[EvanYou]

This is (surprisingly) almost identical to the Reactivity Transform explorations we did in Vue: https://vuejs.org/guide/extras/reactivity-transform.html

  let count = $ref(0)

  const double = $computed(() => count * 2)

  watchEffect(() => {
    console.log(double)
  })

We started experimenting with this ling of thought almost 3 years ago:

- First take (ref sugar), Nov 2020: https://github.com/vuejs/rfcs/pull/228

- Take 2, Aug 2021: https://github.com/vuejs/rfcs/pull/368

- Take 3, Nov 2021: https://github.com/vuejs/rfcs/discussions/369

We provided it as an experimental feature and had a decent number of users trying it out in production. The feedback wasn't great and eventually decided to drop it. https://github.com/vuejs/rfcs/discussions/369#discussioncomm...

[rich_harris]

Hi! First up — not that it matters, but since people will wonder — our design wasn't informed by the Reactivity Transform. We evaluated something close to 50 designs, some of them extremely wacky, before settling on runes, and it wasn't until after that time that the Reactivity Transform was brought to our attention.

Nevertheless, it's interesting and validating that we landed on such similar approaches. While the Reactivity Transform failed, it did so for reasons that I don't think apply to us:

- $state and $ref are quite different. $state doesn't give you access to the underlying object, so there's no conversion necessary between reactive variables and ref objects (either in your head or in code).

- There's strict read/write separation. Anyone with access to a ref has the ability to change its value, which definitely causes problems at scale. It's something that React, Solid and Svelte 5 get right.

- Reactivity Transform introduces things like $() for magic destructuring and $$() for preserving reactivity across boundaries. We're instead encouraging people to use familiar JavaScript concepts like functions and accessors

- There are already a lot of different ways to work with Vue — SFCs vs non-SFCs, template syntax vs JSX, composition API vs options API, `<script>` vs `<script setup>`... on top of that, adding a new compiler mode that needs to interoperate with everything else is inevitably going to be a challenge. This isn't the case with Svelte. While both runes and non-runes mode will be supported for the next two major versions, meaning there will be some short term fragmentation, we've made it clear that runes are the future of Svelte.

[EvanYou]

> $state and $ref are quite different.

I wouldn't say they are "different" - they are fundamentally the same thing: compiler-enabled reactive variables backed by runtime signals! But yes, Vue already exposes the underlying concept of refs, so for users it's two layers of abstractions. This is something that Svelte doesn't suffer from at this moment, but I suspect you will soon see users reinventing the same primitive in userland.

> There's strict read/write separation

I'd argue this is something made more important than it seems to be - we've hardly seen real issues caused by this in real world cases, and you can enforce separation if you want to.

> We're instead encouraging people to use familiar JavaScript concepts like functions and accessors

This is good (despite making exposing state much more verbose). In Vue, we had to introduce destructuring macros because we wanted the transform to be using the same return shape with all the existing composable functions like VueUse.

> There are already a lot of different ways to work with Vue

This is largely because Vue has a longer history, more legacy users that we have to cater to, so it's much harder to get rid of old cruft. We also support cases that Svelte doesn't account for, e.g. use without a compile step. That said, the default way with a compile step is now clearly Composition API + <script setup>. Reactivity Transform also only really applied in this case so the point you raised is kinda moot.

Separate from the above points, the main reason Reactivity Transform wasn't accepted remains in Runes: the fact that compiler-magic now invades normal JS/TS files and alters vanilla semantics. Variable assignments can now potentially be reactive - but there is no obvious indication other than the declaration site. We had users trying Reactivity Transform on large production codebases, and they ended up finding their large composition functions harder to grasp due to exactly this (and not any of the points raised above).

[wentin]

In Svelte 4, the let counter = 0 syntax is already reactive by default, a feature enabled by the compiler. This has been the status quo for Svelte prior to the rune change. The introduction of the $state(0) rune actually provides more hints about its reactivity than before, and restore the original meaning of let counter = 0 (in rune mode). While it's true that the compiler's "invasion" into JS/TS syntax has been a point of discussion, this invasion has been happening for a while, and the initial shock wave has been well-absorbed by the community.

