Environment API for Plugins

Experimental

Initial work for this API was introduced in Vite 5.1 with the name "Vite Runtime API". This guide describes a revised API, renamed to Environment API. This API will be released in Vite 6 as experimental. You can already test it in the latest vite@6.0.0-beta.x version.

Resources:

• Feedback discussion where we are gathering feedback about the new APIs.
• Environment API PR where the new API were implemented and reviewed.

Please share with us your feedback as you test the proposal.

Accessing the current environment in hooks

Given that there were only two Environments until Vite 6 (client and ssr), a ssr boolean was enough to identify the current environment in Vite APIs. Plugin Hooks received a ssr boolean in the last options parameter, and several APIs expected an optional last ssr parameter to properly associate modules to the correct environment (for example server.moduleGraph.getModuleByUrl(url, { ssr })).

With the advent of configurable environments, we now have a uniform way to access their options and instance in plugins. Plugin hooks now expose this.environment in their context, and APIs that previously expected a ssr boolean are now scoped to the proper environment (for example environment.moduleGraph.getModuleByUrl(url)).

The Vite server has a shared plugin pipeline, but when a module is processed it is always done in the context of a given environment. The environment instance is available in the plugin context.

A plugin could use the environment instance to change how a module is processed depending on the configuration for the environment (which can be accessed using environment.config).

  transform(code, id) {
    console.log(this.environment.config.resolve.conditions)
  }

Registering new environments using hooks

Plugins can add new environments in the config hook (for example to have a separate module graph for RSC):

  config(config: UserConfig) {
    config.environments.rsc ??= {}
  }

An empty object is enough to register the environment, default values from the root level environment config.

Configuring environment using hooks

While the config hook is running, the complete list of environments isn't yet known and the environments can be affected by both the default values from the root level environment config or explicitly through the config.environments record. Plugins should set default values using the config hook. To configure each environment, they can use the new configEnvironment hook. This hook is called for each environment with its partially resolved config including resolution of final defaults.

  configEnvironment(name: string, options: EnvironmentOptions) {
    if (name === 'rsc') {
      options.resolve.conditions = // ...

The hotUpdate hook

• Type: (this: { environment: DevEnvironment }, options: HotUpdateOptions) => Array<EnvironmentModuleNode> | void | Promise<Array<EnvironmentModuleNode> | void>
• See also: HMR API

The hotUpdate hook allows plugins to perform custom HMR update handling for a given environment. When a file changes, the HMR algorithm is run for each environment in series according to the order in server.environments, so the hotUpdate hook will be called multiple times. The hook receives a context object with the following signature:

interface HotUpdateContext {
  type: 'create' | 'update' | 'delete'
  file: string
  timestamp: number
  modules: Array<EnvironmentModuleNode>
  read: () => string | Promise<string>
  server: ViteDevServer
}

• this.environment is the module execution environment where a file update is currently being processed.

• modules is an array of modules in this environment that are affected by the changed file. It's an array because a single file may map to multiple served modules (e.g. Vue SFCs).

• read is an async read function that returns the content of the file. This is provided because, on some systems, the file change callback may fire too fast before the editor finishes updating the file, and direct fs.readFile will return empty content. The read function passed in normalizes this behavior.

The hook can choose to:

• Filter and narrow down the affected module list so that the HMR is more accurate.

• Return an empty array and perform a full reload:

  hotUpdate({ modules, timestamp }) {
    if (this.environment.name !== 'client')
      return
  
    // Invalidate modules manually
    const invalidatedModules = new Set()
    for (const mod of modules) {
      this.environment.moduleGraph.invalidateModule(
        mod,
        invalidatedModules,
        timestamp,
        true
      )
    }
    this.environment.hot.send({ type: 'full-reload' })
    return []
  }

• Return an empty array and perform complete custom HMR handling by sending custom events to the client:

  hotUpdate() {
    if (this.environment.name !== 'client')
      return
  
    this.environment.hot.send({
      type: 'custom',
      event: 'special-update',
      data: {}
    })
    return []
  }

  Client code should register the corresponding handler using the HMR API (this could be injected by the same plugin's transform hook):

  if (import.meta.hot) {
    import.meta.hot.on('special-update', (data) => {
      // perform custom update
    })
  }

Per-environment Plugins

A plugin can define what are the environments it should apply to with the applyToEnvironment function.

const UnoCssPlugin = () => {
  // shared global state
  return {
    buildStart() {
      // init per environment state with WeakMap<Environment,Data>, this.environment
    },
    configureServer() {
      // use global hooks normally
    },
    applyToEnvironment(environment) {
      // return true if this plugin should be active in this environment
      // if the function isn't provided, the plugin is active in all environments
    },
    resolveId(id, importer) {
      // only called for environments this plugin apply to
    },
  }
}

Environment in build hooks

In the same way as during dev, plugin hooks also receive the environment instance during build, replacing the ssr boolean. This also works for renderChunk, generateBundle, and other build only hooks.

Shared plugins during build

Before Vite 6, the plugins pipelines worked in a different way during dev and build:

• During dev: plugins are shared
• During Build: plugins are isolated for each environment (in different processes: vite build then vite build --ssr).

This forced frameworks to share state between the client build and the ssr build through manifest files written to the file system. In Vite 6, we are now building all environments in a single process so the way the plugins pipeline and inter-environment communication can be aligned with dev.

In a future major (Vite 7 or 8), we aim to have complete alignment:

• During both dev and build: plugins are shared, with per-environment filtering

There will also be a single ResolvedConfig instance shared during build, allowing for caching at entire app build process level in the same way as we have been doing with WeakMap<ResolvedConfig, CachedData> during dev.

For Vite 6, we need to do a smaller step to keep backward compatibility. Ecosystem plugins are currently using config.build instead of environment.config.build to access configuration, so we need to create a new ResolvedConfig per environment by default. A project can opt-in into sharing the full config and plugins pipeline setting builder.sharedConfigBuild to true.

This option would only work of a small subset of projects at first, so plugin authors can opt-in for a particular plugin to be shared by setting the sharedDuringBuild flag to true. This allows for easily sharing state both for regular plugins:

function myPlugin() {
  // Share state among all environments in dev and build
  const sharedState = ...
  return {
    name: 'shared-plugin',
    transform(code, id) { ... },

    // Opt-in into a single instance for all environments
    sharedDuringBuild: true,
  }
}