Migration to Vue 3

The migration from Vue 2 to 3 is tracked in epic &6252.

To ease migration to Vue 3.x, we have added ESLint rules that prevent us from using the following deprecated features in the codebase.

GitLab can use Vue 3 (@vue/compat)

The GitLab frontend team has enabled Vue 3 (@vue/compat) for development environments like GDK. While not yet production-ready, you can opt-in locally to verify your client code is forward-compatible with Vue 3.

How does it work? When the build tool (Vite or Webpack) detects the VUE_VERSION=3 environment variable, it uses module aliasing to swap out certain dependencies, including Vue itself, for their Vue 3-compatible counterparts.

Some of these replacement libraries are maintained by the team and act as thin wrappers around existing libraries, making them Vue 3-compatible without requiring any changes in consumer code.

Setup GDK to use Vue 3 (@vue/compat)

This guide walks you through configuring the GitLab Development Kit (GDK) to use Vite as the build tool with Vue 3.

Prerequisites

GDK installed and configured
Basic familiarity with Vue.js and Vite
Vite configured in your GDK environment (see GDK Vite Settings)

Initial Setup

Switching Between Vue Versions

To switch between Vue 2 and Vue 3, follow these steps:

Set the desired Vue version:

gdk config set vite.vue_version 3  # or 2

Reconfigure GDK:
```
gdk reconfigure
```

Restart GDK:

gdk restart # or `gdk start` if running for the first time

Important: You can clear caches with yarn clean or gdk kill vite if you face issues switching Vue versions.

Verifying Your Setup

You can verify your Vite configuration by checking your gdk.yml file:

gdk config get vite

This should display your current Vite settings, including the enabled status and Vue version. Your GDK should also be up and running.

---
enabled: true
hot_module_reloading: true
https:
  enabled: true
port: 3038
vue_version: 3

Troubleshooting

General Debugging

When encountering issues, start by checking the Vite logs:

gdk tail vite

This shows real-time Vite output and error messages that can help identify the problem.

Build Errors After Switching Versions

If you encounter build errors after switching Vue versions:

Ensure you’ve cleared the Vite cache with yarn clean
Try clearing node_modules and reinstalling dependencies:
```
rm -rf node_modules
yarn install
```

Vite Not Starting

If Vite fails to start:

Check that vite.enabled is set to true
Verify your Node.js version meets Vite’s requirements
Review GDK logs for specific error messages

Additional Resources

Compatibility changes

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.

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

We have created a factory that you can use to instantiate a new mitt-like 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
  }
})

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.

<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, the props default value factory is passed the raw props as an argument, and can also access injections.

Handling libraries that do not work with `@vue/compat`

Problem

Some libraries rely on Vue.js 2 internals. They might not work with @vue/compat, so we have added an adapter or replacements as a compatibility layer.

Goals

We should add as few changes as possible to existing code to support new libraries. Instead, we should *add- new code, which will act as facade, making the new version compatible with the old one
Switching between new and old versions should be hidden inside tooling (webpack / jest) and should not be exposed to the code
All facades specific to migration should live in the same directory to simplify future migration steps

Testing

For more information about implementing or fixing tests that fail while using Vue 3, read the Vue 3 testing guide.

Migration to Vue 3

GitLab can use Vue 3 (@vue/compat)

Setup GDK to use Vue 3 (@vue/compat)

Prerequisites

Initial Setup

Switching Between Vue Versions

Verifying Your Setup

Troubleshooting

General Debugging

Build Errors After Switching Versions

Vite Not Starting

Additional Resources

Compatibility changes

Vue filters

Event hub

<template functional>

Old slots syntax with slot attribute

Props default function this access

Handling libraries that do not work with @vue/compat

Testing

Old slots syntax with `slot` attribute

Props default function `this` access

Handling libraries that do not work with `@vue/compat`