let count = $ref(0)

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

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

- 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...

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.

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).

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.

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.

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.

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!

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.

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.

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

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.

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.

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.

It's trivial with MobX. The Observer component essentially gives you an "inline" component wherever you need reactivity, without the need to actually componentize.

i.e using a MobX State Tree store:

  const store = types.model('Store', { value: types.string }).create({value:'But I will!'});

  const Page = () => {
    return (
      <div>
        <div><h1>I won't rerender</h1></div>
        <Observer>
          {()=> { 
            const { value } = store;
            return (
              <div>{value}</div>
            )
          }
        </Observer>
      </div>
    )
  }

Observer component is so simple. Just a deferred function invoke. Could have been done with 2016 primitives too.

Also, I will never understand why people like reactive signals. The article even quotes "Knockout being right all along" which, no, reactivity and two way data binding creating a spaghetti mess of changes affecting other changes all over the place unless you're really careful is why React was made in the first place, to have only one way data binding and to re-render the entire page in a smart way. React will soon get its own compiler as well which will made regenerating the DOM even more efficient.

It's like every other framework is slowly rediscovering why React made the decisions it made. I am assuming it's because the users who are making these frameworks now have not used or do not remember the times where "signals" were called observables, or the usage of Rx libraries, and how having too many of these would cause you to lose your mind debugging the intricate webs you inadvertently spun.

That's the part Knockout was right about. It's absolutely true that you can mishandle them and create a spaghetti mess, _if your design allows it_. Svelte 5 doesn't — it uses signals as an implementation detail, but in a way that prevents the sorts of headaches you're describing.

Signals and observables (e.g. RxJS) are related but separate concepts. The framework world is converging on signals as the appropriate mechanism for conveying reactivity — even Angular (which has historically been most closely tied to RxJS) is moving in this direction. I promise you it's not just a mass delusion.

You call signals an "implementation detail", but if someone doesn't know that's what they are, then they'll wind up doing very bad things. This means they either aren't just an implementation detail or they are a hyper-leaky abstraction.

You haven’t even seen Svelte 5 yet, I don’t think it’s fair to say its internals will create trouble yet.

Yes, but having gone through this at previous companies, the average/non-front end specialist developers definitely had an easier time understanding 2 way binding ala Knockout, MobX, Svelte, etc. The Redux style stuff was only pushed by the frontend brainiac types.

I used to say these same things about reactivity. It was highly confusing to me years ago; as a backend developer doing frontend tasks was always reaching outside of my cookie jar and testing the boundaries of what I knew vs what I thought I knew. Many headaches ensued. That said, using Vue 3 and Svelte 4 has made utilizing stores and reactivity a lot more obvious in my opinion. Things render when they're supposed to and my applications remain performant. As a non-frontend person, I mainly like the APIs that Vue and Svelte bring. In React I'm almost always switching to class based components for what I can do in the compositional API in Vue or what's natively possible in Svelte. That brings a lot of repeated code and doesn't look as clear to people not as familiar with React.

I find hooks are flexible enough that I tend to be able to implement more sane and manageable reactivity, not the opposite.

Maybe I’m misunderstand your issue, though.

With classes there was no manual dependency tracking (except comparing old props?) and all re-renders were triggered by state, context or prop changes.

Not making a judgement either way, just stating the trade-offs.

There does seem to be an aspect to modern react where it’s great if you’re willing to put in the effort to figure out all the quirks, but for everyone else who just wants to get things done/has other priorities, it’s a bit of a minefield at times.

Care to elaborate or do you have a link to a blog post explaining further?

Basically, you can only modify state _where it's declared_. If you want to allow the 'outside world' to modify that state, you need to expose a function that does so. This is unlike cases where you're passing around an observable object where anyone with a reference, which they need for reading, can also write to it by doing `thing.value += 1`.

This is something Solid gets right — you can only change a signal's value if you have a reference to its setter.

Surely you mean signals? :)

I was just mentioning in another comment how Rust has a similar problem where, as in C, you can mutate variables via pointers anywhere in the codebase, but Rust counters that by having a borrow checker that explicitly tracks the changes made and makes sure that only one user can modify a variable at any one time. An analogous concept might be quite interesting to implement in all these signal based frameworks as well.

Which is what Svelte 5 is doing with its compiler.

> […] given a large enough codebase, that sort of spaghetti mess will emerge on its own, as not everyone will be so thorough.

LOL! You've just described React projects! Yes, of course you and your team are wonderful and have all the FP experience, so obviously this isn't an issue for YOU.

Most React codebases however turn into big balls of mud. Enormous balls of useMemo-y, useEffect-y, re-render the world-y, spaghettified mud without even any compiler-enabled guardrails that get re-written every 18-24 months but "this time we'll do it right."

I know its not anyones fault (RxJS is amazing), but it does seem to invoke some gray area in the brain, thinking observables would do more heavy lifting around change tracking of inputs / sources, when its not that straightforward. They more rightly should be called SubscriptionStreams maybe

I think its always been somewhat poorly named, based on how often I've had to explain the concept to other developers.

Also observables are more like streams than arrays.

Saying its an observable invokes a messy gray area in people's brains. I know this because I've had to explain observables to co-workers more times than I can count in my career (I'm a big fan of RxJS, I didn't mean what I said as a dismissal), and I like them for async work vs promises in many many cases, but the nomenclature does not invoke easy understanding

When does it start? How do you deal with oversubscription? Late starts? Double subscriptions?

On top of that streams are always a pain in the butt to debug.

The fifteen million obtuse methods/operators didn't help either (I see there are significantly fewer now).

