Error Handler Skill

Implements robust error handling patterns that provide meaningful errors, graceful degradation, and proper logging.

Resources

For detailed code examples, see:

• references/typescript-patterns.md - TypeScript/JavaScript patterns (Express, React)
• references/python-patterns.md - Python patterns (FastAPI, Flask)

Core Principles

1. Fail Fast, Fail Loudly - Catch errors early, make them visible
2. Context is King - Include relevant information in error messages
3. Never Swallow Errors - Always log, re-throw, or handle explicitly
4. User-Friendly Messages - Show generic messages to users, log details server-side
5. Typed Errors - Use custom error classes for different failure types

Quick Patterns

Custom Error Class (TypeScript)

export class AppError extends Error {
  constructor(
    message: string,
    public code: string,
    public statusCode: number = 500,
    public context?: Record<string, unknown>
  ) {
    super(message);
    this.name = this.constructor.name;
  }
}

// Specific types
export class ValidationError extends AppError {
  constructor(message: string, context?: Record<string, unknown>) {
    super(message, 'VALIDATION_ERROR', 400, context);
  }
}

export class NotFoundError extends AppError {
  constructor(resource: string, id: string) {
    super(`${resource} not found`, 'NOT_FOUND', 404, { resource, id });
  }
}

Custom Exception (Python)

class AppError(Exception):
    def __init__(self, message: str, code: str, status_code: int = 500, context: dict | None = None):
        super().__init__(message)
        self.message = message
        self.code = code
        self.status_code = status_code
        self.context = context or {}

class ValidationError(AppError):
    def __init__(self, message: str, context: dict | None = None):
        super().__init__(message, "VALIDATION_ERROR", 400, context)

Error Handling Pattern

async function fetchData(id: string): Promise<Data> {
  if (!id) throw new ValidationError('Invalid ID', { id });

  try {
    const data = await db.find(id);
    if (!data) throw new NotFoundError('Data', id);
    return data;
  } catch (error) {
    if (error instanceof AppError) throw error;
    throw new DatabaseError('Fetch failed', { id, originalError: error.message });
  }
}

Best Practices

DO

• Use custom error classes for different error types
• Include context in errors (but sanitize before sending to client)
• Log errors with structured data (method, path, user ID, etc.)
• Provide user-friendly error messages
• Handle errors at appropriate levels (function, route, global)
• Always clean up resources (use try/finally or context managers)
• Add retry logic for transient failures
• Test error paths (negative tests)

DON'T

• Swallow errors silently (catch (e) {})
• Leak sensitive information in error messages
• Use generic error messages without context
• Ignore promise rejections
• Re-throw errors without adding context
• Return errors as values when exceptions are better
• Use errors for control flow

Error Response Format

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid input data",
    "details": [{ "field": "email", "message": "Invalid format" }]
  }
}

Remember: Good error handling prevents debugging nightmares and provides a better user experience.

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Version

• v1.1.0 (2025-12-05): Split into references (typescript-patterns.md, python-patterns.md)
• v1.0.0 (2025-11-15): Initial version