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When working with environment variables in Vite applications, be explicit about variable loading behavior and precedence. Environment variables from .env files are always loaded regardless of mode, with mode-specific files (.env.[mode]) taking precedence over generic ones.

For type safety, use the TypeScript declaration merging pattern to strongly type your environment variables:

/// <reference types="vite/client" />

// By adding this interface, you can make the type of ImportMetaEnv strict
// to disallow unknown keys.
interface ImportMetaEnv {
  readonly VITE_APP_TITLE: string
  // more env variables...
}

When loading environment variables in config files, be explicit about which prefixes to include:

import { defineConfig, loadEnv } from 'vite'

export default defineConfig(({ mode }) => {
  // Set the third parameter to 'APP_' to load envs with the `APP_` prefix.
  // If necessary, you can set the optional third parameter to '' to load all env regardless of the `VITE_` prefix.
  const env = loadEnv(mode, process.cwd(), 'APP_')

  return {
    // use env variables in config
    define: {
      'process.env.APP_ENV': JSON.stringify(env.APP_ENV)
    }
  }
})

To disable environment variable loading altogether, set envDir: false in your Vite config.