Code Splitting in React

Code-Splitting is a feature supported by bundlers like Webpack, Rollup, and Browserify which can create multiple bundles that can be dynamically loaded at runtime.

As websites grow larger and go deeper into components, it becomes heavier. This is especially the case when libraries from third parties are included. Code Splitting is a method that helps to generate bundles that are able to run dynamically. It also helps to make the code efficient because the bundle contains all required imports and files.

Bundling and its efficiency: Bundling is the method of combining imported files with a single file. It is done with the help of Webpack, Rollup, and Browserify as they can create many bundles that can be loaded dynamically at runtime.
With the help of code splitting, 'lazy load' can be implemented, which means just using the code which is currently needed.

• The default way of importing is as follows: 

import { add } from './math';

console.log(add(x, y));  // Here x, y are two numbers

• Using code-splitting this can be done as follows: 

import("./math").then(math => {
  console.log(math.add(x, y));
});
// Here x, y are two numbers

As soon as Webpack gets this type of syntax, code-splitting is started automatically. When using the Create React App, it is already set up and can be used immediately.
The Webpack guide on code splitting should be followed if using Webpack. The instructions can be found here.
When Babel is being used, it has to be made sure that Babel is not transforming the import syntax, but can parse it dynamically. This can be done using babel-plugin-syntax-dynamic-import.

React.lazy and Suspense

As both React.lazy and Suspense are not available for rendering on the server yet now, it is recommended to use loadable-components for code-splitting in a server-rendered app. React.lazy is helpful for rendering dynamic import as a regular component.
Before: 

import Component from './Component';

After: 

const Component = React.lazy(() => import('./Component'));

The Bundle will be loaded on its own which contains the Component when this component is rendered first. 
The Lazy component should then be rendered inside Suspense Component which helps to reflect some fallback content meanwhile the lazy component loads.

import React, { Suspense } from 'react';
const Component = React.lazy(() => import('./Component'));
function MyComponent() {
  return (
    <div>
      <Suspense fallback={<div>Loading...</div>}>
    </div>);
}

The fallback prop can accept any element of React which will be rendered while waiting for the loading of the Component. The Suspense Component can be placed anywhere above the lazy component. Moreover, multiple lazy components can be wrapped with a single Suspense Component.

import React, { Suspense } from 'react';

const ComponentOne =
     React.lazy(() => import('./ComponentOne'));
const ComponentTwo =
     React.lazy(() => import('./ComponentTwo'));
function MyComponent() {
      return (
        <div>
            <Suspense fallback={<div>Loading...</div>}>
        </div>);
}

Error Boundaries

Error Boundaries are React components that help when some modules fail to load due to any issue, an error will be triggered. These errors can be handled properly and provide a good experience to the user by the use of a suitable error page.

import React, { Suspense } from 'react';
import ErrorBoundary from './ErrorBoundary';

const ComponentOne = React.lazy(() =>
     import('./ComponentOne'));
const ComponentTwo = React.lazy(() =>
     import('./ComponentTwo'));
const MyComponent = () => (
  <div>
    <Suspense fallback={<div>Loading...</div>}>
  </div>
);

Route-Based Code Splitting

It can be difficult to implement code-splitting in code, the bundles can be split evenly, which will improve the experience for the user.
Here you can see the example code for this.

import React,{Suspense, lazy} from 'react';
import {Route, Routes, BrowserRouter } from 'react-router-dom';

const HomePage = lazy(() =>
    import('./routes/HomePage'));
const AboutUs = lazy(() =>
     import('./routes/AboutUs'));
const App = () => 
    (<Suspense fallback={<div>Loading...</div>}>);

Named Exports

React.lazy currently supports only default exports. An intermediate module that re-exports as default has to be created if one wants to import a module that uses named exports. This ensures the working of tree shaking and prevents the pulling in of unused components.

import {React, lazy} from 'react';

// Components.js
export const Component = /* ... */;
export const MyUnusedComponent = /* ... */;

// Component.js
export { Component as default } from "./Components.js";

// MyApp.js
const Component = lazy(() => import("./Component.js"));