Migration to Vue 3

Preparations for a Vue 3 migration are tracked in epic &3174

In order to prepare for the eventual migration to Vue 3.x, we should be wary about adding the following features to the codebase:

Vue filters

Why?

Filters are removed from the Vue 3 API completely.

What to use instead

Component’s computed properties / methods or external helpers.

Event hub

Why?

$on, $once, and $off methods are removed from the Vue instance, so in Vue 3 it can’t be used to create an event hub.

What to use instead

Vue documentation recommends using the mitt library. It’s relatively small (200 bytes, compressed) and has a clear API: 

import mitt from 'mitt'

const emitter = mitt()

// listen to an event
emitter.on('foo', e => console.log('foo', e) )

// listen to all events
emitter.on('*', (type, e) => console.log(type, e) )

// fire an event
emitter.emit('foo', { a: 'b' })

// working with handler references:
function onFoo() {}

emitter.on('foo', onFoo)   // listen
emitter.off('foo', onFoo)  // unlisten

Event hub factory

We have created a factory that you can use to instantiate a new mitt-based event hub. This makes it easier to migrate existing event hubs to the new recommended approach, or to create new ones. 

import createEventHub from '~/helpers/event_hub_factory';

export default createEventHub();

Event hubs created with the factory expose the same methods as Vue 2 event hubs ($on, $once, $off and $emit), making them backward compatible with our previous approach.

<template functional>

Why?

In Vue 3, { functional: true } option is removed and <template functional> is no longer supported.

What to use instead

Functional components must be written as plain functions: 

import { h } from 'vue'

const FunctionalComp = (props, slots) => {
  return h('div', `Hello! ${props.name}`)
}

It is not recommended to replace stateful components with functional components unless you absolutely need a performance improvement right now. In Vue 3, performance gains for functional components are negligible.

Old slots syntax with slot attribute

Why?

In Vue 2.6 slot attribute was already deprecated in favor of v-slot directive. The slot attribute usage is still allowed and sometimes we prefer using it because it simplifies unit tests (with old syntax, slots are rendered on shallowMount). However, in Vue 3 we can’t use old syntax anymore.

What to use instead

The syntax with v-slot directive. To fix rendering slots in shallowMount, we need to stub a child component with slots explicitly. 

<!-- MyAwesomeComponent.vue -->
<script>
import SomeChildComponent from './some_child_component.vue'

export default {
  components: {
    SomeChildComponent
  }
}

</script>

<template>
  <div>
    <h1>Hello GitLab!</h1>
    <some-child-component>
      <template #header>
        Header content
      </template>
    </some-child-component>
  </div>
</template>

// MyAwesomeComponent.spec.js

import SomeChildComponent from '~/some_child_component.vue'

shallowMount(MyAwesomeComponent, {
  stubs: {
    SomeChildComponent
  }
})