What is React.js?

React.js is a JavaScript library used for creating User Interfaces released by the creators of Facebook. React was built to solve one huge problem that anyone who has ever used JavaScript extensively has come across at one point or another:

Building applications with data that changes over time

Simply put, React will update the UI to reflect any new underlying data changes. It essentially "refreshes" the UI and knows to only update the new stuff. This is done using a special diff algorithm (kind of like how Git works) to see what has changed and what needs updated. In addition, React uses a Flux architecture pattern (more on this later) to allow for one-way reactive data flow.

A Different Way of Thinking with React

If we want to understand how React works, we need to understand what the DOM is. When we visit a website, our browser renders the source code of our HTML document. We use the DOM (Document Object Model) as an API to interact with the HTML document. The DOM is a fully object-oriented representation of this document as a structured group of nodes and objects that have properties and methods. It is essentially the connection between a web document (HTML) and a script (JavaScript).

React.js creates something called a Virtual DOM which is an in-memory representation of the DOM (like a snapshot). React's Virtual DOM is much lighter than the real browser DOM so manipulation with Virtual DOM is faster and more efficient. Because of this, React can quickly diff (check for differences) the current state of the UI with the desired state and compute the minimal set of DOM mutations. This golden concept is exactly how React "refreshes" the UI and knows to only update the new stuff.

Terms to Know

Here are some terms that will be helpful in navigating the React ecosystem.

Virtual DOM

An in-memory representation of the DOM. React manipulates this in order to quickly diff the current state of the UI.


JSX is an inline markup that looks like HTML and gets transformed to JavaScript. A JSX expression starts with an HTML-like open tag, and ends with the corresponding closing tag. JSX tags support the XML self close syntax so you can optionally leave the closing tag off.

You can definitely use React without JSX but JSX makes React a lot more elegant. Just like XML, JSX tags have a tag name, attributes, and children. Here's a quick example:

Without JSX, the React code would look like

render: function() {
return (
{className: "commentBox"},
"Hello, world! I am a CommentBox."

This is with JSX

render: function() {
return (
<div className="commentBox">
Hello, world! I am a CommentBox.


Flux or Flux Architecture is a concept/pattern, not a library or framework. The Flux pattern utilizes unidirectional reactive data flow as seen below.

Flux Architecture

This chart may look a bit confusing so let's break it down:

  1. Your view triggers an event (i.e. a user types text into a form input field)
  2. The event triggered by the view updates a model
  3. This model then triggers another event
  4. Finally, the view responds to that model's event by re-rendering with the latest data

This type of pattern is designed to guarantee your stores/models always have the right data.


This is the actual Javascript library. React's code is structured in a Flux pattern (as described above). There are 3 main concepts that makeup React:

  1. Components
  2. Properties
  3. State

React Components

Components are the smallest, yet most fundamental part of React. They’re similar, in concept, to things like widgets and modules. Think of React components in terms of the smallest possible components that you can define.

A React component can maintain internal state data. This can be thought as similar to data-binding (think AngularJS/Ember.js). When a component's state data changes, the rendered markup will be updated.

Here's an example of a basic component:

// This is our React component
var HelloWorld = React.createClass({
render : function() {
return <div>Hello World!</div>;

// We then render the component later
<HelloWorld />,

React Properties

Properties are to React components what attributes are to HTML elements. We can change the behavior based on external information

// This code will grab the attribute 'name' and output
// 'Hello Chris'
var InterfaceComponent = React.createClass({
render : function() {
return <div>Hello {this.props.name}!</div>;

<InterfaceComponent name="Chris" />,

React State

Like React properties, state affects how a component behaves and renders. Properties are defined when components are created. State, on the other hand, is only seen on the inside of component definitions.

When you think of properties, you should be thinking of component initialization. When you think of state, you should think of an internal data-set which affects the rendering of components.

Take a look at an example:

// Our components initial state is set to have
// '{name: Chris, job: Dev }'. When this component is
// rendered it will return 'My name is Chris and I am a Dev'
var InterfaceComponent = React.createClass({
getInitialState : function() {
return {
name : "Chris",
job : "Dev"
render : function() {
return <div>
My name is {this.state.name}
and I am a {this.state.job}.

// Here we render our component to the document's body
<InterfaceComponent />,


While Flux is the architectural pattern React follows, Redux is a library that gives you tools to aid in the implimentation of the Flux pattern.

Enough talk. GIVE ME CODE!!!

A Basic React App

Let's build a quick little React app to get a better understanding of how everything works. We will be building a simple Todo App...suprise!

