diff --git a/.changeset/few-oranges-juggle.md b/.changeset/few-oranges-juggle.md
new file mode 100644
index 00000000..6f2f0fa0
--- /dev/null
+++ b/.changeset/few-oranges-juggle.md
@@ -0,0 +1,5 @@
+---
+"@marko/language-tools": patch
+---
+
+Update Type definitions for for-of loop
diff --git a/packages/language-tools/marko.internal.d.ts b/packages/language-tools/marko.internal.d.ts
index 7ef2da25..47f6b5bc 100644
--- a/packages/language-tools/marko.internal.d.ts
+++ b/packages/language-tools/marko.internal.d.ts
@@ -170,7 +170,7 @@ declare global {
       >(
         input: {
           of: Value | false | void | null;
-          by?: (item: Item, index: number) => string;
+          by?: ((item: Item, index: number) => string) | string;
         },
         content: BodyContent,
       ): ReturnAndScope<BodyContentScope<BodyContent>, void>;