Migration to Vue 3

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


Filters are removed from the Vue 3 API completely.

What to use instead

Component’s computed properties / methods or external helpers.

Event hub


$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 docs recommend using mitt library. It’s relatively small (200 bytes gzipped) and has a simple 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

To make it easier for you to migrate existing event hubs to the new recommended approach, or simply to create new ones, we have created a factory that you can use to instantiate a new mitt-based event hub.

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>


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 will be negligible.

Old slots syntax with slot attribute


In Vue 2.6 slot attribute was already deprecated in favor of v-slot directive but its usage is still allowed and sometimes we prefer using them 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 -->
import SomeChildComponent from './some_child_component.vue'

export default {
  components: {


    <h1>Hello GitLab!</h1>
      <template #header>
        Header content
// MyAwesomeComponent.spec.js

import SomeChildComponent from '~/some_child_component.vue'

shallowMount(MyAwesomeComponent, {
  stubs: {