Height as any? And the Svelte example is returning a function that is either returning an undefined or a function? And checking for el right after he checked for el? What's wrong with you, people?
The thing that he does with the heightRef must be to bust the closure, which could be done more idiomatically with an updater function: (currentHeight) => ref.current && ref.current.offsetHeight !== currentHeight ? ref.current.offsetHeight : currentHeight