Update vue component test example

- It was severely outdated using mountX helpers
- Promotes the practice of using test helpers

doc/development/fe_guide/vue.md

@@ -189,92 +189,97 @@ Each Vue component has a unique output. This output is always present in the ren
 Although we can test each method of a Vue component individually, our goal must be to test the output
 of the render/template function, which represents the state at all times.
 
-Make use of the [axios mock adapter](axios.md#mock-axios-response-in-tests) to mock data returned.
-
-Here's how we would test the Todo App above:
+Here's an example of a well structured unit test for [this Vue component](#appendix---vue-component-subject-under-test):
 
 ```javascript
-import Vue from 'vue';
-import axios from '~/lib/utils/axios_utils';
+import { shallowMount } from '@vue/test-utils';
+import { GlLoadingIcon } from '@gitlab/ui';
 import MockAdapter from 'axios-mock-adapter';
+import axios from '~/lib/utils/axios_utils';
+import App from '~/todos/app.vue';
 
-describe('Todos App', () => {
-  let vm;
+const TEST_TODOS = [
+  { text: 'Lorem ipsum test text' },
+  { text: 'Lorem ipsum 2' },
+];
+const TEST_NEW_TODO = 'New todo title';
+const TEST_TODO_PATH = '/todos';
+
+describe('~/todos/app.vue', () => {
+  let wrapper;
   let mock;
 
   beforeEach(() => {
-    // Create a mock adapter for stubbing axios API requests
+    // IMPORTANT: Use axios-mock-adapter for stubbing axios API requests
     mock = new MockAdapter(axios);
-
-    const Component = Vue.extend(component);
-
-    // Mount the Component
-    vm = new Component().$mount();
+    mock.onGet(TEST_TODO_PATH).reply(200, TEST_TODOS);
+    mock.onPost(TEST_TODO_PATH).reply(200);
   });
 
   afterEach(() => {
-    // Reset the mock adapter
-    mock.restore();
-    // Destroy the mounted component
-    vm.$destroy();
-  });
+    // IMPORTANT: Clean up the component instance and axios mock adapter
+    wrapper.destroy();
+    wrapper = null;
 
-  it('should render the loading state while the request is being made', () => {
-    expect(vm.$el.querySelector('i.fa-spin')).toBeDefined();
+    mock.restore();
   });
 
-  it('should render todos returned by the endpoint', done => {
-    // Mock the get request on the API endpoint to return data
-    mock.onGet('/todos').replyOnce(200, [
-      {
-        title: 'This is a todo',
-        text: 'This is the text',
+  // NOTE: It is very helpful to separate setting up the component from
+  // its collaborators (i.e. Vuex, axios, etc.)
+  const createWrapper = (props = {}) => {
+    wrapper = shallowMount(App, {
+      propsData: {
+        path: TEST_TODO_PATH,
+        ...props,
       },
-    ]);
+    });
+  };
+  // NOTE: Helper methods greatly help test maintainability and readability.
+  const findLoader = () => wrapper.find(GlLoadingIcon);
+  const findAddButton = () => wrapper.find('[data-testid="add-button"]');
+  const findTextInput = () => wrapper.find('[data-testid="text-input"]');
+  const findTodoData = () => wrapper.findAll('[data-testid="todo-item"]').wrappers.map(wrapper => ({ text: wrapper.text() }));
+
+  describe('when mounted and loading', () => {
+    beforeEach(() => {
+      // Create request which will never resolve
+      mock.onGet(TEST_TODO_PATH).reply(() => new Promise(() => {}));
+      createWrapper();
+    });
 
-    Vue.nextTick(() => {
-      const items = vm.$el.querySelectorAll('.js-todo-list div')
-      expect(items.length).toBe(1);
-      expect(items[0].textContent).toContain('This is the text');
-      done();
+    it('should render the loading state', () => {
+      expect(findLoader().exists()).toBe(true);
     });
   });
 
-  it('should add a todos on button click', (done) => {
+  describe('when todos are loaded', () => {
+    beforeEach(() => {
+      createWrapper();
+      // IMPORTANT: This component fetches data asynchronously on mount, so let's wait for the Vue template to update
+      return wrapper.vm.$nextTick();
+    });
 
-    // Mock the put request and check that the sent data object is correct
-    mock.onPut('/todos').replyOnce((req) => {
-      expect(req.data).toContain('text');
-      expect(req.data).toContain('title');
+    it('should not show loading', () => {
+      expect(findLoader().exists()).toBe(false);
+    });
 
-      return [201, {}];
+    it('should render todos', () => {
+      expect(findTodoData()).toEqual(TEST_TODOS);
     });
 
-    vm.$el.querySelector('.js-add-todo').click();
+    it('when todo is added, should post new todo', () => {
+      findTextInput().vm.$emit('update', TEST_NEW_TODO)
+      findAddButton().vm.$emit('click');
 
-    // Add a new interceptor to mock the add Todo request
-    Vue.nextTick(() => {
-      expect(vm.$el.querySelectorAll('.js-todo-list div').length).toBe(2);
-      done();
+      return wrapper.vm.$nextTick()
+        .then(() => {
+          expect(mock.history.post.map(x => JSON.parse(x.data))).toEqual([{ text: TEST_NEW_TODO }]);
+        });
     });
   });
 });
 ```
 
