TypeScript type guards

retool-typescript-control-flow-analysis-5.ts

type UUID = string;

interface User {
   id: UUID;
   first_name: string;
   last_name: string;
   plan: 'free' | 'premium'
}

interface PremiumOptions {
  option1: string
  // ...
}

interface PremiumUser extends User {
  plan: 'premium'
  premiumOptions: {
     option1: string
  }
}


function getPremiumOptions(user: User | PremiumUser): PremiumOptions | null {
  return user.plan === 'premium' ? (user as PremiumUser).premiumOptions : null
}