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Server-Side Rendering

Server-Side Rendering (SSR) is a technique that allows you to serve an initial payload with HTML, JavaScript, CSS, and even application state. You serve a fully rendered HTML page that would make sense even without JavaScript enabled. In addition to providing potential performance benefits, this can help with Search Engine Optimization (SEO).

Even though the idea does not sound that unique, there is a technical cost. The approach was popularized by React. Since then frameworks encapsulating the tricky bits, such as Next.js↗ and razzle↗, have appeared.

To demonstrate SSR, you can use webpack to compile a client-side build that then gets picked up by a server that renders it using React following the principle. Doing this is enough to understand how it works and also where the problems begin.

Setting up Babel with React

To use React, we require specific configuration. Given most of React projects rely on JSX↗ format, you have to enable it through Babel:

npm add babel-loader @babel/core @babel/preset-react -D

Connect the preset with Babel configuration as follows:


  "presets": [
    ["@babel/preset-env", { "modules": false }],

Setting up a React demo

To make sure the project has the dependencies in place, install React and react-dom to render the application to the DOM.

npm add react react-dom

Next, the React code needs a small entry point. For browser, we’ll render a div and show an alert on click. For server, we return JSX to render.

As ES2015 style imports and CommonJS exports cannot be mixed, the entry point has to be written in CommonJS style. Adjust as follows:


const React = require("react");
const ReactDOM = require("react-dom");
const SSR = <div onClick={() => alert("hello")}>Hello world</div>;

// Render only in the browser, export otherwise
if (typeof document === "undefined") {
  module.exports = SSR;
} else {
  ReactDOM.hydrate(SSR, document.getElementById("app"));

Configuring webpack

To keep things nice, we will define a separate configuration file. A lot of the work has been done already. Given you have to consume the same output from multiple environments, using UMD as the library target makes sense:


const path = require("path");
const APP_SOURCE = path.join(__dirname, "src");

module.exports = {
  mode: "production",
  entry: { index: path.join(APP_SOURCE, "ssr.js") },
  output: {
    path: path.join(__dirname, "static"),
    filename: "[name].js",
    libraryTarget: "umd",
    globalObject: "this",
  module: {
    rules: [
        test: /\.js$/,
        include: APP_SOURCE,
        use: "babel-loader",

To make it convenient to generate a build, add a helper script:


  "scripts": {
"build:ssr": "wp --config webpack.ssr.js"
} }

If you build the SSR demo (npm run build:ssr), you should see a new file at ./static/index.js. The next step is to set up a server to render it.

Setting up a server

To keep things clear to understand, you can set up a standalone Express server that picks up the generated bundle and renders it following the SSR principle. Install Express first:

npm add express -D

Then, to get something running, implement a server:


const express = require("express");
const { renderToString } = require("react-dom/server");
const SSR = require("./static");

const app = express();
app.get("/", (req, res) =>
app.listen(parseInt(process.env.PORT, 10) || 8080);

function renderMarkup(html) {
  return `<!DOCTYPE html>
  <head><title>SSR Demo</title><meta charset="utf-8" /></head>
    <div id="app">${html}</div>
    <script src="./index.js"></script>

Run the server now (node ./server.js) and go below http://localhost:8080, you should see a “Hello World”. Clicking the text should show an alert and you should see pre-rendered HTML in the source.

Even though there is a React application running now, it’s difficult to develop. If you try to modify the code, nothing happens. The problem can be solved for example by using webpack-dev-middlewareβ†—.

If you want to debug output from the server, set export DEBUG=express:application.

Open questions

Even though the demo illustrates the basic idea of SSR, it still leaves open questions:

  • How to deal with styles? Node doesn’t understand CSS related imports.
  • How to deal with anything other than JavaScript? If the server side is processed through webpack, this is less of an issue as you can patch it at webpack.
  • How to run the server through something else other than Node? One option would be to wrap the Node instance in a service you then run through your host environment. Ideally, the results would be cached, and you can find more specific solutions for this particular per platform (i.e. Java and others).

Questions like these are the reason why solutions such as Next.js or razzle exist. They have been designed to solve SSR-specific problems like these.

Webpack provides require.resolveWeakβ†— for implementing SSR. It’s a specific feature used by solutions such as react-universal-componentβ†— underneath.

__non_webpack_require__(path) allows you to separate imports that should be evaluated outside of webpack. See the issue #4175β†— for more information.


SSR isn’t the only solution to the SEO problem. Prerendering is an alternate technique that is easier to implement. The point is to use a headless browser to render the initial HTML markup of the page and then serve that to the crawlers. The caveat is that the approach won’t work well with highly dynamic data.

The following solutions exist for webpack:


SSR comes with a technical challenge, and for this reason, specific solutions have appeared around it. Webpack is a good fit for SSR setups.

To recap:

  • Server-Side Rendering (SSR) can provide more for the browser to render initially. Instead of waiting for the JavaScript to load, you can display markup instantly.
  • SSR also allows you to pass initial payload of data to the client to avoid unnecessary queries to the server.
  • Webpack can manage the client-side portion of the problem. It can be used to generate the server as well if a more integrated solution is required. Abstractions, such as Next.js, hide these details.
  • SSR does not come without a cost, and it leads to new problems as you need better approaches for dealing with aspects, such as styling or routing. The server and the client environment differ in essential manners, so the code has to be written so that it does not rely on platform-specific features too much.

In the next chapter, we’ll learn about micro frontends and module federation.