Test array sort in effect to prevent regressions of this use-case (#16175)

Author: raythurnvoid

packages/svelte/tests/runtime-runes/samples/array-sort-in-effect/_config.js

@@ -0,0 +1,52 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	/**
+	 * Ensure that sorting an array inside an $effect works correctly
+	 * and re-runs when the array changes (e.g., when items are added).
+	 */
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		// initial render — array should be sorted
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>add item</button>
+				<p>0</p>
+				<p>50</p>
+				<p>100</p>
+			`
+		);
+
+		// add first item (20); effect should re-run and sort the array
+		flushSync(() => button?.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>add item</button>
+				<p>0</p>
+				<p>20</p>
+				<p>50</p>
+				<p>100</p>
+			`
+		);
+
+		// add second item (80); effect should re-run and sort the array
+		flushSync(() => button?.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>add item</button>
+				<p>0</p>
+				<p>20</p>
+				<p>50</p>
+				<p>80</p>
+				<p>100</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/array-sort-in-effect/main.svelte

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+<script>
+	let arr = $state([100, 0, 50]);
+	let nextValues = [20, 80];
+	let valueIndex = 0;
+
+	$effect(() => {
+		arr.sort((a, b) => a - b);
+	});
+
+	function addItem() {
+		if (valueIndex < nextValues.length) {
+			arr.push(nextValues[valueIndex]);
+			valueIndex++;
+		}
+	}
+</script>
+
+<button onclick={addItem}>add item</button>
+{#each arr as x}
+	<p>{x}</p>
+{/each}