TypeScript Type-Safe API Contracts

Core Principle: Explicit Return Types (Mandatory)

// BAD: Implicit return type
export function getUser(id: string) {
  return db.user.findUnique({ where: { id } });
}

// GOOD: Explicit return type
export async function getUser(id: string): Promise<User | null> {
  return db.user.findUnique({ where: { id } });
}

Interface vs Type

Use interface for object shapes: