Examples
Vue architecture
Style guide
Composition API
Testing Vue Components
- Child components
- Events
Vue.js Expert Role
Vue 2 -> Vue 3 Migration
Appendix - Vue component subject under test

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. To achieve this goal we use Vuex.

You can also read about this architecture in Vue documentation 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 file is the index file of your new feature. The root Vue instance of the new feature should be here.

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 is replaced with a Vue-generated DOM.

The data attributes are only able to accept String values, so you will need to cast or convert other variable types to String.

The advantage of providing data from the DOM to the Vue instance through props or provide in the render function, instead of querying the DOM inside the main Vue component, is that you avoid creating a fixture or an HTML element in the unit test.

`provide` and `inject`

Vue supports dependency injection through provide and inject. In the component the inject configuration accesses the values provide passes down. This example of a Vue app initialization shows how the provide configuration passes a value from HAML to the component:

#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', {
      provide: {
        endpoint
      },
    });
  },
});

The component, or any of its child components, can access the property through inject as:

<script>
  export default {
    name: 'MyComponent',
    inject: ['endpoint'],
    ...
    ...
  };
</script>
<template>
  ...
  ...
</template>

Using dependency injection to provide values from HAML is ideal when:

The injected value doesn’t need an explicit validation against its data type or contents.
The value doesn’t need to be reactive.
Multiple components exist in the hierarchy that need access to this value where prop-drilling becomes an inconvenience. Prop-drilling when the same prop is passed through all components in the hierarchy until the component that is genuinely using it.

Dependency injection can potentially break a child component (either an immediate child or multiple levels deep) if both conditions are true:

The value declared in the inject configuration doesn’t have defaults defined.
The parent component has not provided the value using the provide configuration.

A default value might be useful in contexts where it makes sense.

props

If the value from HAML doesn’t fit the criteria of dependency injection, use props. See the following example.

// 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, make sure this id is unique across the codebase.

For more information on why we explicitly declare the data being passed into the Vue app, refer to our Vue style guide.

Providing Rails form fields to Vue applications

When composing a form with Rails, the name, id, and value attributes of form inputs are generated to match the backend. It can be helpful to have access to these generated attributes when converting a Rails form to Vue, or when integrating components (such as a date picker or project selector) into it. The parseRailsFormFields utility can be used to parse the generated form input attributes so they can be passed to the Vue application. This enables us to integrate Vue components without changing how the form submits.

-# form.html.haml
= form_for user do |form|
  .js-user-form
    = form.text_field :name, class: 'form-control gl-form-input', data: { js_name: 'name' }
    = form.text_field :email, class: 'form-control gl-form-input', data: { js_name: 'email' }

The js_name data attribute is used as the key in the resulting JavaScript object. For example = form.text_field :email, data: { js_name: 'fooBarBaz' } would be translated to { fooBarBaz: { name: 'user[email]', id: 'user_email', value: '' } }

// index.js
import Vue from 'vue';
import { parseRailsFormFields } from '~/lib/utils/forms';
import UserForm from './components/user_form.vue';

export const initUserForm = () => {
  const el = document.querySelector('.js-user-form');

  if (!el) {
    return null;
  }

  const fields = parseRailsFormFields(el);

  return new Vue({
    el,
    render(h) {
      return h(UserForm, {
        props: {
          fields,
        },
      });
    },
  });
};

<script>
// user_form.vue
import { GlButton, GlFormGroup, GlFormInput } from '@gitlab/ui';

export default {
  name: 'UserForm',
  components: { GlButton, GlFormGroup, GlFormInput },
  props: {
    fields: {
      type: Object,
      required: true,
    },
  },
};
</script>

<template>
  <div>
    <gl-form-group :label-for="fields.name.id" :label="__('Name')">
      <gl-form-input v-bind="fields.name" size="lg" />
    </gl-form-group>

    <gl-form-group :label-for="fields.email.id" :label="__('Email')">
      <gl-form-input v-bind="fields.email" type="email" size="lg" />
    </gl-form-group>

    <gl-button type="submit" category="primary" variant="confirm">{{ __('Update') }}</gl-button>
  </div>
</template>

Accessing the `gl` object

We query the gl object for data that doesn’t change during the application’s life cycle in the same place we query the DOM. By following this practice, we can avoid mocking the gl object, which makes 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: {
        avatarUrl: gl.avatarUrl,
      },
    });
  },
});

Accessing feature flags

Use the provide and inject mechanisms in Vue 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, because the flag can be provided to mount/shallowMount from vue-test-utils as a prop.

