TypeScript type guard usage

retool-typescript-control-flow-analysis-7.ts

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



function getPremiumOptions(user: User | PremiumUser): PremiumOptions | null {
  if (isPremiumUser(user)) {
    user
    // `user` is of type `PremiumUser`

    return user.premiumOptions
  } else {
    user
    // `user` is of type `User`

    return null
  }
}