error-handling

# Error Handling & Observability ### When to Load - **Trigger**: Try/catch patterns, retry logic, error responses, circuit breakers, structured logging - **Skip**: No error handling or observability involved in the current task ## Error Handling Workflow Copy this checklist and track progress: ``` Error Handling Progress: - [ ] Step 1: Define error taxonomy (categories and severity) - [ ] Step 2: Implement error handling by layer - [ ] Step 3: Set up structured logging - [ ] Step 4: Add retry and circuit breaker patterns - [ ] Step 5: Configure error tracking service - [ ] Step 6: Define user-facing error messages - [ ] Step 7: Validate against anti-patterns checklist ``` ## Error Handling Patterns by Language ### JavaScript / TypeScript ```typescript // Custom error hierarchy class AppError extends Error { constructor( message: string, public statusCode: number = 500, public code: string = "INTERNAL_ERROR", public isOperational: boolean = true, ) { super(message); this.name = this.constructor.name; } } class NotFoundError extends AppError { constructor(resource: string, id: string) { super(`${resource} with id ${id} not found`, 404, "NOT_FOUND"); } } class ValidationError extends AppError { constructor(public errors: Record<string, string[]>) { super("Validation failed", 400, "VALIDATION_ERROR"); } } // WRONG: Swallowing errors silently try { await saveUser(data); } catch (e) { // nothing here -- bug hides forever } ...