TypeScript Discriminated Unions

retool-typescript-control-flow-analysis-10.ts

type UUID = string;

interface BasicUser {
   id: UUID;
   first_name: string;
   last_name: string;
}

interface User extends BasicUser {
   plan: 'free'
}

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

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

let user : PremiumUser | User | undefined

if (user && user.plan === "premium") {
  user
  // `user` is of type `PremiumUser`

} else {
  user
  // `user` is of type `User`
}

Should be:

interface User extends BasicUser {
   plan: 'free'
}

@fabb true point!

Fixed!