error-handlinglisted

# Error Handling Patterns (TypeScript) Consistent, robust error handling for production TypeScript / Node services. ## When to Activate - Designing error types or exception hierarchies for a new module or service - Adding retry logic or circuit breakers for unreliable external dependencies - Reviewing API endpoints for missing error handling - Implementing user-facing error messages and feedback - Debugging cascading failures or silent error swallowing ## Core Principles 1. **Fail fast and loudly** — surface errors at the boundary where they occur; don't bury them. 2. **Typed errors over string messages** — errors are first-class values with structure. 3. **User messages ≠ developer messages** — show friendly text to users, log full context server-side. 4. **Never swallow errors silently** — every `catch` block must handle, re-throw, or log. 5. **Errors are part of your API contract** — document every error code a client may receive. ## Typed Error Classes ```typescript // Domain error hierarchy export class AppError extends Error { constructor( message: string, public readonly code: string, public readonly statusCode: number = 500, public readonly details?: unknown, ) { super(message) this.name = this.constructor.name // Maintain correct prototype chain so `instanceof` works after transpile. Object.setPrototypeOf(this, new.target.prototype) } } export class NotFoundError extends AppError { constructor(resource: string, id: stri