And to remind you: it took the author of RxJava several months to understand the programming model: https://twitter.com/dmitriid/status/811561007504093184

When I break it down to folks explaining its stream processing over iterables or async work and you subscribe (and can unsubscribe) a pipeline that runs over that stream, people get it quickly.

When I say you can "observe a stream" people start getting other connotations, for some reason. I have ran into this over and over and over. Smart people too, not talking about just juniors here

`observers` do exist in RxJs by the way: https://rxjs.dev/guide/observer

On the other hand, on the other side of the learning curve, Observable is a great name. It's a simple English word that you can say and write a million times (and you will need to when working with them) without extra PascalCase or things like that. Also, when you look at the overall "family" that Observables fit into, it is a name that fits the family: Iterable/AsyncIterable ("PullStreams") versus Observable ("PushStreams"). (In .NET LINQ the family uses Enumerable/AsyncEnumerable and also extends to Queryable and the strangely named but makes sense in context Qbservable. [QueryableObservable; often pronounced like "cube-servable".]) As a family of tools that all generally work well together and are essentially related in a "four quadrant" way, it helps that they all have a nice -able names that sound related.

> why React was made in the first place, to have only one way data binding and to re-render the entire page in a smart way

This last part is why Vue and now Svelte didn't adopt React's model. Yes, reactivity and two-way data binding give you enough rope to thoroughly hang yourself, but it also gives you a very explicit picture of how and when components will update. React, on the other hand, expects you to trust its smart algorithms, which definitely removes a lot of the pain from day-to-day coding but also makes troubleshooting performance problems harder.

I see the distinction as similar to GC languages vs something like Rust: if all you need is to have an app that does a job and you don't care about how it does it, React is a good choice. If you need to have control over exactly what will update and when, Vue's model gives you fine-grained control over updates that's hard to achieve in React.

EDIT: And with that in mind, it's obvious why Svelte, which prides itself on being lean and fast, would avoid the React smart-but-complicated runtime model in favor of the Vue model.

Most devs are of average talent and will hang themselves and everyone else with the stateful webs they weave. Even with careful PR reviews, these things have a way of sneaking in and becoming permanent hinderances.

Performance hasn't been a dealbreaker in most JS apps for years now. What HAS been a problem is getting good code out the door as fast as business needs. MOST React code is disposable based on the whims of the company. The bits that aren't disposable are used enough to justify having them designed and maintained by your best devs who should also have the skill to write performant code.

Performance does matter to the people who depend on our products. Can we please stop throwing people under the bus in the name of churning out more crap?

I don't know, IME it's pretty easy to run into bottlenecks with React. To resolve these, you have to spend time optimizing data flow and preventing unnecessary re-renders, and it can be really difficult to trace where an update originally comes from. Svelte and other reactive frameworks give you good performance by default, so it's very unlikely you ever have to do this in the first place.

I never had to deal with `useMemo` and `useCallback` equivalents in Vue apps. The framework is implemented in such a way that it can let me focus on the task at hand. If I have errors in Vue apps, they are logic errors not "you didn't cater to the framework's rendering model" errors

Looking at the geomean benchmark suite everyone uses, it's 1.33 for Svelte 4 and 1.67 for React. That's a mere 0.2x difference and most of that is due to row swapping performance from React iterating lists of thousands of elements -- hardly typical in most apps.

Your team will make THOUSANDS of decisions in your app that will make WAY more difference than 20%. Incorrectly nest some loops in your data processors and the performance difference will almost instantly outshine the update differences.

And all of this is without mentioning that the React team is working on a compiler that will offer many of the benefits you get from the custom Svelte compiler (though TBH, I dislike needing a special compiler for custom JS).

EDIT: I'd also add that the Stage 2 record/tuple proposal will make comparing props many times cheaper and should give a massive performance boost to React as this comparison is one of the things that slows it down.

Now, I'll grant you that it really depends on the project. Building a simple admin interface? This probably won't matter. But for an interactive dashboard with dynamic charts and data being updated in realtime? You're almost certainly going to spend some time making it fast. And optimizing React code is currently a pretty poor experience.

For the record, I'm not arguing that the performance difference alone makes Svelte worth using over React. In fact, I think the developer experience with Svelte is far better, which is (in the majority of cases) a much better reason to use a framework. I just want to dispel this myth that React is so fast that you'll likely never have to think about performance, because I've had to do a non-trivial amount of optimization work on just about every React codebase I've worked on.

I know you're right; but this still deeply saddens me. I want well crafted software. I want the code I write and the code I depend on to be made in a way that holds reverence and honor for the craft of software engineering.

I don't want to be running crappy, badly performing javascript made by average talent in react, that exists to serve someone else's transient business needs. Gross.

He's not pearl clutching. He's in love with that car. He keeps it immaculate and he has a sparkle in his eye when he talks about it.

Some people are the same with furniture. They buy super expensive, custom stuff and look after it for decades. Not me! All through my 20s I didn't care at all and my various homes were full of cheap ikea furniture. I had bills that need to be paid, and why should I pay more? Most of it is probably in landfill by now.

I'm not an expensive furniture guy. And I'm not an expensive car guy - I don't even own a car. But I am into software. Software is kind of my thing. When software works fast and well, when its clean, reliable, tested and memory efficient - its the best. I know deep down everything is right in the world.

And, obviously, I hate disposable, cheaply made software. I hate when software runs slowly. I hate seeing my expensive computer working hard to do basically nothing because of lazy code. And I hate running into stupid, avoidable bugs. I don't want to create work like that. And I don't want to use with software like that. When software is laggy and unreliable, my skin crawls and I feel disgusting.

Intellectually, I recognize that there's a place for the cheap ikea furniture equivalent in software. But craftsmanship matters to me. And I personally, hate bad software and I don't want it on my computer.