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

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

Accessing a global variable is not required, except in the application’s entry point.

A folder for Components

This folder holds all components that are specific to this new feature. To use or create a component that is likely to be used somewhere else, refer to vue_shared/components.

A good guideline to know when you should create a component is to think if it could 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.
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 recursively converts 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 guideline 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.
Do not add new JavaScript class implementations.
Do use GraphQL, Vuex or a set of components if cannot use 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 utilities, so you can test them separately from your component.

Why

Additional reasons why having a JavaScript class presents maintainability issues on a huge codebase:

After a class is created, it can be extended 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. Because the class is instantiated by the component data function, it is harder to ‘manage’ component and class separately.
Adding Object Oriented Principles (OOP) to a functional codebase adds another way of writing code, reducing consistency and clarity.

Style guide

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

Composition API

With Vue 2.7 it is possible to use Composition API in Vue components and as standalone composables.

Prefer `<script>` over `<script setup>`

Composition API allows you to place the logic in the <script> section of the component or to have a dedicated <script setup> section. We should use <script> and add Composition API to components using setup() property:

<script>
  import { computed } from 'vue';

  export default {
    name: 'MyComponent',
    setup(props) {
      const doubleCount = computed(() => props.count*2)
    }
  }
</script>

Aim to have one API style per component

When adding setup() property to Vue component, consider refactoring it to Composition API entirely. It’s not always feasible, especially for large components, but we should aim to have one API style per component for readability and maintainability.

Composables

With Composition API, we have a new way of abstracting logic including reactive state to composables. Composable is the function that can accept parameters and return reactive properties and methods to be used in Vue component.

// useCount.js
import { ref } from 'vue';

export function useCount(initialValue) {
  const count = ref(initialValue)

  function incrementCount() {
    count.value += 1
  }

  function decrementCount() {
    count.value -= 1
  }

  return { count, incrementCount, decrementCount }
}

// MyComponent.vue
import { useCount } from 'useCount'

export default {
  name: 'MyComponent',
  setup() {
    const { count, incrementCount, decrementCount } = useCount(5)

    return { count, incrementCount, decrementCount }
  }
}

Prefix function and file names with `use`

Common naming convention in Vue for composables is to prefix them with use and then refer to composable functionality briefly (useBreakpoints, useGeolocation etc). The same rule applies to the .js files containing composables - they should start with use_ even if the file contains more than one composable.

Avoid lifecycle pitfalls

When building a composable, we should aim to keep it as simple as possible. Lifecycle hooks add complexity to composables and might lead to unexpected side effects. To avoid that we should follow these principles:

Minimize lifecycle hooks usage whenever possible, prefer accepting/returning callbacks instead.
If your composable needs lifecycle hooks, make sure it also performs a cleanup. If we add a listener on onMounted, we should remove it on onUnmounted within the same composable.
Always set up lifecycle hooks immediately:

// bad
const useAsyncLogic = () => {
  const action = async () => {
    await doSomething();
    onMounted(doSomethingElse);
  };
  return { action };
};

// OK
const useAsyncLogic = () => {
  const done = ref(false);
  onMounted(() => {
    watch(
      done,
      () => done.value && doSomethingElse(),
      { immediate: true },
    );
  });
  const action = async () => {
    await doSomething();
    done.value = true;
  };
  return { action };
};

Avoid escape hatches

It might be tempting to write a composable that does everything as a black box, using some of the escape hatches that Vue provides. But for most of the cases this makes them too complex and hard to maintain. One escape hatch is the getCurrentInstance method. This method returns an instance of a current rendering component. Instead of using that method, you should prefer passing down the data or methods to a composable via arguments.

const useSomeLogic = () => {
  doSomeLogic();
  getCurrentInstance().emit('done'); // bad
};

const done = () => emit('done');

const useSomeLogic = (done) => {
  doSomeLogic();
  done(); // good, composable doesn't try to be too smart
}

