Typescript Optional

optional.ts

// Example type
type UserInfo = {
  username: string,
  email: string
};

type Present<T> = {
  exists: true,
  value: T
};

type Absent = {exists: false};

type Optional<T> = Present<T> | Absent;

function dealWithOptionals(optionalUserInfo: Optional<UserInfo>) {
  if (optionalUserInfo.exists) {
    // Can access value
    optionalUserInfo.value
  } else {
    // Can not access value
    // optionalUserInfo.value // TS2339: Property 'value' does not exist on type 'Optional<UserInfo>'.   Property 'value' does not exist on type 'Absent'.
  }
}

type OptionalWithKey<T, K extends {[k: string] : T}> = {
  exists: true,
} & K | Absent

type OptionalUser = OptionalWithKey<UserInfo, {user: UserInfo}>;

function dealWithOptionalKeyedUser(optionalKeyedUserInfo: OptionalUser) {
  if (optionalKeyedUserInfo.exists) {
    // Can access user
    optionalKeyedUserInfo.user
  } else {
    // Can not access user
    // optionalKeyedUserInfo.user // TS2339: Property 'user' does not exist on type 'OptionalWithKey<UserInfo, { user: UserInfo; }>'.   Property 'user' does not exist on type 'Absent'.
  }
}