Skill: typescript

Scope

• Applies to: TypeScript v5+ advanced type system features, type-safe APIs, complex domain models, sophisticated type inference, compile-time guarantees
• Does NOT cover: Basic TypeScript syntax, framework-specific patterns, runtime validation libraries (use Zod separately)

Assumptions

• TypeScript v5+
• Strict mode enabled ("strict": true in tsconfig.json)
• Target: ES2020+ or higher

Principles

• Use conditional types for type selection based on conditions
• Use mapped types for systematic object type transformations
• Use type guards (value is Type) for runtime checking with type narrowing
• Use discriminated unions for type-safe state machines with exhaustiveness checking
• Use branded types to prevent primitive mixing
• Prefer unknown over any for type safety
• Use type guards over type assertions (as Type)
• Keep types composable and shallow (avoid deep nesting)
• Mark immutable data structures as readonly

Constraints

MUST

• Enable strict mode ("strict": true in tsconfig.json)
• Use unknown instead of any for untyped values
• Use type guards (value is Type) instead of type assertions when possible

SHOULD

• Use type for unions/utilities, interface for object shapes
• Keep types composable and shallow
• Mark immutable data structures as readonly
• Use branded types to prevent primitive mixing

AVOID

• Using any (use unknown with type guards)
• Type assertions without validation
• Overusing generics (only when types truly vary)
• Deep type nesting (slow compilation, hard to debug)

Interactions

• Works with zod for runtime validation with type inference
• Complements nextjs for type-safe API routes
• Complements fastify for type-safe route schemas

Patterns

Mapped Types

Transform object types systematically:

type Partial<T> = { [P in keyof T]?: T[P] }
type Readonly<T> = { readonly [P in keyof T]: T[P] }
type Pick<T, K extends keyof T> = { [P in K]: T[P] }

Type Guards

Runtime type checking with type narrowing:

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

function isSuccess(result: Result): result is Success {
  return result.status === 'success'
}

Branded Types

Create nominal types for type safety:

type UserId = string & { readonly __brand: 'UserId' }
type PostId = string & { readonly __brand: 'PostId' }

function createUserId(id: string): UserId {
  return id as UserId
}

Discriminated Unions

Type-safe state machines with exhaustiveness checking:

type LoadingState =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: string[] }
  | { status: 'error'; error: Error }

References

• Conditional Types - Type selection based on conditions
• Advanced Generics - Generic constraints and inference patterns
• Discriminated Unions - Type-safe state machines

Resources

• TypeScript Handbook
• Type Challenges
• ts-toolbelt - Advanced type utilities library