TypeScript mapped types and template literal types

// Define resources and their actions
type TraceEvents = {
  'trace:created': { traceId: string; title: string };
  'trace:validated': { traceId: string; trustScore: number };
  'trace:deleted': { traceId: string };
};

type UserEvents = {
  'user:registered': { userId: string; email: string };
  'user:promoted': { userId: string; oldScore: number; newScore: number };
};

// Merge all events into one type
type AppEvents = TraceEvents & UserEvents;

// Event name is a key of AppEvents
type EventName = keyof AppEvents;

// Payload type is inferred from event name
type EventPayload<T extends EventName> = AppEvents[T];

// Type-safe event emitter
class EventBus {
  private handlers: { [K in EventName]?: ((payload: AppEvents[K]) => void)[] } = {};

  on<T extends EventName>(event: T, handler: (payload: EventPayload<T>) => void): void {
    (this.handlers[event] ??= []).push(handler as any);
  }

  emit<T extends EventName>(event: T, payload: EventPayload<T>): void {
    this.handlers[event]?.forEach(h => h(payload as any));
  }
}

const bus = new EventBus();

// TypeScript enforces correct payload types
bus.on('trace:created', ({ traceId, title }) => {
  // traceId: string, title: string — correctly inferred
  console.log(`New trace: ${title}`);
});

bus.emit('trace:created', { traceId: '123', title: 'Test' });  // OK
bus.emit('trace:created', { traceId: '123', score: 5 });       // TypeScript error!

// Template literal types for CSS-like APIs
type Spacing = 0 | 1 | 2 | 4 | 8 | 16;
type SpacingProp = `p${'' | 'x' | 'y' | 't' | 'r' | 'b' | 'l'}-${Spacing}`;

// Mapped type: make all properties optional with undefined
type PartialUndefined<T> = { [K in keyof T]?: T[K] | undefined };

// Mapped type: prefix all keys
type Prefixed<T, P extends string> = { [K in keyof T as `${P}${string & K}`]: T[K] };
type PrefixedTraceFields = Prefixed<{ id: string; title: string }, 'trace_'>;
// = { trace_id: string; trace_title: string }