-### `mountComponent` helper
-
-There is a helper in `spec/javascripts/helpers/vue_mount_component_helper.js` that allows you to mount a component with the given props:
-
-```javascript
-import Vue from 'vue';
-import mountComponent from 'spec/helpers/vue_mount_component_helper'
-import component from 'component.vue'
-
-const Component = Vue.extend(component);
-const data = {prop: 'foo'};
-const vm = mountComponent(Component, data);
-```
-
 ### Test the component's output
 
 The main return value of a Vue component is the rendered output. In order to test the component we
@@ -336,3 +341,35 @@ Currently, we recommend to minimize adding certain features to the codebase to p
 - `slot` attributes
 
 You can find more details on [Migration to Vue 3](vue3_migration.md)
+
+## Appendix - Vue component subject under test
+
+This is the template for the example component which is tested in the [Testing Vue components](#testing-vue-components) section:
+
+```html
+<template>
+  <div class="content">
+    <gl-loading-icon v-if="isLoading" />
+    <template v-else>
+      <div
+        v-for="todo in todos"
+        :key="todo.id"
+        :class="{ 'gl-strike': todo.isDone }"
+        data-testid="todo-item"
+      >{{ toddo.text }}</div>
+      <footer class="gl-border-t-1 gl-mt-3 gl-pt-3">
+        <gl-form-input
+          type="text"
+          v-model="todoText"
+          data-testid="text-input"
+        >
+        <gl-button
+          variant="success"
+          data-testid="add-button"
+          @click="addTodo"
+        >Add</gl-button>
+      </footer>
+    </template>
+  </div>
+</template>
+```

doc/development/testing_guide/frontend_testing.md

@@ -832,43 +832,6 @@ 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).
 
-### Vue Helper: `mountComponent`
-
-To make mounting a Vue component easier and more readable, we have a few helpers available in `spec/helpers/vue_mount_component_helper`:
-
-- `createComponentWithStore`
-- `mountComponentWithStore`
-
-Examples of usage:
-
-```javascript
-beforeEach(() => {
-  vm = createComponentWithStore(Component, store);
-
-  vm.$store.state.currentBranchId = 'master';
-
-  vm.$mount();
-});
-```
-
-```javascript
-beforeEach(() => {
-  vm = mountComponentWithStore(Component, {
-    el: '#dummy-element',
-    store,
-    props: { badge },
-  });
-});
-```
-
-Don't forget to clean up:
-
-```javascript
-afterEach(() => {
-  vm.$destroy();
-});
-```
-
 ### Wait until axios requests finish
 
 The axios utils mock module located in `spec/frontend/mocks/ce/lib/utils/axios_utils.js` contains two helper methods for Jest tests that spawn HTTP requests.

doc/telemetry/snowplow.md

@@ -153,11 +153,16 @@ describe('my component', () => {
   let trackingSpy;
 
   beforeEach(() => {
-    const vm = mountComponent(MyComponent);
     trackingSpy = mockTracking('_category_', vm.$el, spyOn);
   });
 
+  const triggerEvent = () => {
+    // action which should trigger a event
+  };
+
   it('tracks an event when toggled', () => {
+    expect(trackingSpy).not.toHaveBeenCalled();
+
     triggerEvent('a.toggle');
 
     expect(trackingSpy).toHaveBeenCalledWith('_category_', 'click_edit_button', {