1. Setup

Let's start off with this basic HTML boilerplate:


<!DOCTYPE html>
<meta charset="utf-8" />
<title>React Todo App</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.14.6/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.14.6/react-dom.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.23/browser.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="content"></div>
<script type="text/babel" src="script.js"></script>

Notice the "text/babel" instead of "text/javascript" in the link to the script.js which we are using. Babel is the Javascript complier we are using to allow us to write in JSX instead of plain Javascript. Now that the foundation is set, let's get to the React code. Create a file called script.js and open that open. The rest of the following code will be going into this file.

2. Our First React Component

Let's start off by creating our first React component. We do this by using the React.createClass() method. This first component will be our main application. You can create the component by using the createClass() method.


var TodoApp = React.createClass({


Inside our TodoApp, we are first going to call React's getInitialState method. This is where we tell React to set the initial state of our application. In this function, we will return an empty array of items. That will the starting state of our todo items.

var TodoApp = React.createClass({

getInitialState: function() {
return { items: [], text: '' };


Just below getInitialState, we are going to add an onChange method. This function watches for any UI changes (value input, checked checkboxes, selected option). Each time the UI is changed, we will set the React state based on our input.

var TodoApp = React.createClass({

getInitialState: function() {
return { items: [], text: '' };

onChange: function(e) {
this.setState({ text: e.target.value });


Next, we will add the handleSubmit function just below onChange. This function will handle our form submit. First, we prevent the default submit event. Then, we set a nextItems variable where we grab the item text from the onChange function along with setting an id using the current date. After, we set the nextText to empty to set it up for the next entry. Finally, we use the setState function to merge current and previous states and prepare for a new entry.

handleSubmit: function(e) {


var nextItems = this.state.items.concat([{
text: this.state.text,
id: Date.now()

var nextText = '';

items: nextItems,
text: nextText

The last thing we are going to do to complete our main component TodoApp is add a render function which returns a tree of React components which will eventually render to HTML. Take note of a few things: First, notice since we are using JSX, instead of using the standard <div class="row"> we have to use <div className="row">. Also notice we are referencing our previous functions inside our returned view. On our form submit we call the handleSubmit function and when there is UI changes to the form input, we call the onChange function.

render: function() {
return (
<div className="row">
<div className="four columns">
<form onSubmit={this.handleSubmit}>
<input className="u-full-width" type="text" onChange={this.onChange} value={this.state.text} />
<button className="button-primary">Add +</button>
<div className="eight columns">
<TodoList items={this.state.items} />

We have a form to input information and handle the events, but still nowhere to render the actual list. Notice the <TodoList items={this.state.items} /> node. This is where we will be calling another React component that will handle rendering list items. What we are basically doing is calling one component inside of another. React encourages "smallest possible components that you can define". Since each component can be isolated, fixes can be applied to one component without inherently affecting the others.

Next, let's build our TodoList component. This will handle rendering the actual list. We will be placing this code outside of our TodoApp component, just above it:

var TodoList = React.createClass({
render: function() {
var createItem = function(item) {
return <li key={item.id}>{item.text}</li>;
return <ul>{this.props.items.map(createItem)}</ul>;

Here we are creating a list item for each todo list item and then returning those inside of an unordered list element. Notice we are referencing the React properties by using this.props to get the items from attribute items in our TodoApp.render() method (<TodoList items={this.state.items} />).

The last step in our app is to render it all to our real DOM. We will call ReactDOM.render() method along with our main app and a place for it to go. In this example, we want to render it to <div id="content"></div>.

<TodoApp />,

And there we have it! Our first React app. Once you get your head around all the terminology and concepts, React is an incredible tool for both large and small applications.

React.js VS AngularJS?

There is an ongoing "debate" taking place between the 2 juggernauts AngularJS and React.js in the Javascript framework universe on which is best. Angular as you may know is supported by Google while React is backed by Facebook. What is the main difference between React.js and AngularJS? There is alot of "Angular vs React" going around the net, but comparing them is similar to comparing a brand new car (Angular) to a brand new engine (React).


  • A Front-end full MVC Framework.
  • Uses HTML, CSS and Javascript code.


  • A View Framework. The V in MVC.
  • Uses just Javascript code with a special syntax called JSX which allows you to use HTML in your Javascript.

So React.js is like a engine while AngularJS is the entire car. Since React is just the V of MVC you could technically use React.js with AngularJS, substituting AngularJS views for that of React.js (take a look at one example here).

Other Things You Can Do With React - React Native

Now that you understand the fundamentals of React, you can use your newfound knowledge to actually build Native Mobile apps! Facebook has also released a great library called React Native which allows you to use React.js in order to build apps on native platforms such as iOS and Android. This means you can use all of the same concepts across multiple platforms. Take a deeper look here.


Suggestions or edits? Feel free to submit a pull request on Github.

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