How to do page speed-optimization with ReactJS 

A modern library like React JS does not develop itself magically. Developers need to understand how React JS works and how components perform through different stages.

With React JS we can benefit greatly from the performance improvements it offers by measuring and optimizing how and when your components appear. And React JS provides the tools and functionality needed to make it easier.

Each react application starts with a root component and is composed of many other components in a tree format.

During the initial rendering process React JS builds a DOM tree like this.

What we want React JS to do is rebuild only the components that are directly affected by the change.

Do not start optimizing the code that you think might slow down your application. Instead, allow React JS to measure performance during the process.
If you are a visual person, then react-pref-tool is what you need, based on the react-addons-perf library. This is a more visual way for us to correct the performance of your application.

For single-page React JS web apps, we end up working our code in a JavaScript file. This is good for small web apps, so when the application starts expanding the sending of this file can become a time-consuming process. If you are using Webpack to build your application you can use its code-sharing capabilities to chunk, this way you can submit your application code to your browser.

There are two types of partitioning: resource splitting and on-demand code splitting. With resource splitting, we divide our code into different files. This type of separation will help in two ways. Helps the browser to store changes and when doing parallel installations reduces installation time.
On-demand code.

Performance optimization for React Apps:

Using Immutable Data Structures.

Function / Stateless Components and React JS.

Multiple Chunk Files.

Using Production Mode Flag in Webpack.

Dependency optimization

Using Immutable Data Structures: can be applied to the design of previous applications. There can be many benefits such as:
Function / Stateless Components and React JS.

Alternatively, to optimize UI updates we can consider converting function components to a PureComponent class. However, if the component does not use the state and other life cycle methods, the initial presentation time is a bit more complicated when compared to the function components with updates.
Multiple Chunk Files:

Your application always starts with several components. It starts by adding new features, thus the application growing to a greater extent.
Using Production Mode Flag in Webpack.

If you are using Webpack 4 as a set of modules for your application, you may want to consider setting the output mode option. This procedure tells the Webpack to use built-in optimization.

Dependency optimization

When considering optimizing application package size, it is worth checking how much code you are using. For instance, you can use Moment.js which includes a localized file for many languages. If you do not need the help of multiple languages then you can use the moment-locales-webpack-plugin to remove unused packages.

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