From 7a1c6ceaef54526fa8f765991bab6900c7a6d7e4 Mon Sep 17 00:00:00 2001
From: Ousama Ben Younes <benyounes.ousama@gmail.com>
Date: Sat, 6 Jun 2026 08:04:55 +0000
Subject: [PATCH] fix(svelte-query): handle shrinking createRawRef array by
 multiple entries (#10341)

The `createRawRef` Proxy used by `createQueries` crashed with
`TypeError: 'deleteProperty' on proxy: trap returned falsish for property 'N'`
whenever the reactive query list was shrunk by two or more items in one update.

Two compounding causes:
1. `update()` iterated `keysToRemove` in ascending index order. The trap
   decrements `target.length` after every deletion, so the next stale index
   was no longer `in target` and the trap returned `false`, which throws in
   strict mode (ES modules).
2. The trap returned `false` for missing props instead of treating delete on
   a nonexistent prop as a spec-compliant no-op.

Fix: sort `keysToRemove` descending for arrays so each delete acts on the
current tail, and return `true` from `deleteProperty` when the prop is
already absent.

Generated by Ora Studio
Vibe coded by ousamabenyounes

Co-Authored-By: Claude <noreply@anthropic.com>
---
 .changeset/fix-svelte-query-shrink-array.md    |  5 +++++
 packages/svelte-query/src/containers.svelte.ts | 12 +++++++++++-
 .../tests/containers.svelte.test.ts            | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fix-svelte-query-shrink-array.md

diff --git a/.changeset/fix-svelte-query-shrink-array.md b/.changeset/fix-svelte-query-shrink-array.md
new file mode 100644
index 00000000000..ee6ca8ff28f
--- /dev/null
+++ b/.changeset/fix-svelte-query-shrink-array.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/svelte-query': patch
+---
+
+Fix `createQueries` crashing with `TypeError: 'deleteProperty' on proxy: trap returned falsish for property 'N'` when two or more items were removed from its reactive array in the same update.
diff --git a/packages/svelte-query/src/containers.svelte.ts b/packages/svelte-query/src/containers.svelte.ts
index 60d27c68431..379fa0f2bd0 100644
--- a/packages/svelte-query/src/containers.svelte.ts
+++ b/packages/svelte-query/src/containers.svelte.ts
@@ -80,7 +80,10 @@ export function createRawRef<T extends {} | Array<unknown>>(
         }
         return true
       }
-      return false
+      // Spec semantics: deleting a missing prop is a no-op. Returning false here
+      // makes `delete` throw in strict mode, which broke #10341 when `update()`
+      // iterated stale indices over an already-shrunk array.
+      return true
     },
   })
 
@@ -88,6 +91,13 @@ export function createRawRef<T extends {} | Array<unknown>>(
     const existingKeys = Object.keys(out)
     const newKeys = Object.keys(newValue)
     const keysToRemove = existingKeys.filter((key) => !newKeys.includes(key))
+    // Arrays: delete in descending index order so each `deleteProperty` trap
+    // sees the slot it is removing as the current tail (length-- stays valid).
+    // Forward iteration would shrink the array under our feet and the next
+    // index would no longer be `in target`, tripping the trap.
+    if (Array.isArray(newValue)) {
+      keysToRemove.sort((a, b) => Number(b) - Number(a))
+    }
     for (const key of keysToRemove) {
       // @ts-expect-error
       delete out[key]
diff --git a/packages/svelte-query/tests/containers.svelte.test.ts b/packages/svelte-query/tests/containers.svelte.test.ts
index 3511dbb5b5d..849dc2849c0 100644
--- a/packages/svelte-query/tests/containers.svelte.test.ts
+++ b/packages/svelte-query/tests/containers.svelte.test.ts
@@ -198,6 +198,24 @@ describe('createRawRef', () => {
     expect(ref).toEqual([7, 8, 9])
   })
 
+  it('should handle shrinking an array by more than one entry at once', () => {
+    // Regression for #10341: createQueries crashed with
+    // `TypeError: can't delete property 'N': proxy deleteProperty handler returned false`
+    // when two or more items were removed from the reactive array in a single update.
+    const [ref, update] = createRawRef([1, 2, 3, 4, 5])
+
+    expect(ref).toEqual([1, 2, 3, 4, 5])
+
+    update([1, 2])
+    expect(ref).toEqual([1, 2])
+
+    update([1, 2, 3, 4])
+    expect(ref).toEqual([1, 2, 3, 4])
+
+    update([])
+    expect(ref).toEqual([])
+  })
+
   it('should behave like a regular object when not using `update`', () => {
     const [ref] = createRawRef<Record<string, unknown>>({ a: 1, b: 2 })