Building UIs with React.js – A Beginner’s Tutorial

React is a library for creating user interfaces with a component‑based approach and a virtual DOM diffing engine.

React 0.13 (released March 2015) added optional ES6 class support but React.createClass remains the common style.

Why React changes front‑end work

Component isolation – each piece of UI is a self‑contained function of props and state.
Virtual DOM – React builds an in‑memory tree and updates only the changed parts on the real DOM, avoiding full page reflows.
One‑way data flow – parent → child props keep data reasoning straight.
Declarative rendering – describe what the UI looks like; React figures out how to update it.

(Check the React v0.13 release notes for details on class support and goals.)

Quick setup

Add React, ReactDOM, and JSX transformer via CDN:

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/react-dom.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.23/browser.min.js"></script>
<div id="app"></div>

Write components in a <script type="text/babel"> block.
The browser runs Babel in‑place to transform JSX into plain JavaScript.

Counter component

Uses React.createClass and local state.

const Counter = React.createClass({
  getInitialState() {
    return { count: 0 };
  },
  add() {
    this.setState({ count: this.state.count + 1 });
  },
  render() {
    return (
      <button onClick={this.add}>
        Clicked {this.state.count} times
      </button>
    );
  }
});

App wrapper

Holds the Counter and shows a heading.

const App = React.createClass({
  render() {
    return (
      <section>
        <h1>Demo: React Counter</h1>
        <Counter />
      </section>
    );
  }
});

Rendering

ReactDOM.render(<App />, document.getElementById('app'));

React builds a virtual DOM tree from App, compares it with the previous tree on every setState, and patches only the node that displays the count.

JSX syntax

JSX lets you write HTML‑like tags in JavaScript. Babel turns this syntax into React.createElement calls.

const element = <h1 className="title">Hello React</h1>;

The code above compiles to:

const element = React.createElement(
  'h1',
  { className: 'title' },
  'Hello React'
);

Props versus state

Props: read‑only data passed from parent to child.
State: mutable data local to a component.

State changes with setState() cause React to re‑render the component and its children. Props flow down; state stays inside.

Key takeaways

Components bundle markup, logic, and state.
State changes trigger efficient re‑renders through the virtual DOM diff.
Composition lets small pieces form complex UIs.

This counter shows the minimum needed to start. Expand with more components, props, and events to grow a full application.