React is a JavaScript UI library created by Facebook. Information on this page is taken from the React guide.


React components accept inputs (props) and return React elements describing what should appear on the screen. Components may be defined with functions or ES6 classes.

class FriendlyButton extends React.Component {
    constructor(props) {
        this.state = { clicked: false };

    render() {
        if (this.state.clicked) {
            return 'Clicked!';
        } else {
            return (
                <button onClick={() => this.setState({ clicked: true })}>
                  Click me, {}!

This component could be rendered with <FriendlyButton name="Elliot" />. In addition to render(), React has other lifecycle methods like componentDidMount() and componentWillUnmount().

Typically, React applications have a single App component at the very top.

Apply React

  1. Place an empty <div> tag in your HTML file.
  2. Include React dependencies in <script> tags.
  3. Load component code. e.g.:
class Example extends React.Component {
    render() {
        return React.createElement('div', ...);

const domContainer = document.querySelector('#container'));  // empty div from above
ReactDOM.render(React.createElement(Example), domContainer);

Note that you may place as many independent DOM containers on one page as you need. See the React guide for more information.


JSX is an eXtension to JavaScript that looks a bit like HTML. JSX produces React elements (i.e. React.createElement() calls). Applications leveraging JSX should use a preprocessor (e.g. babel).


  • Be sure to use React Dev Tools in Chrome.
  • A React toolchain usually includes a package manager (e.g. npm), a bundler (e.g. webpack), and a compiler (e.g. babel). Consider getting started with create-react-app.
  • Capitalize N in the className prop.
  • Instead of collecting data from children, or having two children communicate, declare shared state in the parent component. The parent can pass the state back down to the children with props. Lifting state into a parent component is common when refactoring React components.
  • Favor immutability to enable pure components.
  • Don't be afraid to split components into smaller components.
  • Do not modify state directly, use setState(). Note that this function can either accept an object or a function. The function receives previous state and props.
  • Call e.preventDefault() when handling events. More information here.
  • See this guide for information on using forms in React.