Vue.js style guide

Linting

We default to eslint-vue-plugin, with the plugin:vue/recommended. Please check this rules for more documentation.

Basic Rules

The service has its own file
The store has its own file

Use a function in the bundle file to instantiate the Vue component:

// bad
class {
  init() {
    new Component({})
  }
}

// good
document.addEventListener('DOMContentLoaded', () => new Vue({
  el: '#element',
  components: {
    componentName
  },
  render: createElement => createElement('component-name'),
}));

Do not use a singleton for the service or the store

// bad
class Store {
  constructor() {
    if (!this.prototype.singleton) {
      // do something
    }
  }
}

// good
class Store {
  constructor() {
    // do something
  }
}

Use .vue for Vue templates. Do not use %template in HAML.

Explicitly define data being passed into the Vue app

// bad
return new Vue({
  el: '#element',
  components: {
    componentName
  },
  provide: {
    ...someDataset
  },
  props: {
    ...anotherDataset
  },
  render: createElement => createElement('component-name'),
}));

// good
const { foobar, barfoo } = someDataset;
const { foo, bar } = anotherDataset;

return new Vue({
  el: '#element',
  components: {
    componentName
  },
  provide: {
    foobar,
    barfoo
  },
  props: {
    foo,
    bar
  },
  render: createElement => createElement('component-name'),
}));

We discourage the use of the spread operator in this specific case in order to keep our codebase explicit, discoverable, and searchable. This applies in any place where we would benefit from the above, such as when initializing Vuex state. The pattern above also enables us to easily parse non scalar values during instantiation.

return new Vue({
  el: '#element',
  components: {
    componentName
  },
  props: {
    foo,
    bar: parseBoolean(bar)
  },
  render: createElement => createElement('component-name'),
}));

Naming

Extensions: Use .vue extension for Vue components. Do not use .js as file extension (#34371).

Reference Naming: Use PascalCase for their instances:

// bad
import cardBoard from 'cardBoard.vue'

components: {
  cardBoard,
};

// good
import CardBoard from 'cardBoard.vue'

components: {
  CardBoard,
};

Props Naming: Avoid using DOM component prop names.

Props Naming: Use kebab-case instead of camelCase to provide props in templates.

// bad
<component class="btn">

// good
<component css-class="btn">

// bad
<component myProp="prop" />

// good
<component my-prop="prop" />

Alignment

Follow these alignment styles for the template method:

With more than one attribute, all attributes should be on a new line:

// bad
<component v-if="bar"
    param="baz" />

<button class="btn">Click me</button>

// good
<component
  v-if="bar"
  param="baz"
/>

<button class="btn">
  Click me
</button>

The tag can be inline if there is only one attribute:

// good
  <component bar="bar" />

// good
  <component
    bar="bar"
    />

// bad
 <component
    bar="bar" />

Quotes

Always use double quotes " inside templates and single quotes ' for all other JS.

// bad
template: `
  <button :class='style'>Button</button>
`

// good
template: `
  <button :class="style">Button</button>
`

Props

Props should be declared as an object

// bad
props: ['foo']

// good
props: {
  foo: {
    type: String,
    required: false,
    default: 'bar'
  }
}

Required key should always be provided when declaring a prop

// bad
props: {
  foo: {
    type: String,
  }
}

// good
props: {
  foo: {
    type: String,
    required: false,
    default: 'bar'
  }
}

Default key should be provided if the prop is not required. There are some scenarios where we need to check for the existence of the property. On those a default key should not be provided.

// good
props: {
  foo: {
    type: String,
    required: false,
  }
}

// good
props: {
  foo: {
    type: String,
    required: false,
    default: 'bar'
  }
}

// good
props: {
  foo: {
    type: String,
    required: true
  }
}

Data

data method should always be a function

// bad
data: {
  foo: 'foo'
}

// good
data() {
  return {
    foo: 'foo'
  };
}

Directives

Shorthand @ is preferable over v-on

// bad
<component v-on:click="eventHandler"/>

// good
<component @click="eventHandler"/>

Shorthand : is preferable over v-bind

// bad
<component v-bind:class="btn"/>

// good
<component :class="btn"/>

Shorthand # is preferable over v-slot

// bad
<template v-slot:header></template>

// good
<template #header></template>

Closing tags

Prefer self-closing component tags

// bad
<component></component>

// good
<component />

Component usage within templates

Prefer a component's kebab-cased name over other styles when using it in a template
```
// bad
<MyComponent />

// good
<my-component />
```

Ordering

Tag order in .vue file

<script>
  // ...
</script>

<template>
  // ...
</template>

// We don't use scoped styles but there are few instances of this
<style>
  // ...
</style>

Properties in a Vue Component: Check order of properties in components rule.

