React (a.k.a. ReactJS or React.js) is a JavaScript library for creating user interfaces, open sourced to the world by Facebook and Instagram team in 2013. One might think of it as the “View” in the “Model-View-Controller” pattern.

React’s main goal is to be simple, declarative, and compassable, as it is intended to ease the process of building large applications using data that changes over time.

React was created by Jordan Walke, a software engineer at Facebook, with the influence of XHP, a PHP-based component system that is still in use at Facebook, but also by functional programming ideas. Pete Hunt wanted to use React at Instagram, so he pushed to extract React from Facebook-specific code and open source it. (Editor's note: Some have expressed concern about the React license; see discussions here, here, and most recently (Jan2016) here, with discussion of some alternatives here. We're not lawyers but figure it's important to be aware of this sort of thing.)

React has lately gained a lot of popularity for its concept of a “virtual-DOM,” which allows it to determine which parts of the DOM have changed by diffing the new version with the stored virtual DOM, and using the result to determine how to most efficiently update the browser's DOM.

To get an idea of what’s going on inside React, take a look at the following diagrams demonstrating how React.js rendering and the React Virtual DOM work:

Figure 1: React.js Rendering

Figure 2: The React Virtual DOM

React basically batches DOM updates and then applies minimal diffs to the real DOM.

React components implement a render() method that takes input data and returns what to display.

Here is a simple example of a "Hello" Component:

var Hello = React.createClass({
  render: function () {
    return <div>Hello {this.props.name}</div>;
  }
});

React.render(<Hello name="World" />, document.body);

Note: The XML-like syntax is called JSX.

Here's the same component without JSX:

var Hello = React.createClass({displayName: "Hello",
  render: function() {
    return React.createElement("div", null, "Hello ", this.props.name);
  }
});

React.render(React.createElement(Hello, {name: "World"}), mountNode);

Components API

Lifecycle Methods

States and Properties

Props and states are both plain JS objects; a change within one of them will trigger a render. These objects are deterministic.

<VideoComponent fullscreen={true} />
// props
  this.props.fullscreen //=> true
// state
  this.setState({ user: 'hemanth' });
  this.replaceState({ ... });
  this.state.username //=> 'hemanth'
  render: function () {
    return <div className={this.props.fullscreen ? 'full' : ''}>
      Hello, {this.state.username}
    </div>;
}

Pre-populates states and props:

React.createClass({
  getInitialState: function () {
    return { comments: [] };
  },
  getDefaultProps: function () {
    return { name: "Hello" };
  }
);

Deciding when to use props and went to use state might get tricky. The following table will help to simplify this decision:

React by default doesn’t provide a helper method to manage AJAX requests, but you can use any other third party JavaScript library—like jQuery or Zepto—to make necessary AJAX requests.

Below is a sample code snippet that performs an AJAX request on props.url and on success sets the data state. In case of  an error, it just uses console.error to report the error.

Note: Make sure that the execution context (this) is bound to the success and error callbacks.

componentDidMount: function() {
    $.ajax({
      url: this.props.url,
      dataType: 'json',
      cache: false,
      success: function(data) {
        this.setState({data: data});
      }.bind(this),
      error: function(xhr, status, err) {
        console.error(this.props.url, status, err.toString());
      }.bind(this)
    });
  }

In React, styles are mentioned in line, but unlike the traditional way of inline CSS strings, here we specify each style as an object whose key is the camelCased version of the style name, and whose value is the style's value (usually a string).

var divStyle = {
  color: 'white',
  backgroundImage: 'url(' + imgUrl + ')',
  WebkitTransition: 'all', // note the capital 'W' here
  msTransition: 'all' // 'ms' is the only lowercase vendor prefix
};
React.render(<div style={divStyle}>Hello World!</div>, mountNode);

Most numeric values at the end of a style prop receive an automatic “px” specification added to them (e.g., “width: 10” is read as “width: 10px”). Here is a list of properties that won't get the automatic "px" suffix:

• boxFlex

• boxFlexGroup

• columnCount

• fillOpacity

• flex

• flexGrow

• flexPositive

• flexShrink

• flexNegative

• fontWeight

• lineClamp

• lineHeight

• opacity

• order

• orphans

• strokeOpacity

• widows

• zIndex

• zoom

References

Helps to access the DOM nodes.

