Typescript Exercises 4

Typescript Exercise 4

Exercise 4

Figure out how to help TypeScript understand types in this situation and apply necessary fixes.

interface User {
  type: 'user';
  name: string;
  age: number;
  occupation: string;
}

interface Admin {
  type: 'admin';
  name: string;
  age: number;
  role: string;
}

export type Person = User | Admin;

export const persons: Person[] = [
  {
    type: 'user',
    name: 'Max Mustermann',
    age: 25,
    occupation: 'Chimney sweep',
  },
  { type: 'admin', name: 'Jane Doe', age: 32, role: 'Administrator' },
  { type: 'user', name: 'Kate Müller', age: 23, occupation: 'Astronaut' },
  { type: 'admin', name: 'Bruce Willis', age: 64, role: 'World saver' },
];

export function isAdmin(person: Person) {
  return person.type === 'admin';
}

export function isUser(person: Person) {
  return person.type === 'user';
}

export function logPerson(person: Person) {
  let additionalInformation: string = '';

  if (isAdmin(person)) {
    additionalInformation = person.role;
  }
  if (isUser(person)) {
    additionalInformation = person.occupation;
  }
  console.log(` - ${person.name}, ${person.age}, ${additionalInformation}`);
}

console.log('Admins:');
persons.filter(isAdmin).forEach(logPerson);

console.log();

console.log('Users:');
persons.filter(isUser).forEach(logPerson);

How I Solved This Problem

Though, Type for Person is defined either User or Admin function logPerson dose not know what type parameter person will be passed in

we can see functions like isAdmin and isUser to define whether passed parameter person is belong to Admin or User

This time we can use user-defined type guard to define a function whose return type is a type predicate by adding : [parameter] is part of [Type]

like below

export function isAdmin(person: Person): person is Admin {
  return person.type === 'admin';
}

export function isUser(person: Person): person is User {
  return person.type === 'user';
}