This article is part of my personal wiki where I write personal notes while I am learning new technologies. You are welcome to use it for your own learning!

React.js is a javascript library developed at Facebook to build interactive UIs that focuses on the View part of MVC.

Everything within a React is a component. A component has a state (mutable) and properties (immutable) and produces some arbitrary HTML code.

React provides a special language called JSX, a javascript-meets-html-like language that let’s you embed html templates into javascript code in a very straight-forward manner. jsx files are compiled into javascript and result in building a virtual DOM. Adding this additional layer of abstraction between and application and the real DOM, provides React with a lot of flexibility to improve the performance tied to DOM manipulation.

A Simple React JS Component

A simple example of a react component written in jsx could be this button:

// Create new component
var Button = React.createClass({
  render: function(){
    return (
      <button>Hello World</button>

You can create a simple component by using the React.createClass method and passing an object that contains a render method which returns a JSX template of how the component is going to look like when rendered. You can render the component in the DOM by using React.createElement followed by React.render:

// render component
var element = React.createElement(Button);

// add element to the dom
React.render(element, document.body);

Components Can Have Properties

A react component can have properties that can be displayed in the rendered HTML template. You can pass these properties as a vanilla JavaScript object in the React.createElement method and access them inside the React component via the this.props property. React lets you inject these properties inside the HTML template by using curly braces {this.props.messages}. For instance, this component:

  // define a component
  var Badge = React.createClass({
    render: function(){
      return <button className="btn btn-primary">
      {this.props.title} <strong>{this.props.number}</strong>
  // render component
  var element = React.createElement(Badge, {title: 'Inbox', number: 5});

  // add element to the dom
  React.render(element, document.body);

Would render an HTML like this:

<button class="btn btn-primary">Inbox <strong>5</strong></button>

Nesting Components

Once you have defined a series of components you can easily nest them inside your JSX files. For instance, if you have a navigation item you can nest the previous Badge component in a very straightforward fashion:

  var Nav = React.createClass({
    render: function(){
    return <nav>
        <li><Badge title={this.props.title} number={this.props.number}/></li>
  // render component
  var element = React.createElement(Nav, {title: 'Inbox', messages: 5});

  // add element to the dom
  React.render(element, document.body);

In this example the Nav component includes a nested Badge component. When we render the Nav element we pass an options object whose title and number will be in turn passed to the Badge component via its attributes.

Here is another example with a card that contains an image and a badge:

      var Card = React.createClass({
        render: function() {
          return <div className="thumbnail">
            <img src={this.props.cardImageUrl}/>
            <div className="caption">
              <p><Badge title={this.props.title} /></p>
      var card = React.createElement(Card, {
        title: 'Sing up',
        cardImageUrl: '',
        header: 'awesome',
        description: 'jajajajrajjjafdasf'
      // add component to the DOM
      React.render(card, document.querySelector('.root'));