Does that make me a pearl clutching, elitist snob? Sure, whatever. But I'm done acting like I'm sorry for what I like.

I think if you feel strongly about something you should defend it!

It makes it straightforward to reason about, because you aren't dealing with globals, undeclared deps etc. like in the Knockout days

Performance was the main issue because one incorrect usage of a hook can make a huge difference.

https://fe-tool.com/awesome-react-state-management

That's an article listing the top 18 state management libraries for React.

18?!?

React devs can't even agree on a common development model for state, but you think it's explicit and very easy to reason about?

Folks don't adopt utility libraries for things that were already easy to reason about on their own. I'm glad you've found peace with React's basic notion of state, but it's a far cry from optimal. And the "state ecosystem" of React is a dumpster fire. Layers upon layers of madness that acolytes peer through and proclaim insight.

> These rerenders can only happen down from where in the tree the state change happens.

Yes, after it has DIFFED the whole tree to see where the changes occurred in the first place. Madness. Layers of utter madness.

Despite there being so many, only a handful (redux toolkit, jotai, zustand recoil, and mobx) see much real-world use and they are basically just 3 approaches (reducers, signals, and directed graphs) with varying levels of complexity and features.

> Yes, after it has DIFFED the whole tree to see where the changes occurred in the first place. Madness. Layers of utter madness.

Not true. It only needs to diff the leaves of the part that changed and some of those sub-trees can be skipped too if they are functionally pure (or uses shouldComponentUpdate). This is how they reduce complexity down from O(n^3) to something generally closer to O(n)

Can you explain how all 3 state mechanisms you mentioned, and how they are different ?

I think the more glaring criticism is the lack of any unified component library. The amount of different Button implementations alone is astounding

The real kick to the shins is that React Server Components did a number on most component libraries, too. I've been forced to go to Tailwind (and DaisyUI, which is pretty nice) just so my stuff doesn't require 'use client' all over the place. The end result will be way better, it already is when you actually can get there, but right now it's annoying and awkward.

To be fair, context is not a state management tool [0], an article by acemarke, the Redux maintainer. It does work as a dependency injection tool and for many people, that's enough, but it's not a true state management tool.

[0] https://blog.isquaredsoftware.com/2021/01/context-redux-diff...

> 18?!?

I mean, the top library has more weekly NPM downloads than every other library in the list combined.

> Yes, after it has DIFFED the whole tree to see where the changes occurred in the first place. Madness. Layers of utter madness.

That is not how rendering works in React.

Hooks are also very interesting, they're basically functions that run at specific times, or that is to say, they're `f(...parameters, state)`. They hold state inside the function, sort of like a closure.

Unfortunately, many React apps use Relay (another Meta product) and it opens up a world of “magic” that is sometimes a real pain to reason about.

https://github.com/coralproject/talk

I agree that Relay CAN be simple, but it isn't for what we do. Have a look at this ReplyListContainer and its nested Relay logic. I didn't originally write this, but I can see what they were trying to do and how Relay made it difficult for them. Your thoughts are welcome, I won't judge you for being critical of our code and welcome any other Relay dev's opinions.

https://github.com/coralproject/talk/blob/develop/client/src...

Static analysis seems like a nice to have. All in all I class this category of concern as very far down the list of concerns, one way or two way. Yes bad things are possible. But the martial attitude that oh no people will do it wrong this tech is horrible is, in my view, actively harmful.

99.99999% of times it goes great. We don't need to make coding environments were bad things are impossible.

I'll disagree with you on that. I've seen and used enough 2-way binding in various projects to know that even the best of devs can create tangles of effects everywhere. At some point, you gotta blame the tool for having footguns, which is why Rust was created over C and C++, to solve such footguns. In the same vein, something like that should be made for frontend frameworks too.

And don't get me started on React's "put all the logic you want in our templating language that doesn't even target HTML5" elephant-footgun. Oh! Sorry! Forgot that React's footguns are aKeats elegant and reasonable while everything else's footguns are dealbreakers.

Sorry, but how is JSX a footgun? I have a lot of issues with React, but JSX doesn't even come close to making the list.

It was a bit sad that it didn't get more popular and possibly improved. Some devs liked it but we were a minority compared to those who liked/used Redux.

One strange thing they did was to release a new "version" called Mobx State Tree which was very different. It had a great api but was different and slower.

For Svelte as an "island" tool (i.e. for building islands of interactivity in otherwise static pages, like, say, graphs for the New York Times), that's not a problem, because the extra generated code is made up for by the lack of a bulky runtime dependency. But if you scale that up to complicated SPAs - which has been Vue's main stomping ground - you can end up shipping larger files with Svelte than with other small frameworks.

That said, SFCs in Vue are an ideal source for compile-time compilation, and my understanding is that they're heading more down the route of SolidJS. SolidJS has a similar goal of using a compile step for optimisation, but it leans on its signals library implementation much more heavily, which means the compiled code tends to be much smaller, but you also need to include a runtime. (In practice, there's not much size difference between Svelte and SolidJS at all, but it shows itself at the more extreme ends of very small components and very large apps.)

Vue already has a runtime that's very similar in some ways to Solid's (read: it's all signals, baby), so adopting the rendering ideas from there is an obvious next step. That should drastically speed up rendering, as well as reducing bundle sizes significantly. They've already demoed this a bit, but I think it's not fully released yet - they gave a name for the new system, but I've forgotten what it's called.

This is one of those things that's more of a problem in theory than in practice, but nevertheless it's worth mentioning that Svelte 5 output is _much_ smaller than Svelte 4 output.

I'm very impressed by this stuff - I'm still not sold on the implicit reactivity, but I'm really excited about the pressure this is putting on the ecosystem as a whole and where that's going to go.

