ReactJS overview

ReactJS is a JavaScript library for building user interfaces.

Components and props

Components are the basic building block of ReactJS. Conceptually, they are like JavaScript functions. They accept arbitrary inputs props and return React elements describing what should appear on the screen.

The simplest way to define a component is to write a JavaScript function:

function Welcome(props) {
    return <h1>Hello, {props.name}</h1>;

This function is a valid React component because it accepts a single props (which stands for properties) object argument with data and returns a React element. Such components are called function components because they are literally JavaScript functions.

Another way to define a component is to use an ES6 class:

class Welcome extends React.Component {
    render() {
        return <h1>Hello, {this.props.name}</h1>;

The above two components are equivalent from React's point of view.


React components use JSX, a syntax extension to JavaScript. During the build process, the JSX code is transpiled to regular JavaScript (ES5) code.

The following two examples are equivalent:

// JSX
const element = (
    <h1 className="greeting">
    Hello, world!

// Transpiled to createElement() call
const element = React.createElement(
  {className: 'greeting'},
  'Hello, world!'


New React elements are created from component classes using the createElement function:


This function takes three arguments:

  • type can be either a tag name string (such as div or span), or a component class.

  • props contains a list of attributes passed to the new element.

  • children contains the child node(s) of the new element (which are additional React components).

Safe by design

ReactJS implements security controls by design, for example, string variables in views are escaped automatically.


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