What is Server Side Rendering in React (SSR)?

Server side rendering is normally rendering the requested page at server side of client and sending back the whole html page from server.
i.e: Server-side rendering is all about handling the initial render whenever a request hits the server. For more on basics of SSR please read my first article

Let us see how to implement server side rendering in ReactJs

How to Enable Server Side Rendering using React and ExpressJS

Dependencies:
`React: 16.7.0 Material Ui Library from Google: v3.9.3 redux: 4.0.1 redux-thunk: 4.0.1 express: 4.17.1 react-jss: 8.6.1 react-router-dom: 4.3.1 react-document-meta: 2.1.2 axios: 0.19.0 @babel/plugin-proposal-class-properties: 7.4.0 @babel/preset-env: 7.4.3 @babel/register: 7.4.4 babel-preset-es2015: 6.24.1`
Step1: Create React Application
Create a react app using command create-react-app
Install the create-react-app globally on your machine with below command.

npm install -g create-react-app

Create a project by create-react-app as follows

create-react-app ssr-react-16

Step2: Install necessary npm packages for React server side rendering applicationInstall the following packages by running the following commands, if necessary please use the same version too.

npm install @material-ui/core
npm install  react-redux
npm install redux
npm install redux-thunk 
npm install express
npm install react-router-dom
npm install react-document-meta
npm install react-jss
npm install axios
npm install @babel/plugin-proposal-class-properties
npm install @babel/preset-env
npm install @babel/register
npm install babel-preset-es2015

Thats it now your app is ready with library setup

Step3: babel configs for react server side app
Create a file called .babelrc in root directory of the app and paste the below code.
Babel is a toolchain that is mainly used to convert ECMAScript 2015+ code into a backwards compatible version of JavaScript in current and older browsers or environments. For more you can read the babel doc.

{
  "presets": ["@babel/env", "@babel/preset-react"],
  "plugins": [
    "@babel/plugin-proposal-class-properties"
  ]
}

Step4:
Create a file called www under bin folder which is in root directory
In this file we will be creating a server for client and allocate a port (I am using 5000 here)
Copy the code below to file just created

const app = require('../src/client/src/ssr/clientServer');
const http = require('http');

const port = normalizePort(process.env.PORT || 5000);
app.set('port', port);

const server = http.createServer(app);
server.listen(port);

function normalizePort(val) {
  const port = parseInt(val, 10);
  if (isNaN(port)) {
    // named pipe
    return val;
  }
  if (port >= 0) {
    // port number
    return port;
  }
  return false;
}

In the above code snippet, we are importing a file called clientServer.js that’s covered in the next step.

Step5: ReactJS server side script to initialise server part of client app.
Create a file clientServer.js
For better folder structure, I have organised as shown in the below screenshot where bin and src will falls under root directory. Paste the below code in clientServer file. clientServer is kind of server side script for our client application, it will initiate expressjs and configurations for server side script of client.

require("@babel/register")({
  presets: ["@babel/preset-env"]
});

const express = require('express');
const path = require('path');

const requestHandler = require('./requestHandler');
const app = express();

//static files nd build file reference
app.use(express.static(path.join(__dirname, '../../../../build')));
app.use(express.static(path.join(__dirname, 'public')));

app.set('view engine', 'ejs');
app.set('views', path.join(__dirname, 'public'));

//for server side rendering
app.use(requestHandler);

// catch 404 and forward to error handler
app.use((req, res, next) => {
  const err = new Error('Notdff Found');
  err.status = 404;
  next(err);
});

module.exports = app;

In the above code you can see a requestHandler file, which is very important file and will see there.

Step6: A request handler for ReactJS for server side rendering and invoking (calling) apis at server side
This step is very important for serving the requested page from server side. All magic of serving the requested page is done in this script. When client request for a page, it matches the path and determines whether is there any API call for this page and invokes api then updates HTML body for the respective response. Basically here its creating the react component at server side. Hence React component is fully ready with all the data for the first render itself. If there is no api call for a particular page, it just serves a just a static page

Create a file requestHandler.js Copy the below code.

