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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. They 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:

  1. Set the desired Vue version:

    gdk config set vite.vue_version 3  # or 2

  2. Reconfigure GDK:

    gdk reconfigure

  3. 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:

  1. Ensure you’ve cleared the Vite cache with yarn clean

  2. 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 works 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.

Vue.observable

Why?

Vue.observable creates reactive state that is tied to the Vue version that created it. In the hybrid Vue 2/Vue 3 infection system, modules can be duplicated — one copy for each Vue version. When these modules use Vue.observable(), each copy creates its own separate reactive object, so state changes in one are invisible to the other.

What to use instead

Use observable() from ~/lib/utils/observable:

import { observable } from '~/lib/utils/observable';

// Before
export const state = Vue.observable({ count: 0 });

// After
export const state = observable('unique_key', { count: 0 });

The observable(key, defaults) function:

  • Stores a single canonical state in a global registry keyed by key
  • Creates a per-Vue-context reactive mirror via Vue.observable() internally
  • Returns a Proxy that syncs writes to all mirrors across Vue versions
  • Supports flat objects, getters, and methods

The key must be a unique string identifier (for example, 'super_sidebar_state'). It ensures both module copies share the same underlying state.

An ESLint rule (no-restricted-properties) enforces this — direct Vue.observable usage produces a lint error.

Limitations

  • Flat objects only: Nested mutations like state.nested.prop = value or state.array.push(item) do not sync across Vue versions. Refactor to top-level property replacement instead:

    // Instead of: state.items.push(newItem)
state.items = [...state.items, newItem];

// Instead of: state.config[key] = value
state.config = { ...state.config, [key]: value };

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 acts 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

Migrate to Vue 3

For general Vue 3 migration information, see the Vue 3 official migration guide.

Option 1 (recommended): Migrate your page entrypoint using a feature flag and vue3_migration.yml

GitLab declares the migration state of every page entry under app/assets/javascripts/pages (and the EE and JH equivalents) in a vue3_migration.yml file co-located with the page’s entrypoint, called index.js.

└── pages/
    └── [area]/
        └── [page]/
            ├── index.js              # Entrypoint: imports and calls the initializer
            └── vue3_migration.yml    # Declares the page's migration status

The bundler reads these files to decide whether to emit a Vue 3 chunk, and Rails decides at request time which chunk to render depending on the feature flag.

status: rollout
feature_flag: vue3_migrate_jobs # we recommend naming the flag `vue3_migrate_<page>`
group: group::pipeline authoring # optional
migration_issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/... # optional

When the file exists, it must declare a status field with one of two values:

  • rollout: both Vue 2 and Vue 3 chunks are built (the Vue 3 chunk as a .vue3 sibling entrypoint). Rails serves the Vue 3 chunk when the declared feature flag is enabled, and the Vue 2 chunk otherwise. A feature_flag field is required.
  • migrated: only the Vue 3 chunk is built, under the original entrypoint name, so Rails serves it with no lookup at all. Use this after the feature flag has been fully rolled out and removed.

Optional fields group and migration_issue are accepted for documentation. The schema is defined in config/helpers/vue3_migration_file_validation.js.

How the metadata reaches production

Rails does not read the vue3_migration.yml files at runtime in production: packaged builds (for example, Omnibus) strip app/assets from the Rails application, so the files do not exist there. Instead, the webpack build compiles the rollout entries into a single public/assets/webpack/vue3_migration.json manifest (see config/plugins/vue3_migration_manifest_plugin.js), which ships with the compiled assets in every distribution. At runtime, Gitlab::Vue3Migration:

  • In development and test, reads the source vue3_migration.yml files directly. This works under both the webpack and Vite development servers.
  • In production, reads the compiled vue3_migration.json. A missing manifest raises an error rather than silently serving Vue 2; rake gitlab:assets:compile also fails when webpack does not emit the file, so packaged builds cannot ship without it.

If Vite becomes the production bundler, its build must emit an equivalent manifest.

To verify which apps run under Vue 3 on any environment, query the DOM marker set by the Vue 3 runtime: document.querySelectorAll('[data-gitlab-vue3-app]'). An end-to-end spec (qa/qa/specs/features/browser_ui/4_verify/pipeline/vue3_rollout_spec.rb) uses this marker to verify the rollout mechanism against packaged builds.

When a page has an index.js in more than one root, for example CE and EE, the corresponding vue3_migration.yml files must agree on status and feature_flag. Both the bundler and the validator spec reject inconsistent states.

Migration steps

  1. Identify your page’s entrypoint under app/assets/javascripts/pages (or ee/...). For example, app/assets/javascripts/pages/projects/jobs/show/index.js.

  2. Create or update the feature flag in config/feature_flags/. The migration mechanism uses the current user as the actor.

  3. Create a vue3_migration.yml file next to the page’s index.js, declaring status: rollout with the feature flag name:

    # app/assets/javascripts/pages/projects/jobs/show/vue3_migration.yml
status: rollout
feature_flag: vue3_migrate_jobs

    If the page is shadowed across CE and EE, add the file to whichever directory currently owns the index.js, or to both if both directories contain an index.js. CE and EE YAMLs for the same page must agree on status and feature_flag.

  4. Enable the feature flag and load the page locally.

  5. Verify the message "[gitlab] [V] Using Vue.js 3 (with @vue/compat) for ... appears in the console, this means the page is running on Vue 3.

  6. Verify the app works correctly locally.

  7. Open an MR with your changes and get them merged!

  8. Proceed with the feature flag rollout with the user actor.

  9. Upon removing the feature flag, change the YAML to status: migrated and remove the feature_flag line. You are done!

Option 2: Migrate your page partially using ?vue3

Internally, the Vue 3 entrypoints are generated by appending a ?vue3 query parameter which is read by webpack and vite. This allows you to migrate larger apps partially by adding ?vue3 at any level of the dependency tree.

This requires more boilerplate: you must add the feature flag to your controller manually and your code must add ?vue3 to the relevant imports.

This option should be used only if your app cannot be migrated with Option 1.

Common migration issues

Vue Router props reactivity

Router props passed with the props function are not reactive in Vue 3. Use computed properties that read from this.$route instead.

// Component - use computed property instead of props
computed: {
  currentPath() {
    return this.$route?.params.path || '';
  }
}

Watch expressions

Watch specific route properties using string paths, not the entire $route object. You can still use deep: true with $route, but it adds performance overhead. Watching specific properties is more efficient and explicit about your component’s dependencies.

watch: {
  '$route.params.path'() {
    this.fetchData();
  }
}

Testing

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

Updating @vue/compat patches

See this document for information about how to update our @vue/compat patches, as it can be tricky.