Looks like early experiments were called Vue 'vapor'

Eg if a form input changes, that change triggers an onChange and in a handler function, you can let the change just flow through the data layer (ie through a signal or a store, which is just a hierarchy of signals). This then updates the input, but it already had that value so nothing happens. It’s pretty much the unidirectional loop that React people love to talk about.

React has as much two way binding as vue - in vue v-model is just syntactic sugar for setting up event listeners and setting the value on an input element.

Per the docs [0]

<input v-model="searchText" />

Becomes

<input :value="searchText" @input="searchText = $event.target.value" />

It's such a small thing that it's crazy it's being discussed this much.

[0] https://vuejs.org/guide/components/v-model.html

It’s not the kind of problem you can appreciate when taking any framework for a spin with a form input.

Nope. More like every other framework is slowly rediscovering that observables and dependency tracking offers the best DX and most of the innovations that React introduced have better alternatives.

though, the SSR madness movement including React concerns me, everyone tries to mix CSR and SSR into one now, which makes things way more complex than it needs.

I get it, it gives you flexibility to hydrate the view as close to the user, as late as possible. That sounds cool, but I wonder how many folks really use it and how much you pay for not clearly defining where and when each bit is happening.

I also agree that server components make a lot of sense to solve this problem. It's the obvious optimization (and in some ways, a good re-learning from past wins with PHP, Ruby, etc).

I also still feel we have a long way to go before it becomes elegant and obvious how all this is working. It hasn't become standard in the way that everyone easily groks the concept of how it all works and expects it everywhere on every web stack.

Still waiting to see if we go down another rabbit hole of complexity, or see more cautious and careful improvements towards an easier, more maintainable environment.

So you can't do as much in Astro as you can in Next.js, but it will definitely scratch the itch of compile-time rendering.

https://docs.pmnd.rs/react-three-fiber/getting-started/examp...

Definitely there is convergence in DX now. No need for everyone to admit one or another framework was right first or not - that is just creating antagonism or negating people lived experiences and innovation. But I do find it pretty weird that this convergence is happening. Initially I thought it was diverging with svelte but it seems to be reserving direction.

Convergent evolution [0]. Just as a shark, dolphin, and ichthyosaur all have the same body shape which is efficient for swimming and hunting in the water, so too do technologies converge when the problems are all the same and the solutions are understood, only as humans we can learn from each other rather than randomly mutating our codebases [1]. It's the same reason why we see a lot of languages starting to adopt more functional features such as not using `null` and having exhaustive pattern matching, such as Rust (from OCaml), Dart 3, recent versions of Java, etc.

[0] https://en.wikipedia.org/wiki/Convergent_evolution

[1] https://en.wikipedia.org/wiki/Genetic_programming

[1]: https://en.m.wikipedia.org/wiki/Carcinisation

Programming is not physical. It is generally bound by Turing completeness, the halting problem, 2 generals problem, and the like, but most developers don't even write code on that level. That leaves them with an infinite number of ways to solve problems with the real limiting factor often being the humans instead of the fundamental limits. If you use 5 different design patterns in 5 different frameworks, they all still work, so now you must decide (based on ever-changing factors) which one is actually the best.

Complicating this is the issue that not all people were taught the same or think the same way either, so an approach that makes sense to some people might not make sense to other people. The extremes here based on education and what you learned first are massive even when there are objectively better answers too. An entire generation of devs either retired or died rather than move from assembly to Fortran or C. That generation retired or died rather than move to OOP and the OOP people are retiring/dying rather than moving on to more functional approaches to programming.

In Svelte's case, it feels like they went as far as they could without directly addressing observables until they reached a point where it became one of the largest features that had yet to be addressed.

Open source frameworks, not unlike companies, need to continue attracting new users to grow the ecosystem, which means pleasing an ever-growing user base.

autocorrect style typo, i think

Honestly never understood this - 2 way is less code and way more understandable to me. Redux style codebases are very difficult for me to understand, and never saw benefits praised for. More frequently was "why isn't this updating?".

Different strokes I suppose.

With proper memoization, this is both fast and ergonomic, and it even allows you to turn mutations into data and pass them to a server. I've implemented this as a stand-alone state helper [1] based on JSON patch/diff, and it's magical. Undo/redo, multiplayer, it's all within reach.

[1] https://usegpu.live/docs/reference-live-@use-gpu-state

This will be amazing if works as people are hoping it will. I suspect it's an impossible task (in the general case). My hope is a certain coding style will be the path forward, and not a new version of putting shouldComponentUpdate (and others) all over the place.

You need architecture to avoid a big ball of mud or spaghetti code. Organising your code in layers, modules, each with its own API. Rules which layer can access what other layer. Like backend does for ages. You can and should do this in large frontend SPAs as well. No matter if written Svelte, Angular or React.

And also a tendency to not have experiencing working on large web apps (I’m talking 100k+ cloc) where issues arise, and why those React engineers made the decisions that they did

For me they're a dealbreaker when considering a new framework.

When you had this:

  let count = 0;
  count += 1;

But once it’s this:

  let count = $state(0);
  count += 1;

Yes, stores are unwieldy as you scale them up, and definitely have some practical problems, but that createCounter stuff looks fragile. I’m curious how it all works, because it looks like it’d be needing to do quite a lot of control flow analysis, but not curious enough to investigate at this time. But my intuitions suggest it’s probably markedly more complex and difficult to explain, though perhaps and hopefully more consistent.

Implementation-wise, this is vastly simpler than Svelte 4, because everything is explicit.

One thing we didn't really show today is how this works in your editor in practice — for example, TypeScript thinks `$state` and `$derived` are just the identity function. It makes sense when you use it, but I appreciate that I'm basically asking you to just trust me on that!

