chore: update types for JSHandle (#7650)

Closes #7583

src/common/JSHandle.ts

@@ -191,7 +191,7 @@ export class JSHandle<HandleObjectType = unknown> {
 
   /** Fetches a single property from the referenced object.
    */
-  async getProperty(propertyName: string): Promise<JSHandle | undefined> {
+  async getProperty(propertyName: string): Promise<JSHandle> {
     const objectHandle = await this.evaluateHandle(
       (object: Element, propertyName: string) => {
         const result = { __proto__: null };
@@ -201,7 +201,8 @@ export class JSHandle<HandleObjectType = unknown> {
       propertyName
     );
     const properties = await objectHandle.getProperties();
-    const result = properties.get(propertyName) || null;
+    const result = properties.get(propertyName);
+    assert(result instanceof JSHandle);
     await objectHandle.dispose();
     return result;
   }

test/jshandle.spec.ts

@@ -15,6 +15,7 @@
  */
 
 import expect from 'expect';
+import { JSHandle } from '../lib/cjs/puppeteer/common/JSHandle.js';
 import {
   getTestState,
   setupTestBrowserHooks,
@@ -96,6 +97,19 @@ describe('JSHandle', function () {
       const twoHandle = await aHandle.getProperty('two');
       expect(await twoHandle.jsonValue()).toEqual(2);
     });
+
+    it('should return a JSHandle even if the property does not exist', async () => {
+      const { page } = getTestState();
+
+      const aHandle = await page.evaluateHandle(() => ({
+        one: 1,
+        two: 2,
+        three: 3,
+      }));
+      const undefinedHandle = await aHandle.getProperty('doesnotexist');
+      expect(undefinedHandle).toBeInstanceOf(JSHandle);
+      expect(await undefinedHandle.jsonValue()).toBe(undefined);
+    });
   });
 
   describe('JSHandle.jsonValue', function () {