Migration to Vue 2.7

Vue 2.7 is the latest minor version of Vue 2. It provides built-in support for the Composition API.

Despite Vue 3 now being the default version, we understand that there are still many users who have to stay on Vue 2 due to dependency compatibility, browser support requirements, or simply not enough bandwidth to upgrade. In Vue 2.7, we have backported some of the most important features from Vue 3 so that Vue 2 users can benefit from them as well.

Backported Features

• Composition API
• SFC <script setup>
• SFC CSS v-bind

In addition, the following APIs are also supported:

• defineComponent() with improved type inference (compared to Vue.extend)
• h(), useSlot(), useAttrs(), useCssModules()
• set(), del() and nextTick() are also provided as named exports in ESM builds.

• The emits option is also supported, but only for type-checking purposes (does not affect runtime behavior)

  2.7 also supports using ESNext syntax in template expressions. When using a build system, the compiled template render function will go through the same loaders / plugins configured for normal JavaScript. This means if you have configured Babel for .js files, it will also apply to the expressions in your SFC templates.

Notes on API exposure

• In ESM builds, these APIs are provided as named exports (and named exports only):

  import Vue, { ref } from "vue"; Vue.ref; // undefined, use named export instead

• In UMD and CJS builds, these APIs are exposed as properties on the global Vue object.

• When bundling with CJS builds externalized, bundlers should be able to handle ESM interop when externalizing CJS builds.

Behavior Differences from Vue 3

The Composition API is backported using Vue 2’s getter/setter-based reactivity system to ensure browser compatibility. This means there are some important behavior differences from Vue 3’s proxy-based system:

• All Vue 2 change detection caveats still apply.

• reactive(), ref(), and shallowReactive() will directly convert original objects instead of creating proxies. This means:

  // true in 2.7, false in 3.x reactive(foo) === foo;

• readonly() does create a separate object, but it won’t track newly added properties and does not work on arrays.

• Avoid using arrays as root values in reactive() because without property access the array’s mutation won’t be tracked (this will result in a warning).

• Reactivity APIs ignore properties with symbol keys.

In addition, the following features are explicitly NOT ported:

• ❌ createApp() (Vue 2 doesn’t have isolated app scope)
• ❌ Top-level await in <script setup> (Vue 2 does not support async component initialization)
• ❌ TypeScript syntax in template expressions (incompatible w/ Vue 2 parser)
• ❌ Reactivity transform (still experimental)
• ❌ expose option is not supported for options components (but defineExpose() is supported in <script setup>).

Upgrade Guide

Vue CLI / webpack

1. Upgrade local @vue/cli-xxx dependencies the latest version in your major version range (if applicable):

   • ~4.5.18 for v4
   • ~5.0.6 for v5

2. Upgrade vue to ^2.7.0. You can also remove vue-template-compiler from the dependencies - it is no longer needed in 2.7.

   Note: if you are using @vue/test-utils, you will need to keep vue-template-compiler in the dependencies because test utils rely on some APIs only exposed in this package.

3. Check your package manager lockfile to ensure the following dependencies meet the version requirements. They may be transitive dependencies not listed in package.json.

   • vue-loader: ^15.10.0
   • vue-demi: ^0.13.1

   If not, you will need to remove node_modules and the lockfile and perform a fresh install to ensure they are bumped to the latest version.

4. If you were previously using @vue/composition-api, update imports from it to vue instead. Note that some APIs exported by the plugin, e.g. createApp, are not ported in 2.7.

5. Update eslint-plugin-vue to latest version (9+) if you run into unused variable lint errors when using <script setup>.

6. The SFC compiler for 2.7 now uses PostCSS 8 (upgraded from 7). PostCSS 8 should be backwards compatible with most plugins, but the upgrade may cause issues if you were previously using a custom PostCSS plugin that can only work with PostCSS 7. In such cases, you will need to upgrade the relevant plugins to their PostCSS 8 compatible versions.

Vite

2.7 support for Vite is provided via a new plugin: @vitejs/plugin-vue2. This new plugin requires Vue 2.7 or above and supersedes the existing vite-plugin-vue2.

Note that the new plugin does not handle Vue-specific JSX / TSX transform, which is intentional. Vue 2 JSX / TSX transform for Vite is handled in a separate, dedicated plugin: @vitejs/plugin-vue2-jsx.

Volar Compatibility

2.7 ships improved type definitions so it is no longer necessary to install @vue/runtime-dom just for Volar template type inference support. All you need now is the following config in tsconfig.json:

{
  // ...
  "vueCompilerOptions": {
    "target": 2.7
  }
}

Devtools Support

Vue Devtools 6.2.0 has added support for inspecting 2.7 Composition API state, but the extensions may still need a few days to go through review on respective publishing platforms.

Implications of the 2.7 Release

As stated before, 2.7 is the final minor release of Vue 2.x. After this release, Vue 2 has entered LTS (long-term support) which lasts for 18 months from now, and will no longer receive new features.

This means Vue 2 will reach End of Life on December 31st, 2023. We believe this should provide plenty of time for most of the ecosystem to migrate over to Vue 3. However, we also understand that there could be teams or projects that cannot upgrade by this timeline while still need to fullfil security and compliance requirements. If your team expects to be using Vue 2 beyond end of 2023, make sure to plan head and understand your options: learn more about Vue 2 LTS and Extended Support.