Server-Side Rendering #

Example Projects #

Vite provides built-in support for server-side rendering (SSR). The Vite playground contains example SSR setups for Vue 3 and React, which can be used as references for this guide:

• Vue 3
• React

Source Structure #

A typical SSR application will have the following source file structure:

- index.html
- server.js # main application server
- src/
  - main.js          # exports env-agnostic (universal) app code
  - entry-client.js  # mounts the app to a DOM element
  - entry-server.js  # renders the app using the framework's SSR API

The index.html will need to reference entry-client.js and include a placeholder where the server-rendered markup should be injected:

<div id="app"><!--ssr-outlet--></div>
<script type="module" src="/src/entry-client.js"></script>

You can use any placeholder you prefer instead of <!--ssr-outlet-->, as long as it can be precisely replaced.

Conditional Logic #

If you need to perform conditional logic based on SSR vs. client, you can use

if (import.meta.env.SSR) {
  // ... server only logic
}

This is statically replaced during build so it will allow tree-shaking of unused branches.

Setting Up the Dev Server #

When building an SSR app, you likely want to have full control over your main server and decouple Vite from the production environment. It is therefore recommended to use Vite in middleware mode. Here is an example with express:

server.js

import fs from 'fs'
import path from 'path'
import express from 'express'
import {createServer as createViteServer} from 'vite'

async function createServer() {
  const app = express()

  // Create Vite server in middleware mode and configure the app type as
  // 'custom', disabling Vite's own HTML serving logic so parent server
  // can take control
  const vite = await createViteServer({
    server: { middlewareMode: true },
    appType: 'custom'
  })

  // use vite's connect instance as middleware
  // if you use your own express router (express.Router()), you should use router.use
  app.use(vite.middlewares)

  app.use('*', async (req, res) => {
    // serve index.html - we will tackle this next
  })

  app.listen(5173)
}

createServer()

Here vite is an instance of ViteDevServer. vite.middlewares is a Connect instance which can be used as a middleware in any connect-compatible Node.js framework.

The next step is implementing the * handler to serve server-rendered HTML:

app.use('*', async (req, res, next) => {
  const url = req.originalUrl

  try {
    // 1. Read index.html
    let template = fs.readFileSync(
      path.resolve(__dirname, 'index.html'),
      'utf-8'
    )

    // 2. Apply Vite HTML transforms. This injects the Vite HMR client, and
    //    also applies HTML transforms from Vite plugins, e.g. global preambles
    //    from @vitejs/plugin-react
    template = await vite.transformIndexHtml(url, template)

    // 3. Load the server entry. vite.ssrLoadModule automatically transforms
    //    your ESM source code to be usable in Node.js! There is no bundling
    //    required, and provides efficient invalidation similar to HMR.
    const { render } = await vite.ssrLoadModule('/src/entry-server.js')

    // 4. render the app HTML. This assumes entry-server.js's exported `render`
    //    function calls appropriate framework SSR APIs,
    //    e.g. ReactDOMServer.renderToString()
    const appHtml = await render(url)

    // 5. Inject the app-rendered HTML into the template.
    const html = template.replace(`<!--ssr-outlet-->`, appHtml)

    // 6. Send the rendered HTML back.
    res.status(200).set({ 'Content-Type': 'text/html' }).end(html)
  } catch (e) {
    // If an error is caught, let Vite fix the stack trace so it maps back to
    // your actual source code.
    vite.ssrFixStacktrace(e)
    next(e)
  }
})

The dev script in package.json should also be changed to use the server script instead:

  "scripts": {
-   "dev": "vite"
+   "dev": "node server"
  }

Building for Production #

To ship an SSR project for production, we need to:

1. Produce a client build as normal;
2. Produce an SSR build, which can be directly loaded via import() so that we don't have to go through Vite's ssrLoadModule;

Our scripts in package.json will look like this:

{
  "scripts": {
    "dev": "node server",
    "build:client": "vite build --outDir dist/client",
    "build:server": "vite build --outDir dist/server --ssr src/entry-server.js "
  }
}

Note the --ssr flag which indicates this is an SSR build. It should also specify the SSR entry.

