It is an interesting comparison that JavaScript always ensures that different evaluations of a closure expression results in unique instances. So in general the closures will require allocations for each (unless the allocation is otherwise prevented such as via escape analysis). Of course much of the underlying data may be shared, but the identity itself will be unique.

I don't know how strict JavaScript garbage collection rules are. This was non-observable for the longest time but FinalizationRegistry now exists which makes cleanup observable. It sounds like basically no guarantees are provided when an object will be cleaned up, so presumably an implementation would be allowed to make optimizations such are proposed where for PHP.

The part that would violate guarantees in JavaScript is not function objects being kept alive longer, but function objects which should be distinct not being so.

    function foo() {
        return function() { };
    }
    console.log(foo() === foo()); // This must log `false` in a compliant implementation