Interestingly, the new changes could be seen as a retreat from that initial invasion, likely triggering a different response from the community. In fact, the resistance I've seen (and my own as well) has been in the opposite direction—it's hating this retreat, complaining Svelte becoming less "magical." and more close to regular joe Javascript.

[EvanYou]

I’m specifically taking about non-component context, i.e. plain JS/TS files.

Previously Svelte was able to get a pass on this because magic only happens in svelte files - but in the future, any JS/TS files in a rune-enabled Svelte project will technically be SvelteScript, this never happened before and I doubt the community has already “absorbed” how significant this change is.

[rsimp]

This is a really great point. I think something like a `.svelte.js` file extension is warranted here. This would key tooling to when it needs to interpret runes, and makes it clear which files in a codebase require the svelte compiler. These files clearly aren't just js/ts at this point, but I think its fine as long as they're marked as such. Custom react hooks, for instance, aren't usable outside of the runtime but can be transpiled without issues by esbuild/tsc and interpreted correctly by a js/ts language server.

As long as it's marked separately from js/ts I don't think its a huge issue though. Svelte files already have script tags that aren't completely vanilla js/ts.

[wentin]

Hi Evan You! (Hi from wentin) It's great to see you here in this thread. It's such a wonderful gesture for the open-source community to collaborate in this manner, by sharing valuable lessons learned the hard way.

It’s intriguing to see where the Svelte exploration will lead. Will it face disapproval or achieve success? While I agree that both implementations have arrived at the same place (almost serendipitously), their origins differ. For Vue, it's about adding the syntactic sugar by dropping .value, making it less explicit and more magical. In contrast, Svelte made the change to make things more explicit and reduce the “black magicness” from the svelte compiler, and bring it more similar to javascript. This difference might trigger totally different reaction, time will tell.

Looking forward to more insightful discussions!

[vjerancrnjak]

All of this looks like MobX

[ramesh31]

>All of this looks like MobX

Shhhh. Don't tell the Vue/Svelte community that they're just reinventing the React ecosystem on a 5 year delay. It will spoil all of their fun.

[rich_harris]

Michel and I have been internet acquaintances for years, and we've even talked about this stuff IRL. MobX certainly isn't something we just somehow never learned about!

But anyway: it's absurd to compare this with React+MobX. MobX replaces useState, sure, but you're still re-rendering entire components on each change (which is why MobX explicitly recommends that you break apart your app into many small components, regardless of whether that's a boon to readability and maintainability.

By contrast, Svelte (and Solid, and other frameworks) understand signals on a much deeper and more optimal level. They're really not the same thing at all.

[ramesh31]

> MobX replaces useState, sure, but you're still re-rendering entire components on each change (which is why MobX explicitly recommends that you break apart your app into many small components, regardless of whether that's a boon to readability and maintainability.

This is not true. MobX has had an `<Observer>` component for years now. You can be as fine detailed as you wish with rerenders in a larger component.

https://github.com/mobxjs/mobx-react#observer

[vjerancrnjak]

MobX does not rerender entire components.

Unlike Redux at that time (~2016), it was a first approach where minimum rendering was happening effortlessly.

You can have a list of components where each component references a bit of global state and rerenders only if that bit changes, even though the list might have enlarged or other elements changed.

Last time I used it (2016-2020), they used all of the tricks of the trade, get, set object attributes, dropped decorators and it still worked the same.

[rich_harris]

Notice that you just said "You can have a list of components _where each component_ references a bit of global state". In other words, in order to avoid re-rendering everything, you need to have a component for each item in the list.

In React, the component is the unit of re-rerendering. MobX can't change that fact. The only thing you can do is work around it with hacks that imperatively update the DOM.

[vjerancrnjak]

Yes, this clarifies what you've meant and I can see the case that is not covered with MobX+React but I assume it is covered by Svelte's runes.

You're saying that I can have the whole app written in a single file without any separation and the updates will still happen only in the place that needs it.

That makes sense. With MobX, this could be done but not with React and not without a bunch of boilerplate that obtains html elements that are referencing the state.

[kecupochren]

MobX is not that known unfortunately. It kills me inside.

[tipiirai]

I guess we are all waiting for Rich Harris to step in and comment on this one. I'm sure he has followed this Vue experimentation and has a clear argument to make. At least I hope so.

[Exuma]

Based Vue

[frodowtf]

Funny, I also thought of Vue immediately.