Update docs with commit for link and better explanation

Signed-off-by: Angelo Gulina <agulina@gitlab.com>

doc/development/testing_guide/frontend_testing.md

@@ -235,6 +235,7 @@ possible selectors include:
 
 - A semantic attribute like `name` (also verifies that `name` was setup properly)
 - A `data-testid` attribute ([recommended by maintainers of `@vue/test-utils`](https://github.com/vuejs/vue-test-utils/issues/1498#issuecomment-610133465))
+  optionally combined with [`findByTestId`](#extendedwrapper-and-findbytestid)
 - a Vue `ref` (if using `@vue/test-utils`)
 
 ```javascript
@@ -251,7 +252,8 @@ it('exists', () => {
   // Good (especially for unit tests)
   wrapper.find(FooComponent);
   wrapper.find('input[name=foo]');
-  wrapper.find('[data-testid="foo"]');
+  wrapper.find('[data-testid="my-foo-id"]');
+  wrapper.findByTestId('my-foo-id'); // with the extendedWrapper utility – check below
   wrapper.find({ ref: 'foo'});
   
   // Bad
@@ -262,6 +264,8 @@ it('exists', () => {
 });
 ```
 
+It is recommended to use `kebab-case` for `data-testid` attribute.
+
 It is not recommended that you add `.js-*` classes just for testing purposes. Only do this if there are no other feasible options available.
 
 Do not use a `.qa-*` class or `data-qa-selector` attribute for any tests other than QA end-to-end testing.
@@ -1033,7 +1037,7 @@ testAction(
 );
 ```
 
-Check an example in [`spec/javascripts/ide/stores/actions_spec.jsspec/javascripts/ide/stores/actions_spec.js`](https://gitlab.com/gitlab-org/gitlab/blob/master/spec/javascripts/ide/stores/actions_spec.js).
+Check an example in [`spec/frontend/ide/stores/actions_spec.js`](https://gitlab.com/gitlab-org/gitlab/-/blob/fdc7197609dfa7caeb1d962042a26248e49f27da/spec/frontend/ide/stores/actions_spec.js#L392).
 
 ### Wait until Axios requests finish
 
@@ -1045,6 +1049,29 @@ These are very useful if you don't have a handle to the request's Promise, for e
 
 Both functions run `callback` on the next tick after the requests finish (using `setImmediate()`), to allow any `.then()` or `.catch()` handlers to run.
 
+### `extendedWrapper` and `findByTestId`
+
+Using `data-testid` is one of the [recommended ways to query DOM elements](#how-to-query-dom-elements).
+You can use the `extendedWrapper` utility on the `wrapper` returned by `shalowMount`/`mount`.
+By doing so, the `wrapper` provides you with the ability to perform a `findByTestId`,
+which is a shortcut to the more verbose `wrapper.find('[data-testid="my-test-id"]');`
+
+```javascript
+import { extendedWrapper } from 'jest/helpers/vue_test_utils_helper';
+
+describe('FooComponent', () => {
+  const wrapper = extendedWrapper(shallowMount({
+    template: `<div data-testid="my-test-id"></div>`,
+  }));
+  
+  it('exists', () => {
+    expect(wrapper.findByTestId('my-test-id').exists()).toBe(true);
+  });
+});
+```
+
+Check an example in [`spec/frontend/alert_management/components/alert_details_spec.js`](https://gitlab.com/gitlab-org/gitlab/-/blob/ac1c9fa4c5b3b45f9566147b1c88fd1339cd7c25/spec/frontend/alert_management/components/alert_details_spec.js#L32).
+
 ## Testing with older browsers
 
 Some regressions only affect a specific browser version. We can install and test in particular browsers with either Firefox or BrowserStack using the following steps:

spec/frontend/helpers/vue_test_utils_helper.js

+import { isArray } from 'lodash';
+
 const vNodeContainsText = (vnode, text) =>
   (vnode.text && vnode.text.includes(text)) ||
   (vnode.children && vnode.children.filter(child => vNodeContainsText(child, text)).length);
@@ -34,9 +36,18 @@ export const waitForMutation = (store, expectedMutationType) =>
     });
   });
 
-export const extendedWrapper = wrapper =>
-  Object.defineProperty(wrapper, 'findByTestId', {
+export const extendedWrapper = wrapper => {
+  if (isArray(wrapper) || !wrapper?.find) {
+    // eslint-disable-next-line no-console
+    console.warn(
+      '[vue-test-utils-helper]: you are trying to extend an object that is not a VueWrapper.',
+    );
+    return wrapper;
+  }
+
+  return Object.defineProperty(wrapper, 'findByTestId', {
     value(id) {
       return this.find(`[data-testid="${id}"]`);
     },
   });
+};

spec/frontend/helpers/vue_test_utils_helper_spec.js

 import { shallowMount } from '@vue/test-utils';
-import { shallowWrapperContainsSlotText } from './vue_test_utils_helper';
+import { extendedWrapper, shallowWrapperContainsSlotText } from './vue_test_utils_helper';
 
 describe('Vue test utils helpers', () => {
   describe('shallowWrapperContainsSlotText', () => {
@@ -45,4 +45,48 @@ describe('Vue test utils helpers', () => {
       expect(shallowWrapperContainsSlotText(mockComponent, 'namedSlot', searchText)).toBe(false);
     });
   });
+
+  describe('extendedWrapper', () => {
+    describe('when an invalid wrapper is provided', () => {
+      beforeEach(() => {
+        // eslint-disable-next-line no-console
+        console.warn = jest.fn();
+      });
+
+      it.each`
+        wrapper
+        ${{}}
+        ${[]}
+        ${null}
+        ${undefined}
+        ${1}
+        ${''}
+      `('should warn with an error when the wrapper is $wrapper', ({ wrapper }) => {
+        extendedWrapper(wrapper);
+        /* eslint-disable no-console */
+        expect(console.warn).toHaveBeenCalled();
+        expect(console.warn).toHaveBeenCalledWith(
+          '[vue-test-utils-helper]: you are trying to extend an object that is not a VueWrapper.',
+        );
+        /* eslint-enable no-console */
+      });
+    });
+
+    describe('findByTestId', () => {
+      const testId = 'a-component';
+      let mockComponent;
+
+      beforeEach(() => {
+        mockComponent = extendedWrapper(
+          shallowMount({
+            template: `<div data-testid="${testId}"></div>`,
+          }),
+        );
+      });
+
+      it('should find the component by test id', () => {
+        expect(mockComponent.findByTestId(testId).exists()).toBe(true);
+      });
+    });
+  });
 });