`:key`

When using v-for you need to provide a unique :key attribute for each item.

If the elements of the array being iterated have an unique id it is advised to use it:

<div
  v-for="item in items"
  :key="item.id"
>
  <!-- content -->
</div>

When the elements being iterated don't have a unique ID, you can use the array index as the :key attribute
```
<div
  v-for="(item, index) in items"
  :key="index"
>
  
</div>
```

When using v-for with template and there is more than one child element, the :key values must be unique. It's advised to use kebab-case namespaces.

<template v-for="(item, index) in items">
  <span :key="`span-${index}`"></span>
  <button :key="`button-${index}`"></button>
</template>

When dealing with nested v-for use the same guidelines as above.

<div
  v-for="item in items"
  :key="item.id"
>
  <span
    v-for="element in array"
    :key="element.id"
  >
    <!-- content -->
  </span>
</div>

Useful links:

Vue testing

Over time, a number of programming patterns and style preferences have emerged in our efforts to effectively test Vue components. The following guide describes some of these. These are not strict guidelines, but rather a collection of suggestions and good practices that aim to provide insight into how we write Vue tests at GitLab.

Mounting a component

Typically, when testing a Vue component, the component should be "re-mounted" in every test block.

To achieve this:

Create a mutable wrapper variable inside the top-level describe block.
Mount the component using mount/ shallowMount.
Reassign the resulting Wrapper instance to our wrapper variable.

Creating a global, mutable wrapper provides a number of advantages, including the ability to:

Define common functions for finding components/DOM elements:

import MyComponent from '~/path/to/my_component.vue';
describe('MyComponent', () => {
  let wrapper;

  // this can now be reused across tests
  const findMyComponent = wrapper.find(MyComponent);
  // ...
})

Use a beforeEach block to mount the component (see the createComponent factory for more information).
Use an afterEach block to destroy the component, for example, wrapper.destroy().

The `createComponent` factory

To avoid duplicating our mounting logic, it's useful to define a createComponent factory function that we can reuse in each test block. This is a closure which should reassign our wrapper variable to the result of mount and shallowMount:

import MyComponent from '~/path/to/my_component.vue';
import { shallowMount } from '@vue/test-utils';

describe('MyComponent', () => {
  // Initiate the "global" wrapper variable. This will be used throughout our test:
  let wrapper;

  // Define our `createComponent` factory:
  function createComponent() {
    // Mount component and reassign `wrapper`:
    wrapper = shallowMount(MyComponent);
  }

  it('mounts', () => {
    createComponent();

    expect(wrapper.exists()).toBe(true);
  });

  it('`isLoading` prop defaults to `false`', () => {
    createComponent();

    expect(wrapper.props('isLoading')).toBe(false);
  });
})

Similarly, we could further de-duplicate our test by calling createComponent in a beforeEach block:

import MyComponent from '~/path/to/my_component.vue';
import { shallowMount } from '@vue/test-utils';

describe('MyComponent', () => {
  // Initiate the "global" wrapper variable. This will be used throughout our test
  let wrapper;

  // define our `createComponent` factory
  function createComponent() {
    // mount component and reassign `wrapper`
    wrapper = shallowMount(MyComponent);
  }

  beforeEach(() => {
    createComponent();
  });

  it('mounts', () => {
    expect(wrapper.exists()).toBe(true);
  });

  it('`isLoading` prop defaults to `false`', () => {
    expect(wrapper.props('isLoading')).toBe(false);
  });
})

