Nuxt/docs/content/3.docs/3.migration/20.module-authors.md

# Modules

## Module Compatibility

Nuxt 3 has a basic backward compatibility layer for Nuxt 2 modules using `@nuxt/kit` auto wrappers. But there are usually steps to follow to make modules compatible with Nuxt 3 and sometimes, using Nuxt Bridge is required for cross-version compatibility.

We have prepared a [Dedicated Guide](/docs/advanced/modules) for authoring Nuxt 3 ready modules using `@nuxt/kit`. Currently best migration path is to follow it and rewrite your modules. Rest of this guide includes preparation steps if you prefer to avoid a full rewrite yet making modules compatibile with Nuxt 3.

::alert
You can check for a list of Nuxt 3 ready modules from [Nuxt Modules](https://modules.nuxtjs.org/?version=3.x).
::

### Plugin Compatibility

Nuxt 3 plugins are **not** fully backward compatible with Nuxt 2.

### Vue Compatibility

Plugins or components using the composition API need exclusive Vue 2 or Vue 3 support.

By using [vue-demi](https://github.com/vueuse/vue-demi) they should be compatible with both Nuxt 2 and 3.

## Module Migration

When Nuxt 3 users add your module, a compatible module container layer from `@nuxt/kit` is **automatically injected**, so as long as your code is following the guidelines below, it should continue working as-is.

### Test with `@nuxt/bridge`

Migrating to `@nuxt/bridge` is the first and most important step for supporting Nuxt 3.

If you have a fixture or example in your module, add `@nuxt/bridge` package to its config (see [example](/getting-started/bridge#update-nuxtconfig))

### Migrate from CommonJS to ESM

Nuxt 3 natively supports TypeScript and ECMAScript Modules. Please check [Native ES Modules](/concepts/esm) for more info and upgrading.

### Ensure plugins default export

If you inject a Nuxt plugin that does not have `export default` (such as global Vue plugins), ensure you add `export default () => { }` to the end of it.

::code-group

```js [Before]
// ~/plugins/vuelidate.js
import Vue from 'vue'
import Vuelidate from 'vuelidate'

Vue.use(Vuelidate)
```

```js [After]
// ~/plugins/vuelidate.js
import Vue from 'vue'
import Vuelidate from 'vuelidate'

Vue.use(Vuelidate)

export default () => { }
```

::

### Avoid runtime modules

With Nuxt 3, Nuxt is now a build-time-only dependency, which means that modules shouldn't attempt to hook into the Nuxt runtime.

Your module should work even if it's only added to [`buildModules`](/docs/directory-structure/nuxt.config#buildmodules) (instead of `modules`). For example:

- Avoid updating `process.env` within a Nuxt module and reading by a nuxt plugin; use [`runtimeConfig`](/docs/directory-structure/nuxt.config#publicruntimeconfig) instead.
- (*) Avoid depending on runtime hooks like `vue-renderer:*` for production
- (*) Avoid adding `serverMiddleware` by importing them inside the module. Instead, add them by referencing a file path so that they are independent of the module's context

(*) Unless it is for `nuxt dev` purpose only and guarded with `if (nuxt.options.dev) { }`.

::alert{type=info icon=🔎}
Continue reading about Nuxt 3 modules in the [Modules guide](/docs/advanced/modules).
::

### Use TypeScript (optional)

While it is not essential, most of the Nuxt ecosystem is shifting to use TypeScript, so it is highly recommended to consider migration.

::alert{icon=💡}
You can start migration by renaming `.js` files, to `.ts`. TypeScript is designed to be progressive!
::

::alert{icon=💡}
You can use TypeScript syntax for Nuxt 2 and 3 modules and plugins without any extra dependencies.
::