Introduction to React Javascript

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React is a JavaScript library for building user interfaces, primarily for single-page applications. It's used for handling the view layer in web and mobile apps. React allows you to design simple views for each state in your application, and it will efficiently update and render the right components when your data changes.

What is React?

React was first created by Jordan Walke, a software engineer at Facebook, and it was first deployed on Facebook's newsfeed in 2011 and on Instagram in 2012. It follows the component-based approach which helps in building reusable UI components. It is used for developing complex and interactive web and mobile UI. Even though it was open-sourced only in 2015, it has one of the largest communities supporting it.

Why use React?

React allows developers to create large web applications that can change data, without reloading the page. The main purpose of React is to be fast, scalable, and simple. It works only on user interfaces in the application. This corresponds to the view in the MVC template. It can be used with a combination of other JavaScript libraries or frameworks, such as Angular JS in MVC.

Understanding the Virtual DOM

React creates an in-memory data structure cache which computes the changes made and then updates the browser. This allows a special feature that enables the programmer to code as if the whole page is rendered on each change whereas react library only renders components that actually change.

Setting up a React Environment

Before you start using React, you need to have a working environment set up. This involves installing Node.js and npm (Node Package Manager), and creating a new project using Create React App, a command-line utility for setting up new React projects.

JSX: JavaScript XML

JSX is a syntax extension for JavaScript and comes with the full power of JavaScript. JSX produces React “elements”. You can embed any JavaScript expression in JSX by wrapping it in curly braces. After compilation, JSX expressions become regular JavaScript objects. This means that you can use JSX inside of if statements and for loops, assign it to variables, accept it as arguments, and return it from functions.

In conclusion, React is a powerful library for building interactive user interfaces. It's component-based structure and use of a virtual DOM differentiate it from other JavaScript frameworks. Understanding these aspects of React will be key to becoming proficient in it.