Sep 6, 2025 · Sep 9, 2025 · Sep 12, 2025 · Sep 12, 2025 · Sep 13, 2025 · Sep 14, 2025
diff --git a/.changeset/sharp-snakes-poke.md b/.changeset/sharp-snakes-poke.md
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/_config.js b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/_config.js
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/main.svelte b/packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/main.svelte
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		@@ -0,0 +1,5 @@
	---
	'svelte': patch
	---

	fix: emit `each_key_duplicate` error in production
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Expand Up		@@ -42,6 +42,7 @@ import { active_effect, get } from '../../runtime.js';
		import { DEV } from 'esm-env';
		import { derived_safe_equal } from '../../reactivity/deriveds.js';
		import { current_batch } from '../../reactivity/batch.js';
	import { each_key_duplicate } from '../../errors.js';

		/**
		* The row of a keyed each block that is currently updating. We track this
Expand Down Expand Up		@@ -473,6 +474,21 @@ function reconcile(
		var start = stashed[0];
		var j;

	// full key uniqueness check is dev-only,
	// key duplicates cause crushing only due to `matched` being empty
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	if (matched.length === 0) {
	var map = new Map();
	for (j = 0; j < length; j += 1) {
	var k = get_key(array[j], j);
	if (map.has(k)) {
	k = String(k);
	if (k.startsWith('[object ')) k = null;
	each_key_duplicate(String(j), String(map.get(k)), k);
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	}
	map.set(k, j);
	}
	}

		prev = start.prev;

		var a = matched[0];
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		@@ -0,0 +1,12 @@
	import { flushSync } from 'svelte';
	import { test } from '../../test';

	export default test({
	test({ assert, target }) {
	let button = target.querySelector('button');

	button?.click();

	assert.throws(flushSync, /each_key_duplicate/);
	}
	});
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		@@ -0,0 +1,8 @@
	<script>
	let data = [1, 2, 3];
	</script>

	<button onclick={() => data = [1, 1, 3, 1]}>add</button>
	{#each data as d (d)}
	{d}
	{/each}