[wirahx]

> I don’t follow… in what sense is this JavaScript? > <button on:click={incrementCount}>

it's not. it's html. incrementCount is a pointer to a javascript function.

> Or what about this… is this JavaScript? > $: doubled = count * 2;

Yes. $: is a javascript label. We use it to indicate that a function should be reactive - if the count variable changes, the right hand will be rerun and double updated.

> <button on:click|once={handleClick}> Click me </button>

html again. the |once is a modifier to click which detaches the handler after a single use. It's a directive in the html markup (still valid html) which the compiler picks up and converts into node.removeEventHandler.

[MrJohz]

While this is all technically true, I don't think it's all that helpful a thought process to have when approaching Svelte. Yes, Svelte files are syntactically valid HTML, Javascript, etc, but they have very different semantics.

Take the ability to reference functions in HTML - this is simply not possible in normal HTML, where inline event handlers instead are passed Javascript expressions to evaluate. It's definitely a useful feature, but it's very clearly outside of the normal realm of HTML.

This goes further with Javascript: here the semantics differ considerably, with labels in normal Javascript doing nothing except when dealing with loops, and labels in Svelte are a form of reactivity. You've also got things like dollar signs interacting with stores, and other semantic differences.

As Rich Harris says, Svelte is a language.

[ttfkam]

<div onclick="handleClick">Click me</div>

100% valid HTML and within "the normal realm of HTML". The event is passed as a parameter to the function. Has been available since the introduction of JavaScript. Predates addEventListener(…) and its ilk by a few years.

Yes, "$:" as a labeled break exists outside JavaScript proper. That said, given the rare cases where anyone uses a labeled break and how unlikely that labeled break would be named "$", it seems a pretty safe extension to the language to allow such powerful behavior.

Both "$:" and "$store" are not magical; they are quite deterministic and serve to substantially reduce the total amount of boilerplate. Seems a fair tradeoff, but you are correct, it's a superset of the JavaScript language.

Now let's discuss the "naturalness" of useState, useMemo, and their ilk. I'll take Svelte's minimal "magic" over the verbose, repetitive, and error-prone library API logic, thank you very much.

[MrJohz]

> 100% valid HTML and within "the normal realm of HTML". The event is passed as a parameter to the function. Has been available since the introduction of JavaScript. Predates addEventListener(…) and its ilk by a few years.

Like I said, it's valid HTML, but it doesn't do what you're describing (and you can try it out fairly easily in your browser). The "onclick" handler executes the code string that it's given, working essentially the same way as `eval` or the `new Function("...")` syntax. It does not take a function pointer.

So in this case, the only thing it executes is a variable pointing to a function, which doesn't do anything because the function isn't being executed. You would need to add the function brackets to get any sort of effect.

This is the crux of the issue: Svelte is an additional language that happens to use the same syntax as HTML and JS, just with different (in some cases very different) meanings. You are not just writing "explicit Javascript code", you're writing Svelte code (which in most cases is probably very easy to pick up if you know Javascript, but it's still a different language).

Compare and contrast with, say, Typescript, which is syntactically different from Javascript, but semantically identical - just remove the types, and you have normal Javascript. (In fairness, with old decorators and enums, this isn't quite true, but it holds in general.)

To be clear, I don't think that's saying that one thing is better than the other, or that Svelte is bad because it's a new language or something. I think reactive primitives in Javascript are important, and adding them directly into the syntax of the language is a really clever way of solving a lot of problems in this space. I'm not sure if it's the solution I like the best (I've really enjoyed working with SolidJS signals recently, which provide some of the same ideas with a more typically Javascript-like API), but it's good to see experimentation here.

[lucideer]

> which the compiler picks up

I think this is the crux of the gp's question. The browser doesn't support explicit behaviours for "on:click" or "|once" (the "$" label is a cool trick I'd forgive the gp for not recognising as native JS) - it may be "valid" HTML but it's not "just HTML" (nor just JS), it's a DSL.

[ttfkam]

It does support onclick though. This isn't a far off derivation. Also "|once" is a whole lot cleaner and more terse than the "once" variant of addEventListener, to be sure.

Anyone who already knows HTML and native event handling could pick this up in less than 5 minutes. Good enough.