> Previously, Svelte’s reactivity model was very easy to understand, including its limitations, because it was the result of very simple analysis

I totally understand what you're saying. But in fact the analysis is _anything but_ simple — in places it's frighteningly complicated, to the point that even I sometimes don't understand what's going on! And that's part of the problem — because Svelte 4 is more implicit, you can't really know what's happening in a lot of cases, you just have to hope that the compiler knows what it's doing.

But at the same time I just wish there was some other solution to this while keeping it implicit somehow. I would do anything to keep things implicit while solving the backend complexity some other way. The thing I most love about Svelte is the principle / vector of "as little learning as possible", as well as in many cases transfer of learning, makes it much more user friendly, and I wish evolutions of Svelte continued to evolve along that direction, by reducing more and more.

But things like `$something` are a bit strange. Even `$:` is strange. It adds cognitive load. It's not very English-like and in turn not easily parseable. `on:click` is closer to the attractor of user friendliness. I think all programming languages and frameworks should try to approach English or Python, to allow for maximum "transfer of learning" so things just feel like they flow "without thought". Taking inspiration from UI/UX... things should be as close as possible to 'understanding at a glance'

The gold standard would be to approach something like this level of intuitiveness in the future as crazy as it may seem: https://twitter.com/brianjoseff/status/1617556877218570241 https://twitter.com/mathemagic1an/status/1700232760756207956...

I have many ideas on how, but it would no longer be a programming language.

That seems like a missed opportunity on Svelte’s part… but hard to fault, because TypeScript doesn’t support nominal primitive types very well. Ideally it would be something like Reactive<T> to signal (ha) that it’s not just a plain value.

type Reactive<T> = T; function $state<T>(value: T): Reactive<T>

...then TypeScript will 'unwrap' the type anyway, unless you do funky stuff like this...

type Reactive<T> = T & { [uniquesymbol]: any };

...in which case things like `value += 1` will cause type errors because it coercies `value` from `Reactive<number>` to `number`.

But it also creates problems here:

let message = $state('hello'); obj = { message };

The type of `obj.message` is Reactive<string>, but it's _not_ reactive — it's a non-reactive snapshot of the value when the object was created.

It's possible that we can do some fun stuff with TypeScript plugins, but we haven't dived too deeply into it yet.

Yeah. I’ve spent more time than I’d like to admit trying to find a nice solution to this, and I’ve ultimately arrived at “it needs type system support to work for the general case”. I think you can make your brand (symbol) optional to address this case, but it doesn’t address this:

> The type of `obj.message` is Reactive<string>, but it's _not_ reactive — it's a non-reactive snapshot of the value when the object was created.

And here is the real problem: the types are right (in this case)! It’s Svelte that is wrong (in that it deviates from the JavaScript language semantics, which TypeScript has correctly modeled).

This leads to my other ultimate conclusion with nominal primitive types: most of the time you’re better off just boxing the value… unless, or until, it becomes problematic for other reasons (eg performance, which would probably be a concern here, albeit one worth testing). Boxing allows correct, refined typing; it makes semantics of the value clear. The only downside is convenience… but this sort of convenience is exactly the kind of thing people rightly judge about the JS ecosystem.

I’m not likely to use Svelte (for a variety of other reasons), so I don’t have a particular dog in this race… but I will say I find more explicit types for signals a great deal nicer to work with. Whether the explicit mechanism is a function call (like Solid) or an accessor (like many others).

What is weird is that it’s actually typed as a svelte-compiler-only ‘extended int’ that can have side effects when you increment it, and weird knock on results from reading it.

But I fell in love with Svelte because it was dead simple (according to me). It was a breeze of fresh air and I felt I just program again without wiring enchantments together.

I do agree that its simplicity has downsides too, so perhaps it's just nothing to be afraid of?

But I can't help seeing this as ominous:

> This is just the beginning though. We have a long list of ideas for subsequent releases that will make Svelte simpler and more capable.

Please don't turn Svelte into a black magic box.

I've got a couple of thousand hours of Svelte dev experience, and these changes offer me nothing I really want.

Svelte dependencies outside my .svelte files? No thanks, I'll keep them usable in non-Svelte projects.

A new way to do reactivity to try to appeal to React users? ("When people come to our community in future..." in the vid.) No, thanks! Svelte 3/4 reactivity was very straightforward, one could learn the whole thing in a day from the excellent tutorials. It was better than React.

There was definitely a bit of weirdness with overuse of reactive vars but that's been incentive to keep components small and simple. A good thing!

Personally I'm still stuck on v3 because 4 introduced breaking changes I haven't had a chance to debug, so it'll be a while until any of this impacts me anyway.

Maybe I just need to try it out.

Maybe it's because of using the "runes" word :)

But watch the introduction video. It isn't at all like the functional reactivity that entered other frameworks. It's still the svelte way. Read and assign is plain javascript. Only the initialization syntax changed.

They even promise backwards compatibility, so you don't have to migrate or migrate file by file if you want to. Which is also not how other frameworks handled this in the past.

Given this, I'm curious: couldn't the traditional syntax of `let counter = 0` be made to function similarly to the new let count = $state(0);? Transpile it to let count = $state(0) under the hood? If that can work technically, instead of introducing a new rune for reactivity, why not introduce a "negative" rune to denote non-reactive statements? This way, the change wouldn't break existing code; it would be more of a progressive enhancement.

