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---
stage: none
group: unassigned
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
---
# Migration to Vue 3
Preparations for a Vue 3 migration are tracked in epic [&3174](https://gitlab.com/groups/gitlab-org/-/epics/3174)
In order to prepare for the eventual migration to Vue 3.x, we should not use the following deprecated features in the codebase:
NOTE:
Our linting rules block the use of these deprecated features.
## Vue filters
**Why?**
Filters [are removed](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0015-remove-filters.md) from the Vue 3 API completely.
**What to use instead**
Component's computed properties / methods or external helpers.
## Event hub
**Why?**
`$on`, `$once`, and `$off` methods [are removed](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0020-events-api-change.md) from the Vue instance, so in Vue 3 it can't be used to create an event hub.
**When to use**
If you are in a Vue app that doesn't use any event hub, try to avoid adding a new one unless absolutely necessary. For example, if you need a child component to react to its parent's event, it's preferred to pass a prop down. Then, use the watch property on that prop in the child component to create the desired side effect.
If you need cross-component communication (between different Vue apps), then perhaps introducing a hub is the right decision.
**What to use instead**
Vue documentation recommends using the [mitt](https://github.com/developit/mitt) library. It's relatively small (200 bytes, compressed) and has a clear API:
```javascript
import mitt from 'mitt'
const emitter = mitt()
// listen to an event
emitter.on('foo', e => console.log('foo', e) )
// listen to all events
emitter.on('*', (type, e) => console.log(type, e) )
// fire an event
emitter.emit('foo', { a: 'b' })
// working with handler references:
function onFoo() {}
emitter.on('foo', onFoo) // listen
emitter.off('foo', onFoo) // unlisten
```
**Event hub factory**
We have created a factory that you can use to instantiate a new mitt-based event hub.
This makes it easier to migrate existing event hubs to the new recommended approach, or
to create new ones.
```javascript
import createEventHub from '~/helpers/event_hub_factory';
export default createEventHub();
```
Event hubs created with the factory expose the same methods as Vue 2 event hubs (`$on`, `$once`, `$off` and
`$emit`), making them backward compatible with our previous approach.
## \<template functional>
**Why?**
In Vue 3, `{ functional: true }` option [is removed](https://github.com/vuejs/rfcs/blob/functional-async-api-change/active-rfcs/0007-functional-async-api-change.md) and `<template functional>` is no longer supported.
**What to use instead**
Functional components must be written as plain functions:
```javascript
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.
```html
<!-- 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>
```
```javascript
// MyAwesomeComponent.spec.js
import SomeChildComponent from '~/some_child_component.vue'
shallowMount(MyAwesomeComponent, {
stubs: {
SomeChildComponent
}
})
```
## Props default function `this` access
**Why?**
In Vue 3, props default value factory functions no longer have access to `this`
(the component instance).
**What to use instead**
Write a computed prop that resolves the desired value from other props. This
will work in both Vue 2 and 3.
```html
<script>
export default {
props: {
metric: {
type: String,
required: true,
},
title: {
type: String,
required: false,
default: null,
},
},
computed: {
actualTitle() {
return this.title ?? this.metric;
},
},
}
</script>
<template>
<div>{{ actualTitle }}</div>
</template>
```
[In Vue 3](https://v3.vuejs.org/guide/migration/props-default-this.html#props-default-function-this-access),
the props default value factory is passed the raw props as an argument, and can
also access injections.
|
