feat: allow $derived($state()) for deep reactive deriveds

[paoloricciuti]

This needs discussion, but Dominic and I had this idea. Deriveds are not deeply reactive, but even nowadays you can kinda achieve deep reactivity for deriveds by doing

let local_arr = $derived.by(()=>{
	let _ = $state(arr);
	return _;
});

This is fine, but it's a bit boilerplatey. This PR is to allow a shorthand of this by allowing $state to be used in the init of a derived.

let local_arr = $derived($state(arr));

This basically gets compiled to

let local_arr = $.derived(() => $.get($.state($.proxy($$props.arr))));

So it works exactly the same.

Point of discussions:

1. This is a solution, but it feels a bit weird...shouldn't this be a new rune instead? (I don't think we should add a new rune...just trying to think aloud)
2. While testing this, I've discovered a quirk of this approach (which was obvious in hindsight): since when you proxify a proxy, we just return the original proxy when the prop is $state and not $state.raw you are actually mutating the parent

<script>
	import Component from './Component.svelte';
	let arr = $state([1, 2, 3]);
</script>
{arr}
<Component {arr}/>

<script>
	let { arr } = $props();

	let local_arr = $derived($state(arr));
</script>

<!-- this actually push to the parent state --!>
<button onclick={()=>{
	local_arr.push(local_arr.length + 1);
}}>push</button>

However, this is also true for the original trick that we currently "kinda" recommend.

I look at the server output, and it doesn't need any changes, since $state is just removed anyway.

WDYT should we do this?

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Playground

pnpm add https://pkg.pr.new/svelte@17308

[henrykrinkle01]

YES.
There must have been hundreds of Discord questions and Github issues related to this.
To avoid the parent mutation problem, currently I'm doing it like this:

let local_arr = $derived.by(()=>{
	let _ = $state($state.snapshot(arr));
	return _;
});

Which translates to

let local_arr = $derived($state($state.snapshot(arr)));

It's up to debate whether this should be the default behavior

[sacrosanctic]

There must have been hundreds of Discord questions and Github issues related to this.

I've been collecting.

• The data prop and subsequent $derived not mutable by default kit#13626
• Svelte 5: can't link reactivity from $props() #14536
• Object ownership not added for $state inside $derived #15164
• "Prefer using writable $derived instead of $state and $effect" but it doesn't work #15909
• A deeply reactive derived variable #16189
• Updating derived doesn't trigger reactivity. #16523
• Using a $state variable in an inner object of derived strips reactivity #16804
• Override derived array using push is not reactive #16887
• New state_referenced_locally warnings with svelte 5.45.3 #17289

[Conduitry]

Alternatively, we could expose a proxify function (name TBD) from the Svelte runtime, which wouldn't require any compiler changes. You would then do $derived(proxify(whatever)) when you need a mutable reactive derived instead of the $derived($state(whatever)) currently proposed.

This would also have the advantage of working in a universal load function in a +page.js (where you can't use runes), providing you with data props that you can mutate anywhere in your app and have them be reflected everywhere else.

[kran6a]

Alternatively, we could expose a proxify function (name TBD) from the Svelte runtime.

I have been doing this for months in userland by exporting functions that wrap $state and/or $state + $derived.by from .svelte.ts files. It started because I wanted to create $state on load functions, which I found extremely useful to separate app state (now resides on load functions) from UI state, event handlers, CSS and markup (in .svelte files). Having this built-in and officially supported would be awesome!

[KieranP]

This would be most helpful. After the recent change to warn state_referenced_locally on $state(prop), something like this would be very helpful to achieve a common pattern in our app: creating a new reactive state from an incoming prop.

In terms of naming/conventions, I've also seen the suggestion in some other issues for $state.from(prop), which would operate like $derived($state(prop)). It could also support the $derived.by style of supplying a function. e.g.

<script>
  const { client } = $props()

  let cats = $state.from(() => {
    client.cats.map((cat) => ({
      ...cat,
      _id: Math.random(),
    })),
  })

  function addCat() {
    cats.push({
      _id: Math.random(),
    })
  }
</script>

{#each cats as cat (cat._id)}
  <input type="text" value="{cat.name}" /><br />
{/each}

<button onclick={addCat} title="Add Cat">Add Cat</button>

In the above, if the client prop changed, any $state.from would get recalculated

[sacrosanctic]

Pretty sure any version where $state is the leading text is wrong. $state denotes a source. Which this is not, this derives it's source.

[sillvva]

My preference would be $derived.state or $derived.proxy to clearly describe that, unlike a regular derived, this has similar properties to a deeply reactive $state proxy with dependency tracking.

But I'm also fine with $derived($state(prop))

[brunnerh]

One question with this approach would also be reassignments.

let thing = $derived($state(array));
thing = otherArray;

Is thing still reactive? I suspect not.

