SvelteMap breaks $derived rune

[tilyupo]

Describe the bug

SvelteMap prevents $derived from caching the result when an unrelated key is updated. It requires very specific order of access and initialization. See playground reproduction for exact code.

Reproduction

Playground link

<script lang="ts">
	import {SvelteMap} from 'svelte/reactivity';

	const source = new SvelteMap<string, string>();

	let lastSeenSource: string | undefined = 'none';

	let id = $derived.by(() => {
		const result = source.get('some-random-key');
		if (lastSeenSource === result) {
			console.log("source didn't change, but derived.by called");
		}
		lastSeenSource = result;
		return result;
	});

	$effect(() => {
		let cancelled = false;
		function spin() {
			if (cancelled) return;

			id; // read derived value
			requestAnimationFrame(spin);
		}

                 // $derived works without Promise.resolve(), so it's required for the bug reproduction
		Promise.resolve().then(() => {
			spin();
		});

                 // trigger SvelteMap change
		const timeout = setTimeout(() => {
			source.set('unreleated-key', 'updated');
		}, 10);

		return () => {
			cancelled = true;
			clearTimeout(timeout);
		};
	});
</script>

Result: source didn't change, but derived.by called is logged every time id variable is accessed.
Expected result: id is cached and evaluated only once during initialization.

Logs

System Info

System:
    OS: macOS 14.7.2
    CPU: (10) arm64 Apple M2 Pro
    Memory: 110.47 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.nvm/versions/node/v22.14.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v18.16.0/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.14.0/bin/npm
    pnpm: 10.6.5 - ~/.nvm/versions/node/v18.16.0/bin/pnpm
    bun: 1.1.43 - ~/.bun/bin/bun
  Browsers:
    Chrome: 134.0.6998.166
    Safari: 18.3.1

Severity

blocking an upgrade

[trueadm]

This is to be expected – deriveds can overfire. Deriveds are not guaranteed to be cached if they are unowned and return the same value each time (undefined). In this case, given the derived is never read inside an effect or part of the template where it can be owned – so Svelte has to assume that it might possible be dirty still because it's unowned.

This is the same thing as #15414 really.

[tilyupo]

Thank you for the response.