# Migration Build

# Overview

@vue/compat (aka "the migration build") is a build of Vue 3 that provides configurable Vue 2 compatible behavior.

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.

# Intended Use Cases

  • Upgrading a Vue 2 application to Vue 3 (with limitations)
  • Migrating a library to support Vue 3
  • 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:

# Expectations

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.

# Upgrade Workflow

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 (opens new window). 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

# Installation

  1. Upgrade tooling if applicable.

  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:

    "dependencies": {
    -  "vue": "^2.6.12",
    +  "vue": "^3.1.0",
    +  "@vue/compat": "^3.1.0"
       ...
    },
    "devDependencies": {
    -  "vue-template-compiler": "^2.6.12"
    +  "@vue/compiler-sfc": "^3.1.0"
    }
    
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    Example commit (opens new window)

  3. In the build setup, alias vue to @vue/compat and enable compat mode via Vue compiler options.

    Example Configs

    vue-cli
    // vue.config.js
    module.exports = {
      chainWebpack: config => {
        config.resolve.alias.set('vue', '@vue/compat')
    
        config.module
          .rule('vue')
          .use('vue-loader')
          .tap(options => {
            return {
              ...options,
              compilerOptions: {
                compatConfig: {
                  MODE: 2
                }
              }
            }
          })
      }
    }
    
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    Plain webpack
    // webpack.config.js
    module.exports = {
      resolve: {
        alias: {
          vue: '@vue/compat'
        }
      },
      module: {
        rules: [
          {
            test: /\.vue$/,
            loader: 'vue-loader',
            options: {
              compilerOptions: {
                compatConfig: {
                  MODE: 2
                }
              }
            }
          }
        ]
      }
    }
    
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    Vite
    // vite.config.js
    export default {
      resolve: {
        alias: {
          vue: '@vue/compat'
        }
      },
      plugins: [
        vue({
          template: {
            compilerOptions: {
              compatConfig: {
                MODE: 2
              }
            }
          }
        })
      ]
    }
    
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  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.

    Example commit (opens new window)

  5. After fixing the errors, the app should be able to run if it is not subject to the limitations mentioned above.

    You will likely see a LOT of warnings from both the command line and the browser console. Here are some general tips:

    • 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.

    • You can suppress specific deprecations via compat configuration.

    • 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.

    • If you are using vue-router, note <transition> and <keep-alive> will not work with <router-view> until you upgrade to vue-router v4.

  6. Update <transition> class names. 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.

    Example commit (opens new window)

  7. Update app entry to use new global mounting API.

    Example commit (opens new window)

  8. Upgrade vuex to v4 (opens new window).

    Example commit (opens new window)

  9. Upgrade vue-router to v4 (opens new window). 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 (opens new window).

    Example commit (opens new window)

  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.

    Example commit (opens new window)

  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.

    Example commit (opens new window)

# Compat Configuration

# Global Config

Compat features can be disabled individually:

import { configureCompat } from 'vue'

// disable compat for certain features
configureCompat({
  FEATURE_ID_A: false,
  FEATURE_ID_B: false
})
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Alternatively, the entire application can default to Vue 3 behavior, with only certain compat features enabled:

import { configureCompat } from 'vue'

// default everything to Vue 3 behavior, and only enable compat
// for certain features
configureCompat({
  MODE: 3,
  FEATURE_ID_A: true,
  FEATURE_ID_B: true
})
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# Per-Component Config

A component can use the compatConfig option, which expects the same options as the global configureCompat method:

export default {
  compatConfig: {
    MODE: 3, // opt-in to Vue 3 behavior for this component only
    FEATURE_ID_A: true // features can also be toggled at component level
  }
  // ...
}
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# Compiler-specific Config

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).

