TypeScript TSConfig intro example

retool-typescript-control-flow-analysis-13.ts

function isPremiumUser(user: User | PremiumUser): user is PremiumUser {
  return user.plan === "premium"
}

let user: PremiumUser | User | undefined

// the line below should raise a TypeScript error,
//  but it doesn't!
if (isPremiumUser(user)) {
  // ...
} else {
  // ...
}