According to the comment above the class Animal, the purpose of the Animal[Symbol.hasInstance] method is to "assume that anything with the canEat property is an animal". The condition if (obj.canEat) return true; doesn't fit the purpose mentioned, because it checks whether !!obj.canEat === true. So if we create the object with falsy canEat property value (for example: let obj = { canEat: false};), then obj instanceof Animal will return false, meaning that obj is not the instance of Animal and the condition "anything with the canEat property is an animal" is not true.
I suggest replacing the if (obj.canEat) return true; condition with if ('canEat' in obj) return true;. The in operator returns true if the specified property is in the specified object or its prototype chain. With the new condition, obj instanceof Animal will return true even if the value of the canEat property is false, which is exactly the same as saying "anything with the canEat property is an animal".
P.S. There is also a hasOwnProperty() method, which returns a boolean indicating whether this object has the specified property as its own property (as opposed to inheriting it). This method is not suitable for the implementation Animal[Symbol.hasInstance], because the condition if (obj.hasOwnProperty('canEat')) will be truthy only if the property canEat is a property of the obj itself, but not of its prototypes.
Description
According to the comment above the
class Animal
, the purpose of theAnimal[Symbol.hasInstance]
method is to "assume that anything with the canEat property is an animal". The conditionif (obj.canEat) return true;
doesn't fit the purpose mentioned, because it checks whether!!obj.canEat === true
. So if we create the object with falsycanEat
property value (for example:let obj = { canEat: false};
), thenobj instanceof Animal
will returnfalse
, meaning thatobj
is not the instance ofAnimal
and the condition "anything with the canEat property is an animal" is not true. I suggest replacing theif (obj.canEat) return true;
condition withif ('canEat' in obj) return true;
. Thein
operator returnstrue
if the specified property is in the specified object or its prototype chain. With the new condition,obj instanceof Animal
will returntrue
even if the value of thecanEat
property isfalse
, which is exactly the same as saying "anything with the canEat property is an animal". P.S. There is also ahasOwnProperty()
method, which returns a boolean indicating whether this object has the specified property as its own property (as opposed to inheriting it). This method is not suitable for the implementationAnimal[Symbol.hasInstance]
, because the conditionif (obj.hasOwnProperty('canEat'))
will be truthy only if the propertycanEat
is a property of theobj
itself, but not of its prototypes.Links
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty