fix: handle nested object fields in form data (#14469)

Fixes #14460

correcting object pointer handling in deep_set so nested form fields are properly stored in the same object hierarchy.

Author: LeeWxx

.changeset/fix-nested-form-fields.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: handle nested object fields in form data

packages/kit/src/runtime/utils.js

@@ -123,8 +123,9 @@ export function deep_set(object, keys, value) {
 				throw new Error(`Invalid array key ${keys[i + 1]}`);
 			}
 		} else {
-			object = object[key] ??= is_array ? [] : Object.create(null); // guard against prototype pollution
+			object[key] ??= is_array ? [] : Object.create(null); // guard against prototype pollution
 		}
+		object = object[key];
 	}
 
 	object[keys[keys.length - 1]] = value;

packages/kit/src/runtime/utils.spec.js

@@ -107,4 +107,22 @@ describe('convert_formdata', () => {
 			array: ['a', 'b', 'c']
 		});
 	});
+
+	test('handles multiple fields at the same nested level', () => {
+		const data = new FormData();
+
+		data.append('user.name.first', 'first');
+		data.append('user.name.last', 'last');
+
+		const converted = convert_formdata(data);
+
+		expect(converted).toEqual({
+			user: {
+				name: {
+					first: 'first',
+					last: 'last'
+				}
+			}
+		});
+	});
 });