Vue

To get started with Vue, read through their documentation.

Examples

What is described in the following sections can be found in these examples:

Vue architecture

All new features built with Vue.js must follow a Flux architecture. The main goal we are trying to achieve is to have only one data flow and only one data entry. In order to achieve this goal we use vuex.

You can also read about this architecture in Vue docs about state management and about one way data flow.

Components and Store

In some features implemented with Vue.js, like the issue board or environments table you can find a clear separation of concerns:

new_feature
├── components
│   └── component.vue
│   └── ...
├── store
│  └── new_feature_store.js
├── index.js

For consistency purposes, we recommend you to follow the same structure.

Let's look into each of them:

An `index.js` file

This is the index file of your new feature. This is where the root Vue instance of the new feature should be.

The Store and the Service should be imported and initialized in this file and provided as a prop to the main component.

Be sure to read about page-specific JavaScript.

Bootstrapping Gotchas

Providing data from HAML to JavaScript

While mounting a Vue application, you might need to provide data from Rails to JavaScript. To do that, you can use the data attributes in the HTML element and query them while mounting the application.

You should only do this while initializing the application, because the mounted element will be replaced with Vue-generated DOM.

The advantage of providing data from the DOM to the Vue instance through props in the render function instead of querying the DOM inside the main Vue component is avoiding the need to create a fixture or an HTML element in the unit test, which will make the tests easier.

See the following example, also, please refer to our Vue style guide for additional information on why we explicitly declare the data being passed into the Vue app;

// haml
#js-vue-app{ data: { endpoint: 'foo' }}

// index.js
const el = document.getElementById('js-vue-app');

if (!el) return false;

const { endpoint } = el.dataset;

return new Vue({
  el,
  render(createElement) {
    return createElement('my-component', {
      props: {
        endpoint
      },
    });
  },
}

When adding an id attribute to mount a Vue application, please make sure this id is unique across the codebase

Accessing the `gl` object

When we need to query the gl object for data that won't change during the application's life cycle, we should do it in the same place where we query the DOM. By following this practice, we can avoid the need to mock the gl object, which will make tests easier. It should be done while initializing our Vue instance, and the data should be provided as props to the main component:

return new Vue({
  el: '.js-vue-app',
  render(createElement) {
    return createElement('my-component', {
      props: {
        username: gon.current_username,
      },
    });
  },
});

Accessing feature flags

Use Vue's provide/inject mechanism to make feature flags available to any descendant components in a Vue application. The glFeatures object is already provided in commons/vue.js, so only the mixin is required to use the flags:

// An arbitrary descendant component

import glFeatureFlagsMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';

export default {
  // ...
  mixins: [glFeatureFlagsMixin()],
  // ...
  created() {
    if (this.glFeatures.myFlag) {
      // ...
    }
  },
}

This approach has a few benefits:

Arbitrarily deeply nested components can opt-in and access the flag without intermediate components being aware of it (c.f. passing the flag down via props).

Good testability, since the flag can be provided to mount/shallowMount from vue-test-utils simply as a prop.

import { shallowMount } from '@vue/test-utils';

shallowMount(component, {
  provide: {
    glFeatures: { myFlag: true },
  },
});

No need to access a global variable, except in the application's entry point.

A folder for Components

This folder holds all components that are specific of this new feature. If you need to use or create a component that will probably be used somewhere else, please refer to vue_shared/components.

A good rule of thumb to know when you should create a component is to think if it will be reusable elsewhere.

For example, tables are used in a quite amount of places across GitLab, a table would be a good fit for a component. On the other hand, a table cell used only in one table would not be a good use of this pattern.

You can read more about components in Vue.js site, Component System.

A folder for the Store

Vuex

Check this page for more details.

Mixing Vue and jQuery

Mixing Vue and jQuery is not recommended.
If you need to use a specific jQuery plugin in Vue, create a wrapper around it.
It is acceptable for Vue to listen to existing jQuery events using jQuery event listeners.
It is not recommended to add new jQuery events for Vue to interact with jQuery.

