LibJS: Don't remove non-configurable items in Array when setting length

Author: https://github.com/davidot
Commit: https://github.com/SerenityOS/serenity/commit/b38fb418f8b
Pull-request: https://github.com/SerenityOS/serenity/pull/8245

Userland/Libraries/LibJS/Runtime/IndexedProperties.cpp

@@ -168,13 +168,32 @@ ValueAndAttributes GenericIndexedPropertyStorage::take_last()
 
 void GenericIndexedPropertyStorage::set_array_like_size(size_t new_size)
 {
-    m_array_size = new_size;
+    if (new_size == m_array_size)
+        return;
+
+    if (new_size >= m_array_size) {
+        m_array_size = new_size;
+        return;
+    }
+
+    size_t highest_index = 0;
+    bool any_left = false;
 
     HashMap<u32, ValueAndAttributes> new_sparse_elements;
     for (auto& entry : m_sparse_elements) {
-        if (entry.key < new_size)
+        if (entry.key < new_size || !entry.value.attributes.is_configurable()) {
             new_sparse_elements.set(entry.key, entry.value);
+            any_left = true;
+            highest_index = max(highest_index, (size_t)entry.key);
+        }
     }
+
+    if (any_left) {
+        m_array_size = max(highest_index + 1, new_size);
+    } else {
+        m_array_size = new_size;
+    }
+
     m_sparse_elements = move(new_sparse_elements);
 }
 

Userland/Libraries/LibJS/Tests/builtins/Array/array-length-setter.js

@@ -44,4 +44,14 @@ describe("normal behavior", () => {
         b.length = 0x80000001;
         expect(b.length).toEqual(0x80000001);
     });
+
+    test("should not remove non-configurable values", () => {
+        var a = [1, undefined, 3];
+        Object.defineProperty(a, 1, { configurable: false, value: 2 });
+        expect(a.length).toEqual(3);
+
+        expect((a.length = 1)).toEqual(1);
+        expect(a.length).toEqual(2);
+        expect(a[1]).toEqual(2);
+    });
 });