The migration build runs in Vue 2 mode by default - most public APIs behave exactly like Vue 2, with only a few exceptions. Usage of features that have changed or been deprecated in Vue 3 will emit runtime warnings. A feature's compatibility can also be enabled/disabled on a per-component basis.
- For experienced Vue 2 developers who have not tried Vue 3 yet, the migration build can be used in place of Vue 3 to help learn the difference between versions.
### Known Limitations
While we've tried hard to make the migration build mimic Vue 2 behavior as much as possible, there are some limitations that may prevent your app from being eligible for upgrading:
- Dependencies that rely on Vue 2 internal APIs or undocumented behavior. The most common case is usage of private properties on `VNodes`. If your project relies on component libraries like [Vuetify](https://vuetifyjs.com/en/), [Quasar](https://quasar.dev/) or [ElementUI](https://element.eleme.io/#/en-US), it is best to wait for their Vue 3 compatible versions.
- Internet Explorer 11 support: [Vue 3 has officially dropped the plan for IE11 support](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0038-vue3-ie11-support.md). If you still need to support IE11 or below, you will have to stay on Vue 2.
- Server-side rendering: the migration build can be used for SSR, but migrating a custom SSR setup is much more involved. The general idea is replacing `vue-server-renderer` with [`@vue/server-renderer`](https://github.com/vuejs/core/tree/main/packages/server-renderer). Vue 3 no longer provides a bundle renderer and it is recommended to use Vue 3 SSR with [Vite](https://vitejs.dev/guide/ssr.html). If you are using [Nuxt.js](https://nuxtjs.org/), it is probably better to wait for Nuxt 3.
Please note that the migration build aims to cover only publicly documented Vue 2 APIs and behavior. If your application fails to run under the migration build due to reliance on undocumented behavior, it is unlikely that we'll tweak the migration build to cater to your specific case. Consider refactoring to remove reliance on the behavior in question instead.
A word of caution: if your application is large and complex, migration will likely be a challenge even with the migration build. If your app is unfortunately not suitable for upgrade, do note that we are planning to backport Composition API and some other Vue 3 features to the 2.7 release (estimated late Q3 2021).
If you do get your app running on the migration build, you **can** ship it to production before the migration is complete. Although there is a small performance/size overhead, it should not noticeably affect production UX. You may have to do so when you have dependencies that rely on Vue 2 behavior, and cannot be upgraded/replaced.
The migration build will be provided starting with 3.1, and will continue to be published alongside the 3.2 release line. We do plan to eventually stop publishing the migration build in a future minor version (no earlier than EOY 2021), so you should still aim to switch to the standard build before then.
The following workflow walks through the steps of migrating an actual Vue 2 app (Vue HackerNews 2.0) to Vue 3. The full commits can be found [here](https://github.com/vuejs/vue-hackernews-2.0/compare/migration). Note that the actual steps required for your project may vary, and these steps should be treated as general guidance rather than strict instructions.
### Preparations
- If you are still using the [deprecated named / scoped slot syntax](https://vuejs.org/v2/guide/components-slots.html#Deprecated-Syntax), update it to the latest syntax first (which is already supported in 2.6).
2. In `package.json`, update `vue` to 3.1, install `@vue/compat` of the same version, and replace `vue-template-compiler` (if present) with `@vue/compiler-sfc`:
4. At this point, your application may encounter some compile-time errors / warnings (e.g. use of filters). Fix them first. If all compiler warnings are gone, you can also set the compiler to Vue 3 mode.
- You can filter for specific warnings in the browser console. It's a good idea to use the filter and focus on fixing one item at a time. You can also use negated filters like `-GLOBAL_MOUNT`.
- Some warnings may be caused by a dependency that you use (e.g. `vue-router`). You can check this from the warning's component trace or stack trace (expanded on click). Focus on fixing the warnings that originate from your own source code first.
6. Update [`<transition>` class names](https://v3-migration.vuejs.org/breaking-changes/transition.html). This is the only feature that does not have a runtime warning. You can do a project-wide search for `.*-enter` and `.*-leave` CSS class names.
9. [Upgrade `vue-router` to v4](https://next.router.vuejs.org/guide/migration/index.html). If you also use `vuex-router-sync`, you can replace it with a store getter.
After the upgrade, to use `<transition>` and `<keep-alive>` with `<router-view>` requires using the new [scoped-slot based syntax](https://next.router.vuejs.org/guide/migration/index.html#router-view-keep-alive-and-transition).
