Vite

Configure and optimize Vite for development, production builds, and library bundling.

Vite Patterns

Environment Variables

• Only VITE_ prefixed vars are exposed to client code — DB_PASSWORD stays server-side, VITE_API_URL is bundled
• Access via import.meta.env.VITE_* not process.env — process.env is Node-only and undefined in browser
• .env.local overrides .env and is gitignored by default — use for local secrets
• import.meta.env.MODE is development or production — use for conditional logic, not NODE_ENV

CommonJS Compatibility

• Pure ESM by default — CommonJS packages need optimizeDeps.include for pre-bundling
• require() doesn't work in Vite — use import or createRequire from module for dynamic requires
• Some packages ship broken ESM — add to ssr.noExternal or optimizeDeps.exclude and let Vite transform them
• Named exports from CommonJS may fail — use default import and destructure: import pkg from 'pkg'; const { method } = pkg

Dependency Pre-bundling

• Vite pre-bundles dependencies on first run — delete node_modules/.vite to force rebuild after package changes
• Large dependencies slow down dev server start — add rarely-changing ones to optimizeDeps.include for persistent cache
• Linked local packages (npm link) aren't pre-bundled — add to optimizeDeps.include explicitly
• optimizeDeps.force: true rebuilds every time — only for debugging, kills dev performance

Path Aliases

• Configure in both vite.config.ts AND tsconfig.json — Vite uses its own, TypeScript uses tsconfig
• Use path.resolve(__dirname, './src') not relative paths — relative breaks depending on working directory
• @/ alias is not built-in — must configure manually unlike some frameworks

// vite.config.ts
resolve: {
  alias: { '@': path.resolve(__dirname, './src') }
}

Dev Server Proxy

• Proxy only works in dev — production needs actual CORS config or reverse proxy
• changeOrigin: true rewrites Host header — required for most APIs that check origin
• WebSocket proxy needs explicit ws: true — HTTP proxy doesn't forward WS by default
• Trailing slashes matter: /api proxies /api/users, /api/ only proxies /api//users

server: {
  proxy: {
    '/api': {
      target: 'http://localhost:3000',
      changeOrigin: true,
      rewrite: path => path.replace(/^\/api/, '')
    }
  }
}

Static Assets

• public/ files served at root, not processed — use for favicons, robots.txt, files that need exact paths
• src/assets/ files are processed, hashed, can be imported — use for images, fonts referenced in code
• Import assets to get resolved URL: import logo from './logo.png' — hardcoded paths break after build
• new URL('./img.png', import.meta.url) for dynamic paths — template literals with variables don't work

Build Optimization

• build.rollupOptions.output.manualChunks for code splitting — without it, one giant bundle
• Analyze bundle with rollup-plugin-visualizer — find unexpected large dependencies
• build.target defaults to modern browsers — set 'es2015' for legacy support, but increases bundle size
• build.cssCodeSplit: true (default) — each async chunk gets its own CSS file

Library Mode

• build.lib for npm packages — different config from app mode
• Set external for peer dependencies — don't bundle React/Vue into your library
• Generate types separately with tsc — Vite doesn't emit .d.ts files
• Both ESM and CJS outputs: formats: ['es', 'cjs'] — some consumers still need require()

HMR Issues

• Circular imports break HMR — refactor to break the cycle or full reload triggers
• State lost on HMR means component isn't accepting updates — check for import.meta.hot.accept()
• CSS changes trigger full reload if imported in JS that doesn't accept HMR — import CSS in components that do
• server.hmr.overlay: false hides error overlay — useful for custom error handling but hides issues

SSR Configuration

• ssr.external for Node-only packages — prevents bundling node_modules in SSR build
• ssr.noExternal forces bundling — needed for packages with browser-specific imports
• CSS imports fail in SSR by default — use ?inline suffix or configure css.postcss for SSR