Minimal Vite plugin for environment isolation via macros for server-only and client-only.

npm install -D vite-env-only

// vite.config.ts
import { defineConfig } from "vite"
import envOnly from "vite-env-only"

export default defineConfig({
  plugins: [envOnly()],
})

Marks an expression as server-only and replaces it with undefined on the client. Keeps the expression as-is on the server.

For example:

import { serverOnly$ } from "vite-env-only"

export const message = serverOnly$("i only exist on the server")

On the client this produces:

export const message = undefined

On the server this produces:

export const message = "i only exist on the server"

Marks an expression as client-only and replaces it with undefined on the server. Keeps the expression as-is on the client.

For example:

import { clientOnly$ } from "vite-env-only"

export const message = clientOnly$("i only exist on the client")

On the client this produces:

export const message = "i only exist on the client"

On the server this produces:

export const message = undefined

This plugin eliminates any identifiers that become unreferenced as a result of macro replacement.

For example, given the following usage of serverOnly$:

import { serverOnly$ } from "vite-env-only"
import { readFile } from "node:fs"

function readConfig() {
  return JSON.parse(readFile.sync("./config.json", "utf-8"))
}

export const serverConfig = serverOnly$(readConfig())

On the client this produces:

export const serverConfig = undefined

On the server this produces:

import { readFile } from "node:fs"

function readConfig() {
  return JSON.parse(readFile.sync("./config.json", "utf-8"))
}

export const serverConfig = readConfig()

The macro types capture the fact that values can be undefined depending on the environment.

For example:

import { serverOnly$ } from "vite-env-only"

export const API_KEY = serverOnly$("secret")
//           ^? string | undefined

If you want to opt out of strict type safety, you can use a non-null assertion (!):

import { serverOnly$ } from "vite-env-only"

export const API_KEY = serverOnly$("secret")!
//           ^? string

Vite already provides import.meta.env.SSR which works in a similar way to this plugin in production. However, in development Vite neither replaces import.meta.env.SSR nor performs dead-code elimination as Vite considers these steps to be optimizations.

In general, its a bad idea to rely on optimizations for correctness. In contrast, this plugin considers macro replacement and dead-code elimination to be part of its feature set.

Additionally, this plugin uses function calls to mark expressions as server-only or client-only. That means it can guarantee that code within its macros never ends up in the wrong environment while only transforming a single AST node type: function call expressions.

import.meta.env.SSR is instead a special identifier which can show up in many different AST node types: if statements, ternaries, switch statements, etc. This makes it far more challenging to guarantee that dead-code completely eliminated.

Thanks to these project for exploring environment isolation and conventions for transpilation:

• esm-env
• Qwik
• TanStack bling