`createComponent` best practices

Consider using a single (or a limited number of) object arguments over many arguments. Defining single parameters for common data like props is okay, but keep in mind our JavaScript style guide and stay within the parameter number limit:

// bad
function createComponent(data, props, methods, isLoading, mountFn) { }

// good
function createComponent({ data, props, methods, stubs, isLoading } = {}) { }

// good
function createComponent(props = {}, { data, methods, stubs, isLoading } = {}) { }

If you require both mount and shallowMount within the same set of tests, it can be useful define a mountFn parameter for the createComponent factory that accepts the mounting function (mount or shallowMount) to be used to mount the component:
```
import { shallowMount } from '@vue/test-utils';

function createComponent({ mountFn = shallowMount } = {}) { }
```

Wrap calls to mount and shallowMount in extendedWrapper, this exposes wrapper.findByTestId():

import { shallowMount } from '@vue/test-utils';
import { extendedWrapper } from 'helpers/vue_test_utils_helper';
import { SomeComponent } from 'components/some_component.vue';

let wrapper;

const createWrapper = () => { wrapper = extendedWrapper(shallowMount(SomeComponent)); };
const someButton = () => wrapper.findByTestId('someButtonTestId');

Setting component state

Avoid using setProps to set component state wherever possible. Instead, set the component's propsData when mounting the component:
```
// bad
wrapper = shallowMount(MyComponent);
wrapper.setProps({
  myProp: 'my cool prop'
});

// good
wrapper = shallowMount({ propsData: { myProp: 'my cool prop' } });
```
The exception here is when you wish to test component reactivity in some way. For example, you may want to test the output of a component when after a particular watcher has executed. Using setProps to test such behavior is okay.

Accessing component state

When accessing props or attributes, prefer the wrapper.props('myProp') syntax over wrapper.props().myProp or wrapper.vm.myProp:

// good
expect(wrapper.props().myProp).toBe(true);
expect(wrapper.attributes().myAttr).toBe(true);

// better
expect(wrapper.props('myProp').toBe(true);
expect(wrapper.attributes('myAttr')).toBe(true);

When asserting multiple props, check the deep equality of the props() object with toEqual:

// good
expect(wrapper.props('propA')).toBe('valueA');
expect(wrapper.props('propB')).toBe('valueB');
expect(wrapper.props('propC')).toBe('valueC');

// better
expect(wrapper.props()).toEqual({
  propA: 'valueA',
  propB: 'valueB',
  propC: 'valueC',
});

If you are only interested in some of the props, you can use toMatchObject. Prefer toMatchObject over expect.objectContaining:

// good
expect(wrapper.props()).toEqual(expect.objectContaining({
  propA: 'valueA',
  propB: 'valueB',
}));

// better
expect(wrapper.props()).toMatchObject({
  propA: 'valueA',
  propB: 'valueB',
});

The JavaScript/Vue Accord

The goal of this accord is to make sure we are all on the same page.

When writing Vue, you may not use jQuery in your application.
1. If you need to grab data from the DOM, you may query the DOM 1 time while bootstrapping your application to grab data attributes using dataset. You can do this without jQuery.
2. You may use a jQuery dependency in Vue.js following this example from the docs.
3. If an outside jQuery Event needs to be listen to inside the Vue application, you may use jQuery event listeners.
4. We avoid adding new jQuery events when they are not required. Instead of adding new jQuery events take a look at different methods to do the same task.
You may query the window object one time, while bootstrapping your application for application specific data (for example, scrollTo is ok to access anytime). Do this access during the bootstrapping of your application.
You may have a temporary but immediate need to create technical debt by writing code that does not follow our standards, to be refactored later. Maintainers need to be ok with the tech debt in the first place. An issue should be created for that tech debt to evaluate it further and discuss. In the coming months you should fix that tech debt, with its priority to be determined by maintainers.
When creating tech debt you must write the tests for that code before hand and those tests may not be rewritten. For example, jQuery tests rewritten to Vue tests.
You may choose to use VueX as a centralized state management. If you choose not to use VueX, you must use the store pattern which can be found in the Vue.js documentation.
Once you have chosen a centralized state-management solution you must use it for your entire application. Don't mix and match your state-management solutions.