fix(db): treat objects with Symbol.toStringTag as leaf values in IsPlainObject (#1373)

sleitor · autofix-ci[bot] · web-flow · commit 09c7afc47a5e · 2026-03-24T11:52:34.000+01:00
* fix(db): treat objects with Symbol.toStringTag as leaf values in IsPlainObject Temporal types (Temporal.PlainDate, ZonedDateTime, etc.) have Symbol.toStringTag set but are not in the JsBuiltIns union, causing IsPlainObject to return true and Ref<T> to recursively walk their methods and mangle them to {}. Adds a Symbol.toStringTag check so all class instances (including all Temporal types) are treated as leaf values, preserving their types in query select projections. Fixes #1372 * ci: apply automated fixes --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
diff --git a/.changeset/fix-temporal-isplainobject.md b/.changeset/fix-temporal-isplainobject.md
@@ -0,0 +1,11 @@
+---
+'@tanstack/db': patch
+---
+
+fix(db): treat objects with `Symbol.toStringTag` as leaf values in `IsPlainObject`
+
+Temporal types (e.g. `Temporal.PlainDate`, `Temporal.ZonedDateTime`) have `Symbol.toStringTag` set to a string. Previously, `IsPlainObject` would return `true` for these types because they are objects and not in the `JsBuiltIns` union. This caused the `Ref<T>` mapped type to recursively walk Temporal methods, mangling them to `{}`.
+
+The fix adds a `T extends { readonly [Symbol.toStringTag]: string }` check before returning `true`, causing all class instances with `Symbol.toStringTag` (Temporal types, etc.) to be treated as leaf values with their types fully preserved.
+
+Fixes #1372
diff --git a/packages/db/src/query/builder/types.ts b/packages/db/src/query/builder/types.ts
@@ -909,14 +909,22 @@ export type Prettify<T> = {
 
 /**
  * IsPlainObject - Utility type to check if T is a plain object
+ *
+ * Returns `false` for:
+ * - Arrays (ReadonlyArray)
+ * - JavaScript built-ins (Date, Map, Set, etc.)
+ * - Objects with `Symbol.toStringTag` (class instances like Temporal types,
+ *   TypedArrays not already in JsBuiltIns, etc.) — these are not plain data objects
  */
 type IsPlainObject<T> = T extends unknown
   ? T extends object
     ? T extends ReadonlyArray<any>
       ? false
       : T extends JsBuiltIns
         ? false
-        : true
+        : T extends { readonly [Symbol.toStringTag]: string }
+          ? false
+          : true
     : false
   : false
 
