Typescript Advanced Types

TypeScript Advanced Types

Iron Law: Before applying any type pattern, READ the actual TypeScript file first. Do not suggest type changes from memory — verify the actual code (file:line) before making any claim.

When to Use

• Building type-safe libraries or frameworks
• Creating reusable generic components
• Implementing complex type inference logic
• Designing type-safe API clients
• Building form validation systems
• Creating strongly-typed configuration objects
• Implementing type-safe state management
• Migrating JavaScript codebases to TypeScript

Do Not Use

• The task is unrelated to TypeScript advanced types
• You need a different domain or tool outside this scope (Flutter/Dart, Java, Python)

Core Type Patterns

1. Generics

Create reusable, type-flexible components while maintaining type safety. Use generic constraints (T extends HasLength) to narrow what types are accepted. Supports multiple type parameters, default types, and conditional constraints. See playbook §1 for examples.

2. Conditional Types

Create types that depend on conditions: T extends string ? true : false. Supports infer for extracting type components (e.g., return type, promise type, array element type). Distributive conditional types apply over union members automatically. See playbook §2 for examples.

3. Mapped Types

Transform existing types by iterating over their properties with [P in keyof T]. Supports modifiers (readonly, ?), key remapping via as, and filtering properties by type. Enables patterns like Partial<T>, Readonly<T>, and Getters<T>. See playbook §3 for examples.

4. Template Literal Types

Create string-based types with pattern matching: `on${Capitalize<EventName>}`. Enables type-safe event names, path builders, getter/setter name generation. Built-in helpers: Uppercase, Lowercase, Capitalize, Uncapitalize. See playbook §4 for examples.

5. Utility Types

TypeScript's built-in transformation utilities: Partial<T>, Required<T>, Readonly<T>, Pick<T,K>, Omit<T,K>, Exclude<T,U>, Extract<T,U>, NonNullable<T>, Record<K,T>. Combine with custom mapped types for DeepPartial<T> and DeepReadonly<T>. See playbook §5 and Pattern 4 for examples.

Tech Stack Applicability

Resources

resources/implementation-playbook.md contains full patterns, checklists, and code samples covering:

• Core Concepts: Generics, Conditional Types, Mapped Types, Template Literal Types, Utility Types
• Advanced Patterns:
  • Pattern 1: Type-Safe Event Emitter
  • Pattern 2: Type-Safe API Client
  • Pattern 3: Builder Pattern with Type Safety
  • Pattern 4: Deep Readonly / Deep Partial
  • Pattern 5: Type-Safe Form Validation
  • Pattern 6: Discriminated Unions
• Type Inference Techniques: infer keyword, Type Guards, Assertion Functions
• Best Practices: 10 rules including unknown over any, strict mode, type testing
• Common Pitfalls and Performance Considerations

Related Skills