I agree the move to unify the Svelte <script> tag with regular js/ts files is an improvement. It was indeed a little odd that certain syntactic sugar, like the $, would work exclusively within the Svelte <script> and not in a js/ts file that's right next to it. However, from what I gather, it seems the Svelte team is aligning the Svelte script more with js/ts, rather than bringing the js/ts closer to Svelte's unique syntax. This trajectory seems to be pushing Svelte towards resembling traditional JavaScript frameworks, like React. It's a departure from Svelte's unique strength of having a custom compiler and behaving more like a language. If every syntax in Svelte is expected to mirror its behavior in js/ts, eventually svelte will lose all it secret sauce that made it so unique. Why can't we add a rune into js/ts file, a note in the beginning to tell svelte compiler that this is svelte enhanced js/ts file, compile it like svelte script tag code? Bring js/ts more alike to svelte?

But one of our goals was for you to be able to use Svelte's reactivity inside .js/.ts files, since that's one of the things people have been crying out for since we released Svelte 3. For that to work, reactivity has to be opt-in, not opt-out.

And that's how it _should_ be — someone reading the code should be clued into the fact that this `let` won't behave like a normal `let` in JavaScript, and it should be possible to move code between modules (and between modules and components) without worrying about whether a specific file was opted in to certain behaviour.

In other words this...

> This way, the change wouldn't break existing code

...isn't quite right — it would break _all_ your existing code that wasn't in .svelte files.

> If I'm not mistaken, the compiler allows Svelte to define its syntax to anything they want.

On this point specifically: unfortunately not. The code in a .ts file, for example, has to be valid and typecheckable. You can't insert a preprocessing step ahead of the TypeScript compiler. Again though we concluded that this is a good thing, since it means this all works with all existing ecosystem tooling.

I can't find an issue for that among top 50 open issues on Svelte's GitHub repo. I've also been a member of Svelte's Discord server for years and do occasionally hang out there, I haven't seen people "crying out" for this at all.

On the other hand, there are things like `<style module>` or `forward:class`, etc. that people have ACTUALLY been crying out for (literally one of the most upvoted issues in the repo) which haven't been addressed at all.

What if it is opt-out reactivity in .svelte files and opt-in reactivity in .ts/.js files? Yeah I know it would be a bit more combersome to copy code from .svelte to .js/.ts files but I think it would be worth it

I'm just brainstorming here, but what about an optional $reactive rune where everything in that is reactive by default?

$reactive(() => { let name = 'world'; });

vs

let name = $state('world');

Tbh this seems kinda rude. Maybe check github i am sure the whole journey can be seen there.

$:{ let name = 'world'; };

vs

let name = $state('world');

$: is valid javascript right? So that should also work in .js/.ts files I assume

The $: label syntax is valid JavaScript, but allowing scoped variables to be accessible outside of scope is very invalid JavaScript.

Given the constraints with TypeScript not being further compilable, I've been pondering on Svelte's direction. Personally, I'd lean towards letting go of some of the special touches in js/ts file if it means keeping more magic in Svelte. If we're heading towards making Svelte syntax work exactly the same in js/ts entirely, it feels like we might risk turning Svelte from its unique language-like identity into just another framework in the JS world.

Thanks again for the insights! I have already felt a little better about my current work in migrating an app from another framework to svelte 4. I was worried that I have made a very wasteful decision for my own company.

exactly my thoughts, feels like they could've kept it much simpler

Mobx was ahead of the game here (though, granted, it too draws on Knockout.js). You can use Mobx to declaratively describe reactive state, much like this. But it isn't integrated into any framework - you can use it in vanilla JavaScript with no other runtime or compilation required. You define models with Mobx, then on top of that there's mobx-react, which ties your model into React's update APIs (essentially making render() automatically observe values, and making relevant changes trigger a re-render), or there's mobx-vue, or mobx-svelte, or mobx-preact, etc etc. Decoupling this is super helpful - for example I'm using mobx to model raw state, computed state & reactions to changes in server side Node.js, no problem.

Meanwhile, recreating the same reactive concepts independently in each library instead makes all the resulting code incompatible, so even your pure JS state models are coupled to your UI framework - e.g. createCounter in this post has to import & use $state from Svelte. That makes it far harder to change frameworks in future, hard to share code between apps using different frameworks (plausibly even different versions of the same framework), etc etc.

I'd love to see a native common standard for this, similar to how promises were eventually standarized and suddenly everything had a compatible model for handling async future results (I could swear I've seen initial discussion on exactly that already, but I can't find it anywhere now sadly).

But not only it's an added layer of abstraction, it's also a complex piece of machinery which is not needed when the framework solves reactivity.

When I started using Svelte back in 2019 I was very happy to let go of MobX.

Mobx tc39 decorator support is pretty much done too. I check in on the PR weekly haha.

One huge sadness I have is that when you have a callback in .then or .next, your handler doesn't get a gratis reference to what you were listening to. I'd love to have this be a better record. DOM is so good about having thick events that say what happened, and it's so helpful, sets such a rich base, but js snubbed that path & left passing data in 100% to each event producer to decide as it will. Consumers can craft closures to seal some data into their handlers but it sucks compared to having an extensible data flow system. We're kind of sort of finally fixing this crime the hard way by standardizing async context/async_hooks, which is a whole new ambient way to create context that we can enrich our async events with, since there was no first class object to extend for having omitted passing in the promise to the promise handler. https://github.com/tc39/proposal-async-context

Also worth pointing out, maybe arguable as a hazard tale, observables was proposed a long long time ago. There's still a ton of adoption. But it also feels extremely similar yet notably apart & worse ergonomics than async iteration. It's imo a good thing it didn't get formalized. https://github.com/tc39/proposal-observable

Alas one reactivity I really wish we had had is object.observe (note: different API than observables), or something like it. Undoing good specification work & saying, 'yeah, if users want it, they can implement it themselves with proxies' has lead to no one doing it. And you can't just observe anything; whomever is the producer has to up front make the decision ahead of time for all consumers, which turned a great capability into something first boutique then quickly forgotten. Alas! https://esdiscuss.org/topic/an-update-on-object-observe

I ack your ask, and there's no shortage of options that are pretty damned good. But I feel like we are still in a discovery not deployment phase, still delay competing because we have a lot of terrain and idea space to consider. Before we pick, standardize & ship one.

