The third edition of this book includes React features that were introduced after version 16.6.0. In the following sections, I'll give you a brief introduction to the new functionality. Each feature will be covered in greater detail as you make your way through the book.
For now, we will briefly look at the following:
- Memoizing functional components
- Cook splitting and loading
Let's start exploring them.
Memoizing functional components
The React.memo() function is the modern equivalent of the PureComponent class. Memoized components avoid re-rendering if the component data hasn't changed. In the past, you would extend your class component with PureComponent. This would automatically handle checking whether the component data has changed or not and whether or not the component should re-render.
The challenge with this approach is that it is now common for large React applications to have a lot of functional components. Before React.memo(), there was no way to memorize components so that they could avoid re-rendering if no data changes happened. Now, you can pass your functional components to React.memo() and they'll behave like PureComponent.
You can read more about React.memo() here: https://reactjs.org/docs/react-api.html#reactmemo.
Code splitting and loading
Prior to the React.lazy() function, code splitting in large React applications was cumbersome. Code splitting is important for large applications because it reduces the size of the code bundles that are sent to the browser, which can dramatically improve the user experience. Some features of an application might never be used, which means that the code that implements those features is never delivered to the browser. This is a huge efficiency gain.
With the addition of React.lazy(), React acknowledges that code splitting and the user experience of waiting for pieces of the application to load are integral parts of the application, not an afterthought. By combining React.lazy() and the Suspense component, we get fine-grained control over how our app is split up and what happens while the user waits for it to load.
You can read more about code splitting here: https://reactjs.org/docs/code-splitting.html.
One of the most consequential new features of React is Hooks—functions that extend the behavior of functional React components. Hooks are used to "hook into" the React component machinery from your React components. Instead of relying on classes to build components that have state or that rely on executing side effects when the component is mounted, you can use the React Hooks API to pass functions that handle these cases.
The end result is having more flexibility with how you're able to compose React components since functions are more easily shared between modules than component class methods are. Hooks are the future of how React components are assembled, which will have a big impact on the third edition of this book, where there's a new chapter devoted to Hooks, as well as updated code in all chapters from the second edition.
You can read more about Hooks here: https://reactjs.org/docs/Hooks-intro.html.