TypeScript Type Gymnastics You Actually Need

There's a spectrum in TypeScript land. On one end: any everywhere. On the other: inescapable Turing-complete type puzzles. Here's the sweet spot.

Conditional Types

type IsArray<T> = T extends any[] ? true : false;

type ApiResponse<T> = T extends string
  ? { message: T }
  : T extends object
  ? { data: T; meta: { timestamp: number } }
  : never;

Template Literal Types

type EventName = 'click' | 'hover' | 'focus';
type Handler = `on${Capitalize<EventName>}`;
// Result: 'onClick' | 'onHover' | 'onFocus'

Infer for Extraction

type UnpackPromise<T> = T extends Promise<infer U> ? U : T;
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

These patterns compose beautifully to create APIs where impossible states are genuinely unrepresentable.