10. Pick off individual warnings. Note some features have conflicting behavior between Vue 2 and Vue 3 - for example, the render function API, or the functional component vs. async component change. To migrate to Vue 3 API without affecting the rest of the application, you can opt-in to Vue 3 behavior on a per-component basis with the [`compatConfig` option](#per-component-config).
11. When all warnings are fixed, you can remove the migration build and switch to Vue 3 proper. Note you may not be able to do so if you still have dependencies that rely on Vue 2 behavior.
Features that start with `COMPILER_` are compiler-specific: if you are using the full build (with in-browser compiler), they can be configured at runtime. However if using a build setup, they must be configured via the `compilerOptions` in the build config instead (see example configs above).
| GLOBAL_MOUNT_CONTAINER | ⨂ | Mounted application does not replace the element it's mounted to | [link](https://v3-migration.vuejs.org/breaking-changes/mount-changes.html) |
| CONFIG_DEVTOOLS | ⨂ | production devtools is now a build-time flag | [link](https://github.com/vuejs/core/tree/main/packages/vue#bundler-build-feature-flags) |
| COMPILER_V_IF_V_FOR_PRECEDENCE | ⨂ | `v-if` and `v-for` precedence when used on the same element has changed | [link](https://v3-migration.vuejs.org/breaking-changes/v-if-v-for.html) |
| COMPILER_V_IF_SAME_KEY | ⨂ | `v-if` branches can no longer have the same key | [link](https://v3-migration.vuejs.org/breaking-changes/key-attribute.html#on-conditional-branches) |
| COMPILER_V_FOR_TEMPLATE_KEY_PLACEMENT | ⨂ | `<template v-for>` key should now be placed on `<template>` | [link](https://v3-migration.vuejs.org/breaking-changes/key-attribute.html#with-template-v-for) |
| COMPILER_SFC_FUNCTIONAL | ⨂ | `<template functional>` is no longer supported in SFCs | [link](https://v3-migration.vuejs.org/breaking-changes/functional-components.html#single-file-components-sfcs) | | |
| CONFIG_IGNORED_ELEMENTS | ◐ | `config.ignoredElements` is now `config.compilerOptions.isCustomElement` (only in browser compiler build). If using build setup, `isCustomElement` must be passed via build configuration. | [link](https://v3-migration.vuejs.org/breaking-changes/global-api.html#config-ignoredelements-is-now-config-iscustomelement) |
| COMPILER_INLINE_TEMPLATE | ◐ | `inline-template` removed (compat only supported in browser compiler build) | [link](https://v3-migration.vuejs.org/breaking-changes/inline-template-attribute.html) |
| PROPS_DEFAULT_THIS | ◐ | props default factory no longer have access to `this` (in compat mode, `this` is not a real instance - it only exposes props, `$options` and injections) | [link](https://v3-migration.vuejs.org/breaking-changes/props-default-this.html) |
| INSTANCE_ATTRS_CLASS_STYLE | ✔ | `$attrs` now includes `class` and `style` | [link](https://v3-migration.vuejs.org/breaking-changes/attrs-includes-class-style.html) |
| OPTIONS_DATA_FN | ✔ | `data` must be a function in all cases | [link](https://v3-migration.vuejs.org/breaking-changes/data-option.html) |
| OPTIONS_DATA_MERGE | ✔ | `data` from mixin or extension is now shallow merged | [link](https://v3-migration.vuejs.org/breaking-changes/data-option.html) |
| WATCH_ARRAY | ✔ | watching an array no longer triggers on mutation unless deep | [link](https://v3-migration.vuejs.org/breaking-changes/watch.html) |
| V_FOR_REF | ✔ | `ref` inside `v-for` no longer registers array of refs | [link](https://v3-migration.vuejs.org/breaking-changes/array-refs.html) |
| ATTR_FALSE_VALUE | ✔ | No longer removes attribute if binding value is boolean `false` | [link](https://v3-migration.vuejs.org/breaking-changes/attribute-coercion.html) |
| ATTR_ENUMERATED_COERSION | ✔ | No longer special case enumerated attributes | [link](https://v3-migration.vuejs.org/breaking-changes/attribute-coercion.html) |
| TRANSITION_GROUP_ROOT | ✔ | `<transition-group>` no longer renders a root element by default | [link](https://v3-migration.vuejs.org/breaking-changes/transition-group.html) |
| COMPILER_IS_ON_ELEMENT | ✔ | `is` usage is now restricted to `<component>` only | [link](https://v3-migration.vuejs.org/breaking-changes/custom-elements-interop.html) |
| COMPILER_V_BIND_SYNC | ✔ | `v-bind.sync` replaced by `v-model` with arguments | [link](https://v3-migration.vuejs.org/breaking-changes/v-model.html) |
