What about a pure function that can take anything that has an age as input? Well obviously that wouldn't work for a cat but it's just an example. It requires typescript and I'm not sure how to name the file it would go in, but I think it's interesting to consider this duck-typing style.
function isAdult({age}: {age: int}) {
return age >= 18
}
ps: I replaced const by function because I don't like the IDE saying I can't use something before it is defined. It's not a bug it's an early feature of javascript to be able to use a function before it is defined. Code is just easier to read when putting the caller above the callee.
That wouldn’t be object oriented. In OO you tend to want to ask an object about itself, Yegor talks a bit about this in his book Elegant Objects.
What you are proposing is just functions or data-oriented programming; which is fine if that’s your thing, but I’d be weary because of the reasons you outline above. Can a book be an adult? What about a tv show? Or recipe from the 9th century? isAdult really only applies to users and really belongs on that object.
