React (JavaScript library)

React, JavaScript, library, Facebook

React (also known as React.js or ReactJS) is a free and open-source front-end Javascript Library for building user interfaces or UI components. It is maintained by Facebook and a community of individual developers and companies. React can be used as a base in the development of single-page or mobile applications. However, React is only concerned with state management and rendering that state to the DOM, so creating React applications usually requires the use of additional libraries for routing, as well as certain client-side functionality.

The following is a rudimentary example of React usage in HTML with JSX and JavaScript.

The Greeter the function is a React component that accepts a property greeting. The variable App is an instance of the Greeter component where the greeting property is set to 'Hello World!'. The ReactDOM.render the method then renders our Greeter component inside the DOM element with id myReactApp.

When displayed in a web browser the result will be

Hello World!

Hello World!


React code is made of entities called components. Components can be rendered to a particular element in the DOM using the React DOM library. When rendering a component, one can pass in values that are known as "props":

ReactDOM.render(, document.getElementById('myReactApp'));

The two primary ways of declaring components in React are via function components and class-based components.

Function components

Function components are declared with a function that then returns some JSX.

const Greeter = (props) => 

Hello, {}!



Class-based components

Class-based components are declared using ES6 classes.

class ParentComponent extends React.Component {
  state = { color: 'green' };
  render() {
    return (

Virtual DOM

Another notable feature is the use of a virtual Document Object Model or virtual DOM. React creates an in-memory data structure cache, computes the resulting differences, and then updates the browser's displayed DOM efficiently. This process is called reconciliation. This allows the programmer to write code as if the entire page is rendered on each change, while the React libraries only render subcomponents that actually change. This selective rendering provides a major performance boost. It saves the effort of recalculating the CSS style, layout for the page, and rendering for the entire page.

Lifecycle methods

Lifecycle methods use a form of hooking that allows the execution of code at set points during a component's lifetime.


Main article: JSX

JSX, or JavaScript XML, is an extension to the JavaScript language syntax. Similar in appearance to HTML, JSX provides a way to structure component rendering using syntax familiar to many developers. React components are typically written using JSX, although they do not have to be (components may also be written in pure JavaScript). JSX is similar to another extension syntax created by Facebook for PHP called XHP.

An example of JSX code:

class App extends React.Component {
  render() {
    return (




); } }

Architecture beyond HTML

The basic architecture of React applies beyond rendering HTML in the browser. For example, Facebook has dynamic charts that render to  tags, and Netflix and PayPal use universal loading to render identical HTML on both the server and client



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