fix: prevent deep_set crash on nullish nested values

Author: RazinShafayet2007

.changeset/deep-set-nullish.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prevent `deep_set` crash on nullish nested values

packages/kit/src/runtime/form-utils.js

@@ -510,11 +510,11 @@ export function deep_set(object, keys, value) {
 		const exists = Object.hasOwn(current, key);
 		const inner = current[key];
 
-		if (exists && is_array !== Array.isArray(inner)) {
+		if (exists && inner != null && is_array !== Array.isArray(inner)) {
 			throw new Error(`Invalid array key ${keys[i + 1]}`);
 		}
 
-		if (!exists) {
+		if (!exists || inner == null) {
 			current[key] = is_array ? [] : {};
 		}
 

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

@@ -765,4 +765,20 @@ describe('deep_set', () => {
 		// @ts-ignore
 		expect(Object.prototype.toString.property).toBeUndefined();
 	});
+
+	test('creates nested object when intermediate value is undefined', () => {
+		const target = { nested: undefined };
+
+		deep_set(target, ['nested', 'name'], 'hello');
+
+		expect(target).toEqual({ nested: { name: 'hello' } });
+	});
+
+	test('creates nested object when intermediate value is null', () => {
+		const target = { nested: null };
+
+		deep_set(target, ['nested', 'name'], 'hello');
+
+		expect(target).toEqual({ nested: { name: 'hello' } });
+	});
 });