typescript-patterns

---

name: typescript-patterns description: TypeScript coding style and patterns - type safety, error handling, code organization

Type Safety

• Never use any - create proper types/enums instead
• Avoid unknown too - only use in extreme edge cases where you truly can't know the type
• Leverage existing types with Pick, Omit, Partial, Required rather than creating new ones when possible
• Use const assertions for literal types: as const
• Discriminated unions for state/variant management

Enums and Constants

• Use enums for related constants, then iterate over them instead of hardcoding values
• Const enums for compile-time optimization
• Object literals with as const when you need both type and runtime access

Error Handling

• Return Result/Either types for expected errors, not exceptions
• Custom error classes with proper typing for unexpected errors
• Validate at boundaries (API, user input) - trust internal types after that
• Early returns for error cases - fail fast, avoid deep nesting

Function Design

• Pure functions when possible - no side effects
• Early returns over nested conditionals
• Function declarations for utilities, arrow functions for callbacks
• Single responsibility as a guideline - use judgment, don't blindly split if it makes code less readable

Code Organization

• Types that are reused across multiple places go in types/ or similar shared location
• Types specific to one file/implementation that won't be reused much go at top of file under imports
• Colocate related implementations
• Explicit imports over barrel exports (index files)
• File size guideline is 200-300 lines, but use judgment - split when readability suffers, not by arbitrary line count

Async Patterns

• Always handle Promise rejections
• Promise.all() for parallel operations, not sequential awaits
• Promise.allSettled() when you need all results regardless of failures
• Don't mix callbacks and Promises

Naming Conventions

• Booleans: isLoading, hasError, canSubmit, shouldRetry
• Event handlers: handleClick, handleSubmit (handler implementations)
• Callbacks: onSubmit, onClick (callback props)
• Utilities: verb-first (formatDate, parseResponse, validateEmail)
• Types: PascalCase, purposeful suffixes when needed (UserResponse, CreateUserDto)

Anti-Patterns to Avoid

• Nested ternaries - use if/else or extract to function
• Object mutation - use spread or immutable updates
• Type assertions (as) unless absolutely necessary - fix the types properly
• Premature abstraction - write it twice before extracting
• Deep nesting - max 2-3 levels, then extract
• Magic numbers/strings - use named constants or enums