Svelte 5: make maps and sets reactive

[Rich-Harris]

Describe the problem

It would be nice if these worked...

<script>
  let numbers = $state(new Set([1, 2, 3]));
</script>

<button onclick={() => numbers.add(numbers.size + 1}>add</button>
<button onclick={() => numbers.clear()}>clear</button>

<p>{numbers.size}</p>
<p>{[...numbers].join(', ')}</p>
{#each numbers as n}<p>{n}</p></each}

...and the same for Map.

Describe the proposed solution

Intercept method gets on proxied maps/sets

Alternatives considered

Subclasses. Yuck

Importance

nice to have

[enyo]

I think it's meant to say

let numbers = $state(new Set([1, 2, 3]));

(just adding this note so nobody is confused when finding this issue :) )

[Rich-Harris]

oops, good catch 😆 fixed

[trueadm]

We're going to be doing #10263.