Note to Myself: Non-Obvious Details About Svelte 5 Runes

[newsve]

They might mentioned/documented here and there, I thought it's good to have them in one place. Please correct me if I am wrong and/or let us know if you know about other useful details!

1. Exporting $derived: Instead of exporting $derived or $derived.by just export a function, e.g. () => someRunteState, less verbose/boilerplate-y; do not forget to call the export when using, contrary to $derived Runes
2. Runes in store interoperability: Runes do not work in the old derived store (so the one without $), Runes within do not trigger updates; so old derived stores need to to be migrated as well
3. When to rename .ts files: While you need to rename .ts files to .svelte.ts when you use Runes, you do not need to do so when importing a function which contains Runes, e.g., count (a $state Rune) or derivedCount (a function, not a $derived Rune) from count.svelte.js can be imported to anotherFile.ts and used there; so you need to rename only when Runes are used explicitely
4. When to use object Runes: You can't reassign $state once exported if it contains primitives, you can work-around by using an object; defaulting to objects might seem over-eager and verbose, on the other side, one decision less every time you use them and you can add actions right away; I am not yet decided if it is good to use also a default property key, e.g. .value

[jdvlpr]

I'm also discovering these paradigms as I update from Svelte 4 to 5 and search the new docs. It's helpful to have them in one place, thanks.

For number 2, I think it works the opposite way though, is that correct? E.g. an update to a store inside a $derived rune triggers an update?