. Log errors appropriately:
         - Debug: For development information
         - Info: For tracking normal operation
         - Warn: For potential issues
         - Error: For actual errors affecting operation
         - Fatal: For errors requiring application shutdown
      
      2. Don't expose system errors to users:
         - Log the technical details
         - Return user-friendly messages
      
      3. Include contextual information:
         - What operation was being performed
         - What resources were involved
         - Any IDs or references that help identify the context
      
      4. Handle all error cases - never silently ignore errors

metadata:
  priority: high
  version: 1.0
</rule>

examples:
  - input: |
      // Bad: Untyped error handling
      try {
        const data = await fetchData();
      } catch (error) {
        console.log(error);
      }
    output: |
      // Good: Typed error handling with proper logging and user feedback
      try {
        const data = await fetchData();
      } catch (error) {
        if (error instanceof ApiError) {
          toast.error('Could not connect to the server. Please try again later.');
          logger.error('API Error:', { error, endpoint: '/api/data' });
        } else {
          toast.error('An unexpected error occurred.');
          logger.error('Unexpected error:', { error });
        }
      }
      
  - input: |
      // Bad: No error handling in Go function
      func GetUser(id string) User {
        data := db.Find(id)
        return User{Name: data.Name, Email: data.Email}
      }
    output: |
      // Good: Proper error handling and propagation
      func GetUser(id string) (User, error) {
        data, err := db.Find(id)
        if err != nil {
          return User{}, fmt.Errorf("failed to find user %s: %w", id, err)
        }
        return User{Name: data.Name, Email: data.Email}, nil
      }