typescript-advanced-types

TypeScript Advanced Types

Goal

Use advanced types to improve safety without making code unreadable. Prefer simple types until complexity pays for itself.

Rules

• Use unknown instead of any when input is not trusted.
• Let TypeScript infer simple types.
• Add generics only when a type must connect multiple positions.
• Prefer unions and discriminated unions for variants.
• Prefer interface for object shapes.
• Use type for unions, conditionals, mapped types, and aliases.
• Avoid deeply nested conditional or recursive types.
• Add type tests for reusable utilities when the project supports them.

Core Tools

• Generics: reuse logic across related types.
• Conditional types: choose a type from a type condition.
• Mapped types: transform object properties.
• Template literal types: constrain string patterns.
• Utility types: use Pick, Omit, Partial, Required, Readonly, Record.
• infer: extract inner types from arrays, promises, functions, or objects.
• Type guards: narrow unknown at runtime.
• Assertion functions: throw when runtime validation fails.
• Branded types: distinguish validated strings or IDs.

Generic Rule

If a type parameter appears only once, it is probably unnecessary.

Useful Patterns

type StrictOmit<T, K extends keyof T> = Omit<T, K>;

type ElementType<T> = T extends (infer U)[] ? U : never;

type UserId = string & { readonly __brand: "UserId" };

type User = z.infer<typeof userSchema>;

Avoid

• any as a shortcut.
• Type assertions when a guard is possible.
• Overly clever types that hide intent.
• Unbounded generic parameters.
• Duplicating types already available from schema or API definitions.
• Recursive types that slow the compiler.

Progressive Disclosure

This skill provides detailed examples through context files. Load them when needed:

Context File	When to Load
context/core-examples.md	Need generics, conditionals, mapped types examples
context/patterns.md	Implementing real-world patterns like EventEmitter, API client, or Builder
context/techniques.md	Type inference, guards, assertions, and testing

References

• TypeScript Handbook
• Type Challenges
• TypeScript Deep Dive
• Effective TypeScript

Output

• Type approach.
• Simplest type shape.
• Generic constraints if needed.
• Runtime validation boundary when input is unknown.