Server-Side Rendering (SSR)

Server-side rendering (SSR) can refer to several similar developer stories:

  • Using Snowpack with a server web framework like Rails or Express
  • Using Snowpack to power a server-side frontend framework kit like Next.js or SvelteKit
  • Any project configuration where your HTML is generated at runtime, outside of your static build.

This guide will walk you through three options for setting up Snowpack with your own custom server:

  1. snowpack build --watch - Serve files out of the static build directory.
  2. startServer({ ... }) - Serve files on-demand via Snowpack’s JavaScript API.
  3. getServerRuntime({ ... }) - Run your built JS files server-side, directly inside of Node.js.

Option 1: Static Serving

Serving built files directly out of Snowpack’s build/ directory is the easiest way to get started with Snowpack. Run snowpack build to build your site to a static directory, and then make sure that your HTML server response includes the appropriate script & link tags to load your Snowpack-built JavaScript and CSS:

<!-- Example: If you own the server HTML response, make sure that you host the built assets and load the correct JS/CSS files in your HTML.  -->
<script type="module" src="/dist/index.js"></script>

During development, Snowpack will rebuild files on every change thanks to the --watch command. To enable dev features like automatic page reloads and hot module replacement (HMR), check out the “Custom Server” section of our HMR guide for more info.

This setup also has the benefit of pulling from the same build/ directory in both development and production. You can control this build/ output behavior yourself by passing different --out CLI flags to Snowpack for development vs production. You can even pass entirely different config files via the --config CLI flag, or put custom logic in your snowpack.config.mjs file to behave differently for different builds.

The downside of this static approach is that you need to wait for Snowpack to build the entire build/ directory on startup before your site will run. This is something that all other build tools (like Webpack) have to deal with, but Snowpack has the ability to build files only when they are requested by the browser, leading to ~0ms startup wait time.

Option 2: On Demand Serving (Middleware)

The best developer experience is achieved by loading files on-demand. This removes any need for work on startup, giving you a faster developer environment no matter how large your project grows.

const {startServer} = require('snowpack');
const server = await startServer({ ... });

// Example: Express
// On request, build each file on request and respond with its built contents
app.use((req, res, next) => {
  try {
    const buildResult = await server.loadUrl(req.url);
  } catch (err) {

Note that you’ll still need to set up static file serving out of the build/ directory for production deployments. For that reason, this can be seen as an enhancement over the static setup in Option 1 for faster development speeds.

While our official API is written in JavaScript and requires Node.js to run, you could implement your own API for any language/environment using the snowpack dev CLI command to start the server and loading assets directly by fetching each URL.

Option 3: Server-Side Rendering (SSR)

Some frontend applications are also designed to run on the server. In the two previous sections, we’ve just been loading and serving Snowpack files to the client. In this final section, we’ll look into how your project can run Snowpack-built JavaScript on the server and return server-rendered HTML to the client for a faster first page load.

Snowpack provides an Node.js SSR Runtime API to help you run & render your application server-side. getServerRuntime() returns a runtime instance that can be used to import Snowpack-built modules into your current Node.js process, on-demand. This runtime handles the transformation from browser ESM to Node.js Common.js (CJS) so that it can run directly in server without issues.

const {readFileSync} = require('fs');
const {startServer} = require('snowpack');
const server = await startServer({ ... });
const runtime = server.getServerRuntime();

// Advanced Example: Express + React SSR
app.use(async (req, res, next) => {
  // Server-side import our React component
  const importedComponent = await runtime.importModule('/dist/MyReactComponent.js');
  const MyReactComponent = importedComponent.exports.default;
  // Render your react component to HTML
  const html = ReactDOMServer.renderToString(React.createElement(MyReactComponent, null));
  // Load contents of index.html
  const htmlFile = readFileSync('./index.html', 'utf8');
  // Inserts the rendered React HTML into our main div
  const document = htmlFile.replace(/<div id="app"></div>/, `<div id="app">${html}</div>`);
  // Sends the response back to the client

getServerRuntime() is a lower-level tool to help you implement SSR in your project. However, building a custom SSR setup is an advanced task. If you prefer not to implement this yourself, check out some of the new Snowpack-powered application frameworks and static site generators like SvelteKit and Microsite.