fix: emit `each_key_duplicate` error in production by 7nik · Pull Request #16724 · sveltejs/svelte

7nik · 2025-09-06T16:22:46Z

It looks like duplicate keys cause crashing only in one place.
In some cases it still can successfully render the list but will crash soon anyway, so I don't see reasons to add other checks.

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svelte-docs-bot · 2025-09-06T16:23:17Z

Preview: https://svelte-dev-git-preview-svelte-16724-svelte.vercel.app/

github-actions · 2025-09-06T16:25:36Z

Playground

pnpm add https://pkg.pr.new/svelte@16724

packages/svelte/src/internal/client/dom/blocks/each.js

Rich-Harris · 2025-09-10T23:04:19Z

I feel a bit weird about adding a less-cryptic error for when matched is empty, but still not erroring at all if there are duplicate keys that don't result in an error. Is there some other proxy, besides matched being empty, for those other cases?

I also realised as a result of this PR that validate_each_keys doesn't always work, because the render_effect could run after the block update — for the test case in this PR, it fails with a is undefined. We could change that by turning the render_effect in validate_each_keys to a block, but maybe it would be better to move the check into reconcile inside an if (DEV) block?

7nik · 2025-09-11T10:45:58Z

Checking other cases requires computations. We cannot even do array.length === state.items.size - to_destroy.length (key duplication often results into a shortened list) because state.items may still contain transiting items from previous runs. Probably the simplest computation is a set of seen keys in the reconcile - validate keys during reconciling. ~~Another place is inside create_item.~~ If we want to add any computation to prod.

Co-authored-by: Rich Harris <rich.harris@vercel.com>

dummdidumm · 2025-09-12T21:47:28Z

I vote for merging this. I just ran into this and it's really annoying to debug. Yes it's not going to catch all cases but I'd rather fail with a runtime error that gives me a hint at what goes wrong compared to a cryptic "undefined" runtime error.

7nik · 2025-09-12T21:57:44Z

I tried to move validate_each_keys inside reconcile but some tests failed, and I didn't get time to dig into why.

7nik · 2025-09-13T12:08:44Z

I moved validate_each_keys inside #each's block because, if thrown in reconcile, the error isn't caught by the boundary in async mode. Guess it is because reconcile is called in batch callbacks.

7nik · 2025-09-13T14:40:57Z

Btw, in prod the error isn't caught by boundary. I need help here.

7nik · 2025-09-14T21:14:54Z

Added the error emitting for the non-crashing case.
~~It doesn't emit the error at hydration in prod, but I guess it's fine.~~

7nik · 2025-09-24T11:29:32Z

Added ensuring the thrown error is caught by a boundary - is this the right approach?
I removed the tests because DEV is always true and thus we cannot really test the prod checks.
No idea why the TSGo test decided to die on an unrelated file.

Rich-Harris · 2026-01-29T03:50:35Z

Interestingly, #16840 works for DEV in user code, but not in Svelte code, which means we still can't add a test for this. Any ideas? If we can't get it to behave then we might be better off reverting #16840

7nik · 2026-01-29T16:35:32Z

Opened #17590

* chore: bump playwright * maybe this will help somehow? * err whatever * fix

* Revert "chore: allow testing in production env (#16840)" This reverts commit ffd65e9. * new approach

* fix: handle renderer run rejections * add test * changeset * simplify * explanatory comment --------- Co-authored-by: Antonio Bennett <abennett@mabelslabels.com> Co-authored-by: Rich Harris <rich.harris@vercel.com>

* fix: only create async functions in SSR output when necessary * actually... * simplify generated code a bit more * simplify

…tent (#17587) * fix: merge consecutive text nodes during hydration for large text content Fixes #17582 Browsers automatically split text nodes exceeding 65536 characters into multiple consecutive text nodes during HTML parsing. This causes hydration mismatches when Svelte expects a single text node. The fix merges consecutive text nodes during hydration by: - Detecting when the current node is a text node - Finding all consecutive text node siblings - Merging their content into the first text node - Removing the extra text nodes This restores correct hydration behavior for large text content. * add test, fix * fix * fix * changeset --------- Co-authored-by: Miner <miner@example.com> Co-authored-by: Rich Harris <rich.harris@vercel.com>

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

This reverts commit 65f77ef.

Rich-Harris

eventually figured out all we needed was a length > keys.size check in the block effect

7nik · 2026-02-04T19:49:57Z

Oh, I missed that keys was added by the refactoring. Good catch!

fix: emit each_key_duplicate error in production

ec016eb

bytecodemanipulator reviewed Sep 9, 2025

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fix: preserve key

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Rich-Harris reviewed Sep 10, 2025

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7nik and others added 2 commits September 12, 2025 13:08

Update packages/svelte/src/internal/client/dom/blocks/each.js

93354e0

Co-authored-by: Rich Harris <rich.harris@vercel.com>

Update packages/svelte/src/internal/client/dom/blocks/each.js

492b301

Co-authored-by: Rich Harris <rich.harris@vercel.com>

fix: ensure keys are validated

e541296

7nik added 2 commits September 15, 2025 00:11

fix silly test name

c208ef0

fix: cover other case of duplicate keys

cb89bb8

7nik added 4 commits September 15, 2025 11:04

emit error on hydration

89a3f0f

ensure the error is handled

3100402

drop useless tests

65f77ef

unused

ba50206

7nik mentioned this pull request Sep 25, 2025

chore: allow testing in production env #16840

Merged

6 tasks

Rich-Harris added 2 commits January 28, 2026 22:07

attempt merge

a11820f

finish merge

be59fed

7nik and others added 11 commits January 29, 2026 19:04

add lost check back

ae97347

chore: bump playwright (#17565)

dd67655

* chore: bump playwright * maybe this will help somehow? * err whatever * fix

chore: allow testing in production env 2 (#17590)

2339a73

* Revert "chore: allow testing in production env (#16840)" This reverts commit ffd65e9. * new approach

fix: handle renderer.run rejections (#17591)

16ea0ce

* fix: handle renderer run rejections * add test * changeset * simplify * explanatory comment --------- Co-authored-by: Antonio Bennett <abennett@mabelslabels.com> Co-authored-by: Rich Harris <rich.harris@vercel.com>

fix: only create async functions in SSR output when necessary (#17593)

b2191c3

* fix: only create async functions in SSR output when necessary * actually... * simplify generated code a bit more * simplify

Version Packages (#17585)

f2f297a

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Revert "drop useless tests"

3e2d52b

This reverts commit 65f77ef.

update tests

fdfec68

Merge branch 'main' into fix-15339

8895953

fix test

4cbaa68

Rich-Harris mentioned this pull request Jan 31, 2026

fix: ensure infinite effect loops are cleared after flushing #17601

Merged

6 tasks

Rich-Harris added 8 commits February 4, 2026 13:16

Merge branch 'main' into fix-15339

6ec3190

we don't need to expose this function any more

c83464b

figured it out... we cant have errors during reconcile

b765352

simplify

ff23884

tweak

97ab460

unused

77eb159

revert no-longer-needed change

8664326

unused

38a1857

Rich-Harris approved these changes Feb 4, 2026

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7nik merged commit d7a8e3d into main Feb 4, 2026
18 checks passed

7nik deleted the fix-15339 branch February 4, 2026 19:51

github-actions bot mentioned this pull request Feb 4, 2026

Version Packages #17615

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