Composables and Vuex

We should always prefer to avoid using Vuex state in composables. In case it’s not possible, we should use props to receive that state, and emit events from the setup to update the Vuex state. A parent component should be responsible to get that state from Vuex, and mutate it on events emitted from a child. You should never mutate a state that’s coming down from a prop. If a composable must mutate a Vuex state, it should use a callback to emit an event.

const useAsyncComposable = ({ state, update }) => {
  const start = async () => {
    const newState = await doSomething(state);
    update(newState);
  };
  return { start };
};

const ComponentWithComposable = {
  setup(props, { emit }) {
    const update = (data) => emit('update', data);
    const state = computed(() => props.state); // state from Vuex
    const { start } = useAsyncComposable({ state, update });
    start();
  },
};

Testing composables

Testing Vue Components

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 each method of a Vue component can be tested individually, our goal is to test the output of the render function, which represents the state at all times.

Visit the Vue testing guide for help.

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

import { GlLoadingIcon } from '@gitlab/ui';
import MockAdapter from 'axios-mock-adapter';
import { shallowMountExtended } from 'helpers/vue_test_utils_helper';
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();
    mock.restore();
  });

  // It is very helpful to separate setting up the component from
  // its collaborators (for example, Vuex and axios).
  const createWrapper = (props = {}) => {
    wrapper = shallowMountExtended(App, {
      propsData: {
        path: TEST_TODO_PATH,
        ...props,
      },
    });
  };
  // Helper methods greatly help test maintainability and readability.
  const findLoader = () => wrapper.findComponent(GlLoadingIcon);
  const findAddButton = () => wrapper.findByTestId('add-button');
  const findTextInput = () => wrapper.findByTestId('text-input');
  const findTodoData = () =>
    wrapper
      .findAllByTestId('todo-item')
      .wrappers.map((item) => ({ text: item.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', async () => {
      findTextInput().vm.$emit('update', TEST_NEW_TODO);
      findAddButton().vm.$emit('click');

      await wrapper.vm.$nextTick();

      expect(mock.history.post.map((x) => JSON.parse(x.data))).toEqual([{ text: TEST_NEW_TODO }]);
    });
  });
});

Child components

Test any directive that defines if/how child component is rendered (for example, v-if and v-for).
Test any props we are passing to child components (especially if the prop is calculated in the component under test, with the computed property, for example). Remember to use .props() and not .vm.someProp.
Test we react correctly to any events emitted from child components:

  const checkbox = wrapper.findByTestId('checkboxTestId');

  expect(checkbox.attributes('disabled')).not.toBeDefined();

  findChildComponent().vm.$emit('primary');
  await nextTick();

  expect(checkbox.attributes('disabled')).toBeDefined();

Do not test the internal implementation of the child components:

  // bad
  expect(findChildComponent().find('.error-alert').exists()).toBe(false);

  // good
  expect(findChildComponent().props('withAlertContainer')).toBe(false);

Events

We should test for events emitted in response to an action in our component. This testing verifies 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 firing a Vue event, use emit.

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

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="confirm"
          data-testid="add-button"
          @click="addTodo"
        >Add</gl-button>
      </footer>
    </template>
  </div>
</template>

Vue

Examples

Vue architecture

Components and Store

An index.js file

Bootstrapping Gotchas

Providing data from HAML to JavaScript

provide and inject

props

Providing Rails form fields to Vue applications

Accessing the gl object

Accessing feature flags

A folder for Components

A folder for the Store

Vuex

Mixing Vue and jQuery

Mixing Vue and JavaScript classes (in the data function)

Why

Style guide

Composition API

Prefer <script> over <script setup>

Aim to have one API style per component

Composables

Prefix function and file names with use

Avoid lifecycle pitfalls

Avoid escape hatches

Composables and Vuex

Testing composables

Testing Vue Components

Child components

Events

Vue.js Expert Role

Vue 2 -> Vue 3 Migration

Appendix - Vue component subject under test

ヘルプとご意見

An `index.js` file

`provide` and `inject`

Accessing the `gl` object

Prefer `<script>` over `<script setup>`

Prefix function and file names with `use`