Then, in server.js we need to add some production specific logic by checking process.env.NODE_ENV:

• Instead of reading the root index.html, use the dist/client/index.html as the template instead, since it contains the correct asset links to the client build.

• Instead of await vite.ssrLoadModule('/src/entry-server.js'), use import('./dist/server/entry-server.js') instead (this file is the result of the SSR build).

• Move the creation and all usage of the vite dev server behind dev-only conditional branches, then add static file serving middlewares to serve files from dist/client.

Refer to the Vue and React demos for working setup.

Generating Preload Directives #

vite build supports the --ssrManifest flag which will generate ssr-manifest.json in build output directory:

- "build:client": "vite build --outDir dist/client",
+ "build:client": "vite build --outDir dist/client --ssrManifest",

The above script will now generate dist/client/ssr-manifest.json for the client build (Yes, the SSR manifest is generated from the client build because we want to map module IDs to client files). The manifest contains mappings of module IDs to their associated chunks and asset files.

To leverage the manifest, frameworks need to provide a way to collect the module IDs of the components that were used during a server render call.

@vitejs/plugin-vue supports this out of the box and automatically registers used component module IDs on to the associated Vue SSR context:

// src/entry-server.js
const ctx = {}
const html = await vueServerRenderer.renderToString(app, ctx)
// ctx.modules is now a Set of module IDs that were used during the render

In the production branch of server.js we need to read and pass the manifest to the render function exported by src/entry-server.js. This would provide us with enough information to render preload directives for files used by async routes! See demo source for full example.

Pre-Rendering / SSG #

If the routes and the data needed for certain routes are known ahead of time, we can pre-render these routes into static HTML using the same logic as production SSR. This can also be considered a form of Static-Site Generation (SSG). See demo pre-render script for working example.

SSR Externals #

Many dependencies ship both ESM and CommonJS files. When running SSR, a dependency that provides CommonJS builds can be "externalized" from Vite's SSR transform / module system to speed up both dev and build. For example, instead of pulling in the pre-bundled ESM version of React and then transforming it back to be Node.js-compatible, it is more efficient to simply require('react') instead. It also greatly improves the speed of the SSR bundle build.

Vite performs automated SSR externalization based on the following heuristics:

• If a dependency's resolved ESM entry point and its default Node entry point are different, its default Node entry is probably a CommonJS build that can be externalized. For example, vue will be automatically externalized because it ships both ESM and CommonJS builds.

• Otherwise, Vite will check whether the package's entry point contains valid ESM syntax - if not, the package is likely CommonJS and will be externalized. As an example, react-dom will be automatically externalized because it only specifies a single entry which is in CommonJS format.

If this heuristics leads to errors, you can manually adjust SSR externals using ssr.external and ssr.noExternal config options.

In the future, this heuristics will likely improve to detect if the project has type: "module" enabled, so that Vite can also externalize dependencies that ship Node-compatible ESM builds by importing them via dynamic import() during SSR.

SSR-specific Plugin Logic #

Some frameworks such as Vue or Svelte compiles components into different formats based on client vs. SSR. To support conditional transforms, Vite passes an additional ssr property in the options object of the following plugin hooks:

• resolveId
• load
• transform

Example:

export function mySSRPlugin() {
  return {
    name: 'my-ssr',
    transform(code, id, options) {
      if (options?.ssr) {
        // perform ssr-specific transform...
      }
    }
  }
}

The options object in load and transform is optional, rollup is not currently using this object but may extend these hooks with additional metadata in the future.

SSR Target #

The default target for the SSR build is a node environment, but you can also run the server in a Web Worker. Packages entry resolution is different for each platform. You can configure the target to be Web Worker using the ssr.target set to 'webworker'.

SSR Bundle #

In some cases like webworker runtimes, you might want to bundle your SSR build into a single JavaScript file. You can enable this behavior by setting ssr.noExternal to true. This will do two things:

• Treat all dependencies as noExternal
• Throw an error if any Node.js built-ins are imported

Vite CLI #

The CLI commands $ vite dev and $ vite preview can also be used for SSR apps. You can add your SSR middlewares to the development server with configureServer and to the preview server with configurePreviewServer.