Returning $state from a $derived should not be allowed?

[RuthgerD]

consider the following code:

let dep = $state(123)

let derived_state = $derived.by(() => {
    let state = $state(dep)

    return { get current() { return state }, set current(value) { state = value } }
})


derived_state.current = 456

can I get someone to clarify if this is encouraged or not

[brunnerh]

Edit: Starting with Svelte 5.25.0, it is possible to write to $derived variables declared with let.

---

If you need something to update when a dependency changes and also be reactive locally, this is a reasonable thing to do. If the alternative would be using an $effect, then maybe this would even be recommended.

With proxied state, this can additionally be simplified to:

let derived_state = $derived.by(() => {
    let state = $state({ current: dep });
    return state;
});