Mixing Vue and JavaScript classes (in the data function)

In the Vue documentation the Data function/object is defined as follows:

The data object for the Vue instance. Vue will recursively convert its properties into getter/setters to make it “reactive”. The object must be plain: native objects such as browser API objects and prototype properties are ignored. A rule of thumb is that data should just be data - it is not recommended to observe objects with their own stateful behavior.

Based on the Vue guidance:

Do not use or create a JavaScript class in your data function, such as user: new User().
Do not add new JavaScript class implementations.
Do use GraphQL, Vuex or a set of components if cannot use simple primitives or objects.
Do maintain existing implementations using such approaches.
Do Migrate components to a pure object model when there are substantial changes to it.
Do add business logic to helpers or utils, so you can test them separately from your component.

Why

There are additional reasons why having a JavaScript class presents maintainability issues on a huge codebase:

Once a class is created, it is easy to extend it in a way that can infringe Vue reactivity and best practices.
A class adds a layer of abstraction, which makes the component API and its inner workings less clear.
It makes it harder to test. Since the class is instantiated by the component data function, it is harder to 'manage' component and class separately.
Adding OOP to a functional codebase adds yet another way of writing code, reducing consistency and clarity.

Style guide

Please refer to the Vue section of our style guide for best practices while writing and testing your Vue components and templates.

Testing Vue Components

Please refer to the Vue testing style guide for guidelines and best practices for testing your Vue components.

Each Vue component has a unique output. This output is always present in the render function.

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.

Here's an example of a well structured unit test for this Vue component:

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';

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(() => {
    // IMPORTANT: Use axios-mock-adapter for stubbing axios API requests
    mock = new MockAdapter(axios);
    mock.onGet(TEST_TODO_PATH).reply(200, TEST_TODOS);
    mock.onPost(TEST_TODO_PATH).reply(200);
  });

  afterEach(() => {
    // IMPORTANT: Clean up the component instance and axios mock adapter
    wrapper.destroy();
    wrapper = null;

    mock.restore();
  });

  // 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,
      },
    });
  };
  // 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();
    });

    it('should render the loading state', () => {
      expect(findLoader().exists()).toBe(true);
    });
  });

  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();
    });

    it('should not show loading', () => {
      expect(findLoader().exists()).toBe(false);
    });

    it('should render todos', () => {
      expect(findTodoData()).toEqual(TEST_TODOS);
    });

    it('when todo is added, should post new todo', () => {
      findTextInput().vm.$emit('update', TEST_NEW_TODO)
      findAddButton().vm.$emit('click');

      return wrapper.vm.$nextTick()
        .then(() => {
          expect(mock.history.post.map(x => JSON.parse(x.data))).toEqual([{ text: TEST_NEW_TODO }]);
        });
    });
  });
});

Test the component's output

The main return value of a Vue component is the rendered output. In order to test the component we need to test the rendered output. Visit the Vue testing guide.

Events

We should test for events emitted in response to an action within our component, this is useful to verify the correct events are being fired with the correct arguments.

For any DOM events we should use trigger to fire out event.

// Assuming SomeButton renders: <button>Some button</button>
wrapper = mount(SomeButton);

...
it('should fire the click event', () => {
  const btn = wrapper.find('button')

  btn.trigger('click');
  ...
})

When we need to fire a Vue event, we should use emit to fire our event.

wrapper = shallowMount(DropdownItem);

...

it('should fire the itemClicked event', () => {
  DropdownItem.vm.$emit('itemClicked');
  ...
})

We should verify an event has been fired by asserting against the result of the emitted() method

Vue.js Expert Role

You should only apply to be a Vue.js expert when your own merge requests and your reviews show:

Deep understanding of Vue and Vuex reactivity
Vue and Vuex code are structured according to both official and our guidelines
Full understanding of testing a Vue and Vuex application
Vuex code follows the documented pattern
Knowledge about the existing Vue and Vuex applications and existing reusable components

Vue 2 -> Vue 3 Migration

This section is added temporarily to support the efforts to migrate the codebase from Vue 2.x to Vue 3.x

Currently, we recommend to minimize adding certain features to the codebase to prevent increasing the tech debt for the eventual migration:

filters;
event buses;
functional templated
slot attributes

You can find more details on Migration to Vue 3

Appendix - Vue component subject under test

This is the template for the example component which is tested in the Testing Vue components section:

<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>

Vue