Svelte onMount return vs onDestroy

Whether it runs in a server-side component (SvelteKit)

Similarities

Function returned from onMount and callback passed to onDestroy are both called before a Svelte component is unmounted(destroyed).

If a function is returned from onMount, it will be called when the component is unmounted. — onMount, Svelte Docs

Therefore, they can be used to clear intervals or remove event listeners.

Schedules a callback to run immediately before the component is unmounted. — onDestroy, Svelte Docs

Difference

onDestroy runs inside a server-side component. This affects SSR(server-side-rendering). Therefore in SvelteKit, it is advised to clear intervals and remove event listeners using onMount instead.

Out of onMount, beforeUpdate, afterUpdate and onDestroy, this is the only one that runs inside a server-side component. — onDestroy, Svelte Docs