One thing that I am definitely happy to see is the removal of $: as it should help Typescript users. Personally, I was quite sick of writing:

  let input = 'Hello';
  // ...
  let loudInput: string;
  $: loudInput = `${input)!`;

  let input = 'Hello';
  // ...
  $: loudInput: string = `${input)!`;

  let input = $state('hello');
  // ...
  let loudInput: string = `${input)!`;

This is the primary place where we have to go outside the framework in React. Only a large linear list has this problem -- if it was a decently balanced component tree, then we could skip updating huge swathes of it by skipping at a top level node. But it is not possible in an array. You have it iterate through each element and do a shallow comparison to see if the references have changed.

That said, it is fairly easy to escape to DOM from inside a React component with useRef and the portal pattern. Then we can write vanilla JS which makes updates to only the values it changes.

If Svelte solves this in an elegant manner, then it would be a very compelling feature over React. I'm asking here because last I checked, most of the examples in Svelte's documentation pointed to simple counters and regular objects, and I couldn't find an example of a large linear list.

But it doesn't help in the rendering phase - aka when the virtual DOM is constructed when `render` is called on the root component, and all our JSX code is executed across the component hierarchy. Virtual DOM reconciliation cost is only a part of the performance penalty, re-running the "view as a function of state" computation is another major chunk.

Svelte:

    <button on:click={nextLight}>Next light</button>

    <button @click="nextLight">
      Next light
    </button>

Actually, it was Svelte who coined the term and sold us on the idea of reactivity by default. I don’t think anybody asked for “Reactivity by default”. Svelte advanced this idea, and it helped the framework gain traction. It was easy to get started, and gave Svelte this sense of better ergonomics than other frameworks. I was always skeptical about the performance claims, amortized, and the real selling point of Svelte was ergonomics and the dev experience.

The problem with the Node.js ecosystem is the devs are borderline marketing and sales type. They’ll justify, rationalize and make things sound good after the fact. Previously, Svelte was persuading us that Svelte was better than the rest because of reactivity by default. Now they did a literal 180. It’s probably in the right direction, and maybe how things should have been. A related symptom of the Node.js ecosystem is reinventing and rediscovering the wheel. The problem here is a lost of trust. Anything else which Svelte purports it’s got figured out or is more enlightened about should be taken with a grain of salt.

So it seems those boring FANG engineers with React has it right all along. They had experiencing building sufficiently complex apps where verbose but explicit code was necessary.

> Because the compiler can 'see' where count is referenced, the generated code is highly efficient

Yeah, I don’t believe such claims anymore. Sure, in cherry picked and constrained settings, the performance benchmarks might seem good. As much as I hate to admit, I will reach for the production ready and battle tested React, as boring as it is.

In Svelte 3, you had to opt in to running code on updates, using things like the `$:` label. It was designed to be conservative about updating components, unlike virtual DOM solutions that like to update everything unless you opt _out_.

I too am very sceptical of benchmarks — they can certainly obscure more than they reveal at times. But you don't have to take my word for it, or the benchmark results showing that Svelte 5 is faster than every other framework in existence (https://twitter.com/Rich_Harris/status/1688581184018583558) — you just need to understand the different mechanisms in play.

I've spent a lot of time building an intuition for how Javascript code executes and how I can combine its primitives to make reasonable abstractions. The fact that this _looks_ like Javascript but suddenly operates differently is almost more confusing to me than if Svelte was just a non-Javascript language.

React's `useState`, `useMemo`, etc. are perhaps more verbose but they're just functions. Dependency arrays are messy but it's fairly easy to extrapolate their behavior from a basic description.

For me personally I tried svelte in the past and bounced off because there was too much implicitly happening that I needed to have a deep understanding of to model correctly. This solves basically all those problems for me.

I thought your video[1] especially did a great job walking through the pros this change brings. Thanks for all your great work on this!

1. Link for the curious: https://www.youtube.com/watch?v=RVnxF3j3N8U&t=6s

1. primitives for managing state - $state vs useState

2. removal of lifecycle mechanisms - onMount vs componentDidMount

3. replacing lifecycle mechanisms with new primitives - $effect vs useEffect

It's like the Svelte team took a leaf out of React team's book on how to upgrade a framework - this is evident by the way these features are presented as opt-in like how React marketed Hooks as opt-in. I would go on to predict that the upgrade to Runes will just like the upgrade to Hooks. Developers will use it and then love it - because it presents improvements to the way codebases will be structured and maintained just like React did with Hooks. This is really a Hooks moment for Svelte. Good job Rich and the Svelte team!

<script> let time = $state(new Date().toLocaleTimeString());

  $effect(() => {
    const timer = setInterval(() => {
      time = new Date().toLocaleTimeString();
    }, 1000);

    return () => clearInterval(timer);
  });

<p>Current time: {time}</p>

Example: https://www.solidjs.com/examples/counter

`onMount` and `onDestroy` feel like really useful, dependable callbacks. `$effect` is scary because if you add a reference to state in it, you can't depend on it being called like `onMount`.

They claim most (but not all) of current usage of `onMount` will be better off using `$effect` instead

I'm guessing runes will make it easier for the compiler to track the reactivity which will enable stuff like automatic partial hydration? Maybe even something like qwik?

I only read the blog post and didn't watch the video... was there any mention on perf and size improvements?

