fix: check boundary exists before calling error handler in async derived#18384
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Rich-Harris merged 7 commits intoJun 5, 2026
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When an async derived value rejects, the error is handled by the boundary. However, if the boundary is destroyed before the rejection settles (e.g. due to component unmount), effect.b becomes null and calling effect.b.error() throws a TypeError. This fix adds a null check for effect.b in invoke_error_boundary, allowing the error to bubble up to the parent boundary instead of crashing. Fixes sveltejs#18383
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This PR was opened by the [Changesets release](https://github.com/changesets/action) GitHub action. When you're ready to do a release, you can merge this and the packages will be published to npm automatically. If you're not ready to do a release yet, that's fine, whenever you add more changesets to main, this PR will be updated. # Releases ## svelte@5.56.3 ### Patch Changes - fix: ignore errors that occur in destroyed effects ([#18384](#18384)) - fix: type BigInts in `$state.snapshot(...)` return values ([#18388](#18388)) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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fixes #18383
I traced the issue to
packages/svelte/src/internal/client/error-handling.js, specifically theinvoke_error_boundaryfunction. The problem is at line 56 whereeffect.b.error(error)is called without checking ifeffect.bexists.What happens is: when an async
$derivedrejects inside a<svelte:boundary>, the error is handled correctly the first time. But if the boundary is destroyed before the rejection fully settles (e.g. component unmounts), a subsequent settle tries to calleffect.b.error()on a destroyed boundary whereeffect.bisnull, causing aTypeError.The fix adds a null check for
effect.bbefore callingerror(). If the boundary has been destroyed, we skip it and continue bubbling up to the parent boundary. This way the error still gets handled properly instead of crashing.This could also be tested by creating a component with an async
$derivedthat rejects after the component unmounts, but I wanted to get the fix up first for review.