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`clone()` reached the `value instanceof Date` branch before the `toJSON` branch, and `structuredClone` reads a Date's internal slot rather than going through the prototype. A `SvelteDate` therefore registered no dependency when it was read through `$state.snapshot`, so a reaction never re-ran when the date changed. `SvelteMap` and `SvelteSet` were unaffected, because their branches copy by iteration and so go through the reactive machinery. Cloning as `new Date(value.getTime())` reads through the prototype, which is what makes the reactive subclass register a dependency, and is equivalent to `structuredClone` for a Date, invalid dates included.
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Install the latest version of pnpm add https://pkg.svelte.dev/svelte/c/99a356e721f6fbf8e36c14bf3189b5aa261d7738Open in Note This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed. |
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Thank you for the issue and the fix, though I like the solution in #18700 a bit better, therefore closing in favor of that. |
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Fixes #18698
Problem
clone()reaches thevalue instanceof Datebranch before thetoJSONbranch, andstructuredClonereads a Date's internal slot rather than going through the prototype. ASvelteDatetherefore registers no dependency when read through$state.snapshot, so a reaction never re-runs when the date changes.SvelteMapandSvelteSetare unaffected, because their branches copy by iteration and so go through the reactive machinery — which is what makes this look like an oversight rather than a deliberate asymmetry.Fix
Clone as
new Date(value.getTime()). The read goes through the prototype, which is what makes the reactive subclass register a dependency, and the result is equivalent tostructuredClonefor a Date — invalid dates included, sincegetTime()returnsNaNandnew Date(NaN)is an Invalid Date.Moving the
Datebranch after thetoJSONbranch would also fix it, but would be breaking: a plainDatewould then snapshot to a string.Test plan
Added a case to
packages/svelte/src/reactivity/date.test.tsthat reads aSvelteDatethroughsnapshot()inside arender_effectand asserts the effect re-runs on mutation. It fails onmain— the log holds only the initial value — and passes with the fix.Suites run locally:
src/reactivity+tests/snapshot— 88 passedtests/runtime-runes— 2699 passed, 47 skipped