And the output from the compiler is much smaller. You can compare the output on the Svelte 4 REPL to the output on the Svelte 4 REPL. The runtime has grown a bit, but the output overall is still much smaller even when factoring that in

Shared-everything does not scale. It ends in whack-a-mole and an escalating blame game about whether team members are fit to work on the project. It’s bunk. It’s deflecting from the real problem which was dereliction of duty by the “architects” who took a laissez-faire stance on state management instead of providing structure. Worked on a couple of those. Never again.

Practically the whole point of encapsulation is gated access to data, putting constraints on sharing, compressing the possible state space of the system closer to the desired state space.

Svelte for Apps. Svelte for Sites.

I found `$:` to be extremely confusing, full of weird quirks, and completely turned me off to using svelte for anything more than basic sites.

Runes seem like a clear improvement, but brings Svelte a step closer to React -- which hurts its appeal to me.

The difference between `let counter = $state(0)` and `const [counter, setCounter] = useState(0)` is near its initial value -- zero.

I do love the view library competitive landscape and enjoy seeing different paradigms leveraging reactivity. I also like the compiler optimizations that Svelte is bringing to the entire ecosystem.

I wrote my initial thoughts on sveltekit when I built neovimcraft: https://bower.sh/my-experience-with-svelte-kit

> The magic of Svelte isn't `let count = 0`, it's `count += 1`

That part hasn't changed!

That said, svelte5 does solve a lot of problems that stop me from trying it.

Does not Svelte from 2018 works today?

They got the DX right from the start, that other libraries are still trying to emulate.

The DX isn't "right". It's just the one they are sticking to. There are many issues with the actual developer experience when it comes to hooks in React.

So it's a good thing that other frameworks are iterating and finding new and possibly improved ways of doing things. E.g. neither Svelte nor Solid require dependency arrays to effects or other reactive primitives. Their effects work everywhere, and not just inside component code. You're not beholden to the order and number of effects, unlike with hooks. Etc.

I feel they lost that as people gradually rotated out of the team, around the time they announced concurrent-mode, now suspense and the half baked RSC "release".

DX Amaze.

Let's compare Svelte's and Solid's approach to nested reactivity. Both of them implement the same nested reactivity todo example:

Svelte: https://svelte-5-preview.vercel.app/docs/fine-grained-reacti...

Solid: https://www.solidjs.com/tutorial/stores_nested_reactivity?so...

In Solid, converting something to use nested reactivity is one step. In Svelte, it is two steps. And that second step is really verbose and annoying:

  todos = [...todos, {
    get done() { return done },
    set done(value) { done = value },
    get text() { return text },
    set text(value) { text = value }
   }];

It is nice that runes allows nested reactivity in Svelte, but it feels nicer to use in Solid.

function createSignal(initial) { let value = $state(initial); return [() => value, (v) => value = $state(v)]; }

Note that the Solid change is _not_ 'one step' — the `completed` property is being turned from a property to a function, which means you must update all the usage sites as well. Using getters and setters also allows you to use Svelte's convenient `bind:value` approach, which is much less verbose than using the equivalent event handler code. And don't get me started on the [type narrowing issues](https://www.typescriptlang.org/play?#code/C4TwDgpgBA4hzAgJwD...).

There's nothing _wrong_ with the Solid approach, but the ergonomics aren't to our liking. If we need to take a hit in terms of verbosity, we'd rather do it once at the declaration site than n times at every usage site.

1. It's really nice that it's so easy to make a `createSignal`

2. I didn't realize that in Solid you had to update all the usage sites too, so now I'd rather stick to the Svelte usage. Nested reactivity is not as effortless as I thought in Solid.

  get done() { return done },
  set done(value) { done = value },
  get text() { return text },
  set text(value) { text = value }

This might be common enough that some syntax sugar might be worth it?

The short hand for:

  todos = [...todos, {
    get done() { return done },
    set done(value) { done = value },
    get text() { return text },
    set text(value) { text = value }
  }];

  todos = [...todos, {
    $done,
    $set
  }];

They do an excellent job of some pretty substantial tech. But also the way he/they explain it is so powerful.

That includes the blog posts, the videos, the framework itself and the playground.

This does seem like a unifying step. Seems like (at least) Svelte and Angular have both declared this model superior to their existing implementations. The video gave credit to prior art in general but would have been better to give the specific projects credit - not just ethically but also to be explicit about what is and is not similar.

Nope, nope. Been there, done that, with 2-way data binding and never going back.

We used to know that, too. We also used to know that entirely event-driven architectures were a level of chaos you shouldn’t invite into your enterprise. Don’t need to look hard to see those either.

Whole industry has been cycles of collective amnesia going back to at least 1975.

[Edit] but in this case what we seem to be doing is having another go at Aspect Oriented Programming, which I was interested in for some time but concluded it, like event-only systems, are a poor fit for the average developer’s mental model, and also a terrible way to encode business requirements. Particularly from a testing perspective. They are usually dabbling at functional core, imperative wrapper architectures, but tend to retain global shared state which is a reliability nightmare waiting to happen.

> Like every other framework, we've come to the realisation that Knockout was right all along.

> Svelte 5's reactivity is powered by signals, which are essentially what Knockout was doing in 2010. More recently, signals have been popularised by Solid and adopted by a multitude of other frameworks.

> We're doing things a bit differently though. In Svelte 5, signals are an under-the-hood implementation detail rather than something you interact with directly.

That's a sad development and further makes the framework more complect (as defined by Rich Hickey), with more hidden layers and "magic".

But if these guys are only now reaching Knockout, I still have to wait for them to catch up to Backbone.js, I guess.

Hell, even querying the DOM is still easier with jQuery.