Introduction

React.js is all about building user interfaces, and, in the context of this book, it's about building web user interfaces specifically.

Web user interfaces are ultimately all about the Document Object Model (DOM). You can use JavaScript to read or manipulate the DOM. This is what allows you to build interactive websites: you can add, remove, or edit DOM elements after a page was loaded. This can be used to add or remove overlay windows or to read values entered into input fields.

This was already discussed in Chapter 1, React – What and Why, and, as you learned there, React is used to simplify this process. Instead of manipulating the DOM or reading values from DOM elements manually, you can use React to describe the desired state. React then takes care of the steps needed to achieve this desired state.

However, there are scenarios and use cases wherein, despite using React, you still want to be able to directly reach out to specific DOM elements—for example...