19 — Error Handling Patterns

Core Concepts

• Result/Either type — represent success or failure without exceptions
• Custom error classes — typed error hierarchies with instanceof checking
• Type-safe try/catch — catch block typing and error narrowing
• Error hierarchies — base error class with specific subtypes
• Exhaustive error handling — discriminated union for error types
• Async error patterns — typed Promise rejection, error boundaries

Why Typed Errors Matter

• JavaScript catch gives you unknown — no type info
• Thrown errors can be anything (string, number, Error, etc.)
• Result types make error handling explicit in the type system
• Discriminated error unions ensure you handle every case