Technically, svelte (and, for that matter, Vue 3) are supersets of HTML, not JS. This means either one of three things:

• Ruby2JS when run with a svelte filter would output svelte enhanced HTML, not JS; and that intermediate output would then be need to be passed through a svelte compiler in order to produce the JS that ultimately will be served to the browser.
• Ruby2JS will produce svelte enhanced HTML internally, and then transparently call out to the svelte compiler and return the generated JS as the result
• Ruby2JS's svelte filter compiles direct to JS, essentially producing the equivalent of what the svelte compiler would have produced, but doing so directly.

I would suggest focusing on the first option initially, and then perhaps proceeding to either the second or the third option.

I don't have enough (or for that matter any) svelte experience to design a reasonable mapping of svelte to Ruby, but if somebody could develop a set of specs (and by all that I mean a list of "this input should produce that output"), I can help with the implementation.

i'm not a svelte expert (yet) but as far as i understand it a transformation of

<script lang="ruby">  ...   svelte_dollar_block do      ...   end<script>

to

<script>   ...  $: {  }<script>

would be enough

transformations of conditional/loop svelte blocks (outside of <script> ...</script> could be omitted in the first version (or even at all)

{#if }{/if}

but i'm really not sure if it would be all ...

I'm not sure where/how this would fit into the existing project, but to keep the discussion going, here is a script that will convert source that contains<script lang="ruby"> blocks into a source where the ruby scripts are replaced with #"auto" data-snippet-clipboard-copy-content="require 'nokogiri'require 'ruby2js'if ARGV.length == 0 source = STDIN.readelse source = ARGV.map {|file| IO.read file}.joinendfragment = Nokogiri::HTML5.fragment(source)fragment.search('script[lang=ruby]').each do |script| script.remove_attribute 'lang' script.content = Ruby2JS.convert(script.content).to_sendputs fragment.to_html">

require'nokogiri'require'ruby2js'ifARGV.length ==0source=STDIN.readelsesource=ARGV.map{|file|IO.readfile}.joinendfragment=Nokogiri::HTML5.fragment(source)fragment.search('script[lang=ruby]').eachdo |script|script.remove_attribute'lang'script.content=Ruby2JS.convert(script.content).to_sendputsfragment.to_html

Why not just convert ruby symbol (:id) into svelte's $:id? There's anyway no direct equivalent of ruby's symbol in js.