Article: Composition Alone Can't Replace Inheritance - runtime hierarchy check

runtimeHierarchyCheck.js

const monsterSentinel = {}

export const isMonster = obj => obj.typeSentinel === monsterSentinel

export class Slime {
  typeSentinel = monsterSentinel
  ...
}

export class Skeleton {
  typeSentinel = monsterSentinel
  ...
}
  
isMonster(new Slime()) // true
isMonster(new Skeleton()) // true
isMonster({ x: 2 }) // false