This is a 2020-updated version of An Introduction to React Server-Side Rendering by @alligatorio.

0. In Terminal, choose existing (or create new) folder, and make it current.

1. To create new React app named react-ssr-intro, run—

   $ npx create-react-app react-ssr-intro
   

2. Check if the app runs without errors—

   $ yarn start
   

// ./src/Home.js

import React from "react";

export default ({ name }) => <h1>Hello, {name}</h1>;

Use Home component in the App—

// ./src/App.js

import React from "react";
import Home from "./Home";

export default () => <Home name="Alligator" />;

Use ReactDOM's hydrate method to facilitate server side rendering—

// ./src/index.js

import React from "react";
import ReactDOM from "react-dom";
import App from "./App";

ReactDOM.hydrate(<App />, document.getElementById("root"));

Express is a good candidate for a simple server that will render the app output and send it along.

0. Add the component—

   $ yarn add express
   

1. Create the server folder, on the src level, and the index.js file inside—

   $ mkdir server && touch server/index.js
   

2. Modify this index.js—

   // ./server/index.js
   
   import path from "path";
   import fs from "fs";
   
   import React from "react";
   import express from "express/lib/express";
   import ReactDOMServer from "react-dom/server";
   
   import App from "../src/App";
   
   const PORT = process.env.PORT || 3006;
   const app = express();
   
   const appDataFolder =
     process.env.APPDATA ||
     (process.platform === "darwin"
       ? process.env.HOME + "/Library/Preferences"
       : process.env.HOME + "/.local/share");
   console.log("AppData folder:", appDataFolder);
   
   // app.get("/", express.static("build"));
   
   app.get("/*", (req, res) => {
     const app = ReactDOMServer.renderToString(<App />);
     const indexFile = path.resolve("build/index.html");
     fs.readFile(indexFile, "utf8", (err, data) => {
       if (err) {
         console.error("Something went wrong:", err);
         return res.status(500).send("Oops, better luck next time!");
       }
   
       return res
         .status(200)
         .send(
           data.replace('<div id="root"></div>', `<div id="root">${app}</div>`)
         );
     });
   });
   
   app.listen(PORT, () => {
     console.log(`😎 Server is listening on port ${PORT}`);
   });

0. Install components as devDependencies—

   $ yarn add --dev @babel/core @babel/preset-env @babel/preset-react babel-loader nodemon webpack-cli webpack-node-externals concurrently
   

1. In the project root, create Babel config file, .babelrc—

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

2. In the project root, create webpack.server.js config file—

   const path = require("path");
   const nodeExternals = require("webpack-node-externals");
   
   module.exports = {
     entry: "./server/index.js",
     target: "node",
     externals: [nodeExternals()],
     output: {
       path: path.resolve("server-build"),
       filename: "index.js"
     },
     module: {
       rules: [
         {
           test: /\.js$/,
           exclude: /node_modules/,
           use: "babel-loader"
         }
       ]
     }
   };

   Webpack transpiles the server (written in ES6) to ES5 executable by NodeJS, by generating server-build/index.js.

Open package.json, and update its scripts node with all three dev entries—

"scripts": {
  "start": "react-scripts start",
  "build": "react-scripts build",
  "test": "react-scripts test",
  "eject": "react-scripts eject",
  "dev:build-server": "NODE_ENV=development webpack --config webpack.server.js --mode=development -w",
  "dev:start": "nodemon ./server-build/index.js",
  "dev": "concurrently \"yarn build\" \"yarn dev:build-server\" \"yarn dev:start\""
}

$ yarn dev

Your app should be available on http://localhost:3006.