So having a $dervide.xxx() rune that takes care of the state wrapping internally might be better. The function approach should also work.

thing = $state(otherArray); // not allowed
thing = proxify(otherArray); // OK

[sacrosanctic]

The proxify fn seems better than overloading the terms state or derive. It's also more honest about what is happening.

function proxify(value) {
    const proxified = $state(value);
    return proxified;
}

let foo = $derived(proxify(...));

src: #16189 (comment)

[sillvva]

One question with this approach would also be reassignments.

let thing = $derived($state(array));
thing = otherArray;

Is thing still reactive? I suspect not.

If you reassign to the derived, it will break reactivity, yes. If you reassign to the prop/state it's based on it will work correctly.

https://svelte.dev/playground/1a0230ade38a4ddd8ebc64ecfa8c5d95?version=5.45.6

This foot gun would exist regardless of whether you use $derived($state(thing)) or $derived(proxify(thing)). Svelte might need another warning if you attempt to reassign to a proxified derived.

[henrykrinkle01]

To avoid reassignment:

	function box<T>(value: T): { current: T } {
		const boxed = $state({
			current: $state.snapshot(value)
		});
		return boxed;
	}
	const thing = $derived(box(array));
	thing.current = otherArray;

[brunnerh]

This foot gun would exist regardless of whether you use $derived($state(thing)) or $derived(proxify(thing)).

The difference is that in one case you can avoid the reactivity loss using the same syntax (proxify(thing)) and in the other you cannot because the syntax is not allowed (using $state on already declared variable).

And as noted, if the wrapping happens inside a dedicated rune, the problem would not exist at all.

[henrykrinkle01]

I think it's possible to make $derived($state(...)) or $derived.xxx work with reassignment without using a box. That would be an improvement from the current $derived.by(...) and a justification for a new rune/syntax change. Need more compiler magic!

[sillvva]

This foot gun would exist regardless of whether you use $derived($state(thing)) or $derived(proxify(thing)).

The difference is that in one case you can avoid the reactivity loss using the same syntax (proxify(thing)) and in the other you cannot because the syntax is not allowed (using $state on already declared variable).

That still results in reactivity loss.

https://svelte.dev/playground/663f9bf9fe4e420c8b398b6b83256d16?version=5.45.6

To avoid reassignment:

	function box<T>(value: T): { current: T } {
		const boxed = $state({
			current: value
		});
		return boxed;
	}
	const thing = $derived(box(array));
	thing.current = otherArray;

That does more intuitively help you avoid the foot gun. Though the foot gun still exists if you reassign to thing. However, it's at least mitigated because you're far more likely to reassign to .current.

If not making a new rune, this is the method I'd probably go with. Otherwise, as stated, you'd probably need another warning.

[frederikhors]

Sorry if this question makes you smile (I'm the least appropriate person to answer these technical questions): why can't we have a deep reactive $derived() like $state()?

[sillvva]

My assumption is that $derived is not proxified by default, because it's less performant. Same reason $state.raw exists.

[cofl]

For minimal magic-guts-exposing (exposing the behavior without lifting the nuts-and-bolts of how reactivity proxies work right to the user's face), maybe some extensions on derived would serve well?

const thing = $derived.proxy(array);
const thing = $derived.proxyOf(() => {
    // if possible
    return array
});

That would satisfy:

1. Having a discoverable function ("hm, what's thing on $derived?")
2. Have it be $derived (not $state)
3. Naming it to encourage deeper learning of the system ("proxy?")
4. Without requiring knowledge of that system to reason about at the surface level ("of course $derived($state(array)) works because $state() transforms array into a proxy with deep reactivity, and $derived ensures the toplevel state is recreated when array changes" -- did I even get that right?)

Edit: not as a replacement for what this does, but an extension.

[henrykrinkle01]

Having a discoverable function ("hm, what's thing on $derived?")

This is the most compelling argument I've seen for a new rune. $derived($state(...)) is still kind of obscure

[KieranP]

I'm not sure proxy/proxify functions are the best approach. They expose terms that ideally should only exist within Svelte internals. And the use of $derived wrapping a $state sounds problematic because of the reassignment issue. So perhaps a new rune (or variation of existing) is the best path forward, one that returns a $state proxy, not a $derived one.

Some options being tossed around + some new ones:

1. $state.from(prop)
2. $state.derivedFrom(prop) (like above but more explicit as to purpose)
3. $derived.state(prop)
4. $derived(prop).toState() (convert from derived proxy to state proxy)

My personal preference is for options 1 or 3, but given how often I need this in our codebase, I'd be happy with any of them :-)

Another alternative would be to allow a way to make $derived deeply reactive (substitute deep for whatever):

1. $derived.deep(prop)
2. $derived(prop, { deep: true })
3. $derived(prop).deeplyReactive()