# Feature Reference

# Compatibility Types

  • ✔ Fully compatible
  • ◐ Partially Compatible with caveats
  • ⨂ Incompatible (warning only)
  • ⭘ Compat only (no warning)

# Incompatible

Should be fixed upfront or will likely lead to errors

ID Type Description Docs
GLOBAL_MOUNT_CONTAINER Mounted application does not replace the element it's mounted to link
CONFIG_DEVTOOLS production devtools is now a build-time flag link (opens new window)
COMPILER_V_IF_V_FOR_PRECEDENCE v-if and v-for precedence when used on the same element has changed link
COMPILER_V_IF_SAME_KEY v-if branches can no longer have the same key link
COMPILER_V_FOR_TEMPLATE_KEY_PLACEMENT <template v-for> key should now be placed on <template> link
COMPILER_SFC_FUNCTIONAL <template functional> is no longer supported in SFCs link

# Partially Compatible with Caveats

ID Type Description Docs
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
COMPILER_INLINE_TEMPLATE inline-template removed (compat only supported in browser compiler build) link
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
INSTANCE_DESTROY $destroy instance method removed (in compat mode, only supported on root instance)
GLOBAL_PRIVATE_UTIL Vue.util is private and no longer available
CONFIG_PRODUCTION_TIP config.productionTip no longer necessary link
CONFIG_SILENT config.silent removed

# Compat only (no warning)

ID Type Description Docs
TRANSITION_CLASSES Transition enter/leave classes changed link

# Fully Compatible

ID Type Description Docs
GLOBAL_MOUNT new Vue() -> createApp link
GLOBAL_EXTEND Vue.extend removed (use defineComponent or extends option) link
GLOBAL_PROTOTYPE Vue.prototype -> app.config.globalProperties link
GLOBAL_SET Vue.set removed (no longer needed)
GLOBAL_DELETE Vue.delete removed (no longer needed)
GLOBAL_OBSERVABLE Vue.observable removed (use reactive) link
CONFIG_KEY_CODES config.keyCodes removed link
CONFIG_WHITESPACE In Vue 3 whitespace defaults to "condense"
INSTANCE_SET vm.$set removed (no longer needed)
INSTANCE_DELETE vm.$delete removed (no longer needed)
INSTANCE_EVENT_EMITTER vm.$on, vm.$off, vm.$once removed link
INSTANCE_EVENT_HOOKS Instance no longer emits hook:x events link
INSTANCE_CHILDREN vm.$children removed link
INSTANCE_LISTENERS vm.$listeners removed link
INSTANCE_SCOPED_SLOTS vm.$scopedSlots removed; vm.$slots now exposes functions link
INSTANCE_ATTRS_CLASS_STYLE $attrs now includes class and style link
OPTIONS_DATA_FN data must be a function in all cases link
OPTIONS_DATA_MERGE data from mixin or extension is now shallow merged link
OPTIONS_BEFORE_DESTROY beforeDestroy -> beforeUnmount
OPTIONS_DESTROYED destroyed -> unmounted
WATCH_ARRAY watching an array no longer triggers on mutation unless deep link
V_FOR_REF ref inside v-for no longer registers array of refs link
V_ON_KEYCODE_MODIFIER v-on no longer supports keyCode modifiers link
CUSTOM_DIR Custom directive hook names changed link
ATTR_FALSE_VALUE No longer removes attribute if binding value is boolean false link
ATTR_ENUMERATED_COERCION No longer special case enumerated attributes link
TRANSITION_GROUP_ROOT <transition-group> no longer renders a root element by default link
COMPONENT_ASYNC Async component API changed (now requires defineAsyncComponent) link
COMPONENT_FUNCTIONAL Functional component API changed (now must be plain functions) link
COMPONENT_V_MODEL Component v-model reworked link
RENDER_FUNCTION Render function API changed link
FILTERS Filters removed (this option affects only runtime filter APIs) link
COMPILER_IS_ON_ELEMENT is usage is now restricted to <component> only link
COMPILER_V_BIND_SYNC v-bind.sync replaced by v-model with arguments link
COMPILER_V_BIND_PROP v-bind.prop modifier removed
COMPILER_V_BIND_OBJECT_ORDER v-bind="object" is now order sensitive link
COMPILER_V_ON_NATIVE v-on.native modifier removed link
COMPILER_V_FOR_REF ref in v-for (compiler support)
COMPILER_NATIVE_TEMPLATE <template> with no special directives now renders as native element
COMPILER_FILTERS filters (compiler support)