The truth about Svelte

what-is-svelte.md

I've been deceiving you all. I had you believe that Svelte was a UI framework — unlike React and Vue etc, because it shifts work out of the client and into the compiler, but a framework nonetheless.

But that's not exactly accurate. In my defense, I didn't realise it myself until very recently. But with Svelte 3 around the corner, it's time to come clean about what Svelte really is.

Svelte is a language.

Specifically, Svelte is an attempt to answer a question that many people have asked, and a few have answered: what would it look like if we had a language for describing reactive user interfaces?

A few projects that have answered this question:

• Elm
• Imba
• Idyll
• Marko
• ...probably many others

(Idyll is an outlier as it's geared towards a specific use case, rather than general purpose app development, but I think it qualifies as an example.)

These projects are all very cool, but there's a reason they haven't hit mass adoption: they want to control the entire world. You can't adopt Elm or Imba incrementally, and they need dedicated tooling far beyond just the compiler itself (e.g. syntax highlighting, unless you like your code monochrome). In some cases (Elm stands out), interop with the JS ecosystem is less than seamless.

Beyond that, they have a steep learning curve, which is hard to justify when there are so many options that are more accessible.

Thinking inside the box

What if we had a language that was designed for building reactive user interfaces, but that also worked with your existing tools? What if you didn't need you to discard your years of experience using HTML, CSS and JavaScript, because it extended those languages?

• It would extend HTML by adding JavaScript expressions in markup, directives for controlling behaviour and reacting to input, syntax for using conditions, loops and asynchronous values
• It would extend CSS by adding a scoping mechanism that kept styles from clobbering each other
• It would extend JavaScript by making reactivity a language primitive

How do we make reactivity a language primitive without introducing invalid syntax, or breaking the relationship with existing tooling (like TypeScript)? By hacking existing syntax:

• We instrument assignments to variables and properties
• We add $: statements, using the little-known label construct, that run whenever their inputs change value
• We tie it all together with an opinionated scheduler

This, to me, is the best of all possible worlds: we can lean on decades of accumulated wisdom by extending well-known languages, author components in a delightfully concise and expressive way, and yet still generate apps that are bleeding-edge in terms of performance and everything that goes with it.

Sometimes I wonder about why we can't have detached Fragment in the code like in react, but in a way what svelte offers covers well most of the use cases, I would like to find some use case where it's a critical limitation. I guess that this limitation opens the door for an optimal component pattern. Some code-base in react use all the possible ways possible to render fragments at the same time and it becomes uncomfortable. With svelte I got a feeling of predictability. Svelte components follow more or less the MVC pattern with the script only containing the state (props,lets) controller (handlers/reactive statements) stores, the document/style is the view. You don't declare blocks / functions / state member there, you can only consume,listen or bind. This means that you can't obfuscate the intents and the how, it constrains the developer/teams into a clear path. Le lun. 13 mars 2023 à 23:14, Eckehard ***@***.***> a écrit :

…

***@***.**** commented on this gist. ------------------------------ ***@***.*** <https://github.com/Rich-Harris>: "Svelte is a language"*? Maybe, that is the weakest point of Svelte. It isn´t really a "programming" language! Or maybe, a fairly limited one. There are - for good reason - about 10 different ways to use loops in Javascript. Svelte knows ? 1 ? Don´t get me wrong: Svelte is absolute fantastic and possibly the best we could get for the web community today, but I absolutely miss the option to do all the things we can do with a "real" programming language. What, if we coud turn things around and not embed scripting elements into HTML, but vice versa: In Svelte, you write: <script> const percentage = [0, 25, 50, 75, 100]</script> {#each percentage as p,i}<button on:click="{() => progress.set(p/100.0)}"> {p}%</button> which is nice and easy to understand. But "#each" is not as powerful as loops in JS ar. If we could use JS directly, this could be: <script> for (p of [0, 25, 50, 75, 100]){ <button on:click="{() => progress.set(p/100.0)}"> {p}% </button> }</script> I do not pray for the JSX-syntax, this is a false friend as well. But using a full featured programming language would bring much more power to the task. As my grandma said: Never do a thing for a computer, that a computer can do for you. — Reply to this email directly, view it on GitHub <https://gist.github.com/0f910048478c2a6505d1c32185b61934#gistcomment-4501441> or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACLIE5DW6EYZA2MSZAIHEH3W36L33BFKMF2HI4TJMJ2XIZLTSKBKK5TBNR2WLJDHNFZXJJDOMFWWLK3UNBZGKYLEL52HS4DFQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZVRZXKYTKMVRXIX3UPFYGLK2HNFZXIQ3PNVWWK3TUUZ2G64DJMNZZDAVEOR4XAZNEM5UXG5FFOZQWY5LFVA4TGMRUGMYDCMNHORZGSZ3HMVZKMY3SMVQXIZI> . You are receiving this email because you commented on the thread. Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub> .

<script>
      for (p of [0, 25, 50, 75, 100]){
         <button on:click="{() => progress.set(p/100.0)}">
	      {p}%
         </button>
      }
</script>

You can pretty much already do this:

<script>
	let progress = 0;
</script>

<progress value={progress} />

{#each [0, 25, 50, 75, 100] as p}
	<button on:click="{() => progress = p/100.0}">
		{p}%
	</button>
{/each}

https://svelte.dev/repl/f3a28a0ab1134c6fa877f89faaa0c9f5?version=4.0.5

Cool