Example of when to use Type Guards

models.ts

export interface Noun {
  id: string;
  created?: number;
  name: string;
}

export interface Character extends Noun {
  gender: Gender;
}

export interface Setting extends Noun {
  location: string;
}

// This is the Type Guard to help us correct the errors
export const isCharacter = (noun: Noun): noun is Character => {
  return 'gender' in noun;
};

select-character-attributes.ts

// Example 1 - extended type
function getGender(noun: Noun) {
  return noun.gender;
  // Property 'gender' does not exist on type 'Noun'.ts(2339)
}

// Example 2 - union type
function getGender(noun: Character | Setting) {
  return noun.gender;
  // Property 'gender' does not exist on type 'Character | Setting'.
  // Property 'gender' does not exist on type 'Setting'.ts(2339)
}

// How to correct with a type guard
function getGender(noun: Noun) {
  if (isCharacter(noun)) return noun.gender;
  
  throw 'This noun is not a character';
}