<input ref="firstName">
this.refs.firstName
React.findDOMNode(this.refs.firstName).focus()
React.findDOMNode(this.refs.firstName).value
DOM Events: Helps to handle DOM events.
<input type="text" value={this.state.value} onChange={this.handleChange} />
handleChange: function(event) {
  this.setState({ value: event.target.value });
}

Two-way data bindings with mixins

Email: <input type="text" valueLink={this.linkState('email')} />
React.createClass({
  mixins: [React.addons.LinkedStateMixin]
});
this.state.email

Validating properties

Primitive types: .string, .number, .func, and .bool.

React elements: .element, .node.

Enumerables: .oneOf, .oneOfType.

Arrays and objects: array[Of], .object[Of], .instanceOf, .shape.

Sample usage:

React.createClass({
  propTypes: {email: React.PropTypes.string, firstName: React.PropTypes.string, age:  React.PropTypes.number, gender: React.PropTypes.oneOf(['M','F','NA']) 
    node: React.PropTypes.node, cb: React.PropTypes.func.isRequired,
  }
});

Custom Validation

propTypes: {
  customProp: function(props, propName, componentName) {if (!/matchme/.test(props[propName])) {return new Error('Validation failed!');}
  }
}

React Addons

React.addons are useful utilities for building React apps, these are currently experimental, and not yet part of Core React.

• TransitionGroup and CSSTransitionGroup, for dealing with animations and transitions

• LinkedStateMixin helps in two way data binding.

• cloneWithProps, to make shallow copies of React components and change their props.

• createFragment, helps to create a set of externally-keyed children.

• update, helps to deal with immutable data.

• PureRenderMixin, a performance booster under certain situations.

Apart from these there are few which are available in the development (unminified) version of React only:

• TestUtils, simple helpers for writing test cases

• Perf, for measuring performance and giving you hint where to optimize.

To get the add-ons, use react-with-addons.js (and its minified counterpart) rather than the common react.js.

When using the react package from npm, simply require('react/addons') instead of require('react') to get React with all of the add-ons.

Creating Your Own Mixins

var TimeOutMixin = {
  componentWillMount: function() { .. }
}
var TickTock = React.createClass({
  mixins: [TimeOutMixin]
}

React on ES2015/ES6:

These are experimental and you must use a transpiler for this to work

Classes

class Animal extends React.Component {
  render() {
      return <img alt={this.props.name} src={this.props.src} />;
  }
}

Property Initializers

var Video = React.createClass({
  getDefaultProps: function() {
                                    return {
                                    autoPlay: false,
                                    maxLoops: 10,
                                    };
  },
  getInitialState: function() {
                                    return {
                                    loopsRemaining: this.props.maxLoops,
                                    };
  },
  propTypes: {
                                    autoPlay: React.PropTypes.bool.isRequired,
                                    maxLoops: React.PropTypes.number.isRequired,
                                    posterFrameSrc: React.PropTypes.string.isRequired,
                                    videoSrc: React.PropTypes.string.isRequired,
  },
});

Arrow Functions

class PostInfo extends React.Component {
  handleOptionsButtonClick = (e) => {
                                    this.setState({showOptionsModal: true});
  }
}

Dynamic property names with template strings

class Form extends React.Component {
  onChange(inputName, e) {
                                    this.setState({
                                    [`${inputName}Value`]: e.target.value,
                                    });
  }
}

Destructuring & spread attribute

class AutoloadingPostsGrid extends React.Component {
  render() {
                                    var {
                                    className,
                                    ...others,  // all properties of this.props except for className
                                    } = this.props;
                                    return <PostsGrid {...others} />
  }
}

CoffeeScript and React [React v0.13.0+]

div = React.createFactory 'div'
class Counter extends React.Component
  @propTypes = initialCount: React.PropTypes.number
  @defaultProps = initialCount: 0
  constructor: (props) ->
                                    super props
                                    @state = count: props.initialCount
  tick: =>
                                    @setState count: @state.count + 1
  render: ->
                                    div onClick: @tick,
                                    'Clicks: '
                                    @state.count

• React DOM elements are expected to be in camelCased.

• the camelCasing of DOM matches what you would write for custom components: <Typeahead onClick=../> and <div onClick=.. />

• These camelCased attributes are usually what you write when updating the DOM via JS (input.maxLength).

• One current confusing exception is that class= is normalized automatically into className= at transform time.

• Use className attribute instead of class

• Use htmlFor attribute instead of for

• Use <textarea value="something"> instead of <textarea>something</textarea>

• Custom HTML attributes may be data-attr and all lower case.

• Make use of ternary operators, wherever  required for example: React.render(<div id={condition ? 'msg' : ''}>Hello World!</div>, mountNode);

Anti-pattern

Props in getInitialState Is an Anti-Pattern:

Instead of:

var MessageBox = React.createClass({
  getