Description

According to the comment above theclass 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