TypeScript Advanced Patterns

Expert guidance for leveraging TypeScript's advanced type system features to build robust, type-safe applications with sophisticated type inference, compile-time guarantees, and maintainable domain models.

When to Use This Skill Building type-safe APIs with strict contracts and validation Implementing complex domain models with compile-time enforcement Creating reusable libraries with sophisticated type inference Enforcing business rules through the type system Building type-safe state machines and builders Developing framework integrations requiring advanced types Implementing runtime validation with type-level guarantees Core Concepts

TypeScript's type system enables compile-time safety through:

Conditional Types: Type selection based on conditions (type-level if/else) Mapped Types: Transform object types systematically (Partial, Readonly, Pick, Omit) Template Literal Types: String manipulation at compile time Type Guards: Runtime checking with type narrowing (value is Type) Discriminated Unions: Type-safe state machines with exhaustiveness checking Branded Types: Nominal types for preventing primitive mixing Builder Pattern: Type-safe fluent APIs with progressive type constraints Advanced Generics: Constraints, inference, and higher-kinded type patterns Utility Types: Deep transformations and compositions Type Inference: Const assertions and contextual typing Quick Reference

Load detailed references on-demand:

Topic Reference File Conditional Types skills/typescript-advanced-patterns/references/conditional-types.md Mapped Types skills/typescript-advanced-patterns/references/mapped-types.md Template Literal Types skills/typescript-advanced-patterns/references/template-literal-types.md Type Guards skills/typescript-advanced-patterns/references/type-guards.md Discriminated Unions skills/typescript-advanced-patterns/references/discriminated-unions.md Branded Types skills/typescript-advanced-patterns/references/branded-types.md Builder Pattern skills/typescript-advanced-patterns/references/builder-pattern.md Advanced Generics skills/typescript-advanced-patterns/references/advanced-generics.md Utility Types skills/typescript-advanced-patterns/references/utility-types.md Type Inference skills/typescript-advanced-patterns/references/type-inference.md Decorators skills/typescript-advanced-patterns/references/decorators.md Performance Best Practices skills/typescript-advanced-patterns/references/performance-best-practices.md Common Pitfalls skills/typescript-advanced-patterns/references/common-pitfalls.md Testing Types skills/typescript-advanced-patterns/references/testing-types.md Implementation Workflow 1. Identify Pattern Need Analyze type safety requirements Identify runtime vs compile-time constraints Choose appropriate pattern from Quick Reference 2. Load Reference Read specific reference file for pattern Review examples and use cases Understand trade-offs 3. Implement Pattern Start simple, add complexity as needed Use strict mode (tsconfig.json with "strict": true) Test with type assertions 4. Validate Ensure type errors caught at compile time Verify runtime behavior matches types Check performance (avoid excessive type complexity) 5. Document Add JSDoc comments for public APIs Document type constraints and assumptions Provide usage examples Common Mistakes to Avoid

Using any instead of unknown: Loses all type safety

Use unknown and type guards instead

Type assertions without validation: Unsafe runtime behavior

Prefer type guards (value is Type) over as Type

Overusing generics: Unnecessary complexity

Only use generics when types truly vary

Deep type nesting: Slow compilation, hard to debug

Keep types composable and shallow

Forgetting readonly: Accidental mutations

Mark immutable data structures as readonly

Not enabling strict mode: Missing null checks and type errors

Always use "strict": true in tsconfig.json

Mixing type and interface incorrectly: Confusing semantics

Use type for unions/utilities, interface for object shapes Quick Patterns Type-Safe ID type UserId = string & { readonly __brand: 'UserId' }; function createUserId(id: string): UserId { return id as UserId; }

Discriminated Union type State = | { status: 'loading' } | { status: 'success'; data: string } | { status: 'error'; error: Error };

Mapped Type Transformation type Readonly = { readonly [P in keyof T]: T[P] }; type Partial = { [P in keyof T]?: T[P] };

Type Guard function isString(value: unknown): value is string { return typeof value === 'string'; }

Resources TypeScript Handbook: https://www.typescriptlang.org/docs/handbook/ Type Challenges: https://github.com/type-challenges/type-challenges ts-toolbelt: Advanced type utilities library zod: Runtime validation with TypeScript inference tsd: Test TypeScript type definitions

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