'use strict';
import React from 'react';
import { Provider } from 'react-redux';
import thunk from 'redux-thunk';
import { applyMiddleware, createStore } from 'redux';
import { renderToString } from 'react-dom/server';
import { SheetsRegistry } from 'jss';
import JssProvider from 'react-jss/lib/JssProvider';
import path from 'path'
import fs from 'fs'
import {
  MuiThemeProvider,
  createMuiTheme,
  createGenerateClassName,
} from '@material-ui/core/styles';
import green from '@material-ui/core/colors/green';
import red from '@material-ui/core/colors/red';
import { StaticRouter, matchPath } from 'react-router-dom';
import DocumentMeta from 'react-document-meta';
import reducers from '../reducers/index';
import routes from '../routes';
import routesConfigs from './routesConfig';

const middleware = applyMiddleware(thunk);

function renderView(req, res, state) {

  // Create a theme instance.
  const theme = createMuiTheme({
    palette: {
      primary: green,
      accent: red,
      type: 'light',
    },
  });

  // STEP-1 CREATE A REDUX STORE ON THE SERVER
  const store = createStore(reducers, state, middleware);
  const sheetsRegistry = new SheetsRegistry();
  // Create a sheetsManager instance.
  const sheetsManager = new Map();
  const generateClassName = createGenerateClassName();
  // STEP-2 GET INITIAL STATE FROM THE STORE
  const initialState = JSON.stringify(store.getState()).replace(/<\/script/g, '<\\/script').replace(/<!--/g, '<\\!--');
  // STEP-3 IMPLEMENT REACT-ROUTER ON THE SERVER TO INTERCEPT CLIENT REQUESTs AND DEFINE WHAT TO DO WITH THEM
  const context = {};
  const reactComponent = renderToString(
    <JssProvider registry={sheetsRegistry} generateClassName={generateClassName}>
      <MuiThemeProvider theme={theme} sheetsManager={sheetsManager}>
        <Provider store={store}>
          <StaticRouter
            location={req.url}
            context={context}>
            {routes}
          </StaticRouter>
        </Provider>
      </MuiThemeProvider>
    </JssProvider>
  );
  const css = sheetsRegistry.toString()
  const reactMetaComponent = DocumentMeta.renderToStaticMarkup();
  if (context.url) {
    // can use the `context.status` that
    // we added in RedirectWithStatus
    redirect(context.status, context.url);
  } else {
    //https://crypt.codemancers.com/posts/2016-09-16-react-server-side-rendering/
    //res.status(200).render('index', { reactComponent, reactMetaComponent, initialState });
    fs.readFile(path.resolve('build/index.html'), 'utf8', (err, data) => {
      if (err) {
        return res.status(500).send('An error occurred')
      }
      const replacedData = data.replace(
        '<div id="root"></div>',
        `<div id="root">${reactComponent}</div>
        <style id="jss-server-side">${css}</style>
        <script>
          window.INITIAL_STATE = ${initialState}
        </script>`
      );
      const replacedMetaTagData = replacedData
        .replace(`<meta id="reactMetaTags"/>`,
          `${reactMetaComponent}`);
      res.send(replacedMetaTagData);
    })
  }
}

function handleRender(req, res) {
  const components = routesConfigs
    .filter(route => matchPath(req.path, route)) // filter matching paths
    .map(route => route.component); // check if components have data requirement
  let promiseObj = null;
  if (components.length > 0 && (components[0].fetchData instanceof Function)) {
    /* fetchData is the function defined in each component and make it like class 
       function and it will be called at server side
    */
    components[0]
      .fetchData(req.query)
      .then((response) => {
        renderView(req, res, response);
      })
      .catch((error) => {
        console.log('***--- handleRender error ', error);
        renderView(req, res, {});
      });
  } else {
    renderView(req, res, {});
  }
}

module.exports = handleRender;

By default it will be executing a handleRender function in the above code. In the handleRender function, we read a routesConfig file which is just for keeping track of respective component for each route path.

import LandingComponent from '../pages/landing/components/LandingComponent';
import AboutComponent from '../pages/about/components/AboutComponent';
export default [
  {
    path: "/",
    component: LandingComponent,
    exact: true,
  },
  {
    path: '/about',
    component: AboutComponent,
    exact: true,
  }
];

routesConfigs will be used by requestHandler to find dynamically which component needs to be rendered for the request. It also checks for fetchData, if fetchData is defined by the component then requestHandler will invoke it which internally invokes api. Note here is all this is happening at server side not on the browser, component object is created in server and fetchData action will be invoked in server. You can see the brief description of fetchData and how to use this in the component in the next step.

When renderView is called, every times it creates is own redux store and a material ui theme provider, thats why we follow this code(which is also mention in the above code snip).

const theme = createMuiTheme({
    palette: {
      primary: green,
      accent: red,
      type: 'light',
    },
  });

  // STEP-1 CREATE A REDUX STORE ON THE SERVER
  const store = createStore(reducers, state, middleware);
  const sheetsRegistry = new SheetsRegistry();
  // Create a sheetsManager instance.
  const sheetsManager = new Map();
  const generateClassName = createGenerateClassName();

Next major step is reading a build/index.html file which you will get by running npm run build,
Here we will be injecting the component created at server side into index.html.

const replacedData = data.replace(
        '<div id="root"></div>',
        `<div id="root">${reactComponent}</div>
        <style id="jss-server-side">${css}</style>
        <script>
          window.INITIAL_STATE = ${initialState}
        </script>`
 );

where in index.html of public folder should compulsory has this tag

<div id="root"></div>

For facebook sharing, when you share the url a image and the title and description of the page has to be displayed, for that all those content has to be in the meta tag inside the head tag, hence we need to replaced meta tag with server generated meta tag code.
```
const replacedMetaTagData = replacedData
  .replace(`<meta id="reactMetaTags"/>`,
  `${reactMetaComponent}`);
```
Where in index.html of public folder, meta tag with id is should be unchanged
```
<meta id="reactMetaTags"/>
```
Finally we will serve the requested page with updated meta tags and updated html for the first rendering cycle it self.
```
res.send(replacedMetaTagData);
```
Step 7
Every component should define an action which will be invoked from server side script, however in server side the request handler will be a generic script and we have to dynamically invoke an action at server side, to achieve that, each component has to define an action and assign action function pointer to fetchData, fetchData is a static class variable kept in component. Check the below code snippet , fetchData is defined at AboutComponent class level and a pointer to action is assigned to it.
```
AboutComponent.fetchData = fetchAboutData;
export default connect(mapStateToProps, null)(AboutComponent);
```

You can refer to the fetchAboutData action in the aboutActions.js which will make the API call to some server, for testing iI am using the JSON placeholder get API which will get the list of posts as response, refer the code below

import axios from 'axios';
export const fetchAboutData = (params) => {
  return new Promise((resolve, reject) => {
    const url = 'https://jsonplaceholder.typicode.com/posts';
    axios
      .get(url)
      .then(response => {
        resolve({
          about: {
            posts: response.data
          }
        });
      })
      .catch((error) => {
        console.log('Error while fetching posts from network', error);
        reject(null);
      });
  });
}

In the positive view, it goes to then block(success block) of the axios api call, you can observe that it’s not direct resolve, its resolving a reducer named about and that contain the posts as one of the key, this set up has to be same in both client side reducer as well in axios call in order to use it component (About Component), Observe the below codes, I have shown the code snips of axios call, aboutReducer to mapStateToProps in AboutComponent,
In axios call
```
resolve({
    about: {
       posts: response.data
    }
});
```
In about reducer
```
const INITIAL_STATE = {
posts: []
};
```
In about component
```
const mapStateToProps = (state) => {
    return {
       posts:state.about.posts
    };
}
```
So in the render method, you can access the posts array like this.props.posts, in the return method of render function i have written one more function which will return the list of posts(only title) and it creates a html code
```
getAllPosts = () => {
   const postView = this.props.posts.map(post =>
   <li>{post.title}</li>
 );
 return postView;
}
```
Once the html is ready, then its in the success block of the fetchData function in the handleRender function of requestHandler file , then followed by the code it calls the renderView method along with data(html) returned from the component.

Step8 :
Update the package.json start script to point to server side rendering beginning of the file, thats www under bin directory. Now server side rendering is really ready.

  "scripts": {
    "start": "node bin.www",

Step9 : Running project:

In the requestHandler file we will be pointing to index.html inside the build folder, hence we need to generate a build from react by running the command
```
npm run build
```
Run the start command of your script by
```
npm run start
```
OR
```
nodemon
```
OR
```
node bin/www
```
Since we are running this project on 5000 port, run http://localhost:5000 which is just a page without requesting for API, you simply see as like below
I have made one more component with calling API and printing the results on screen, run http://localhost:5000/about . I am using an open dummy api for printing list of posts,
Ie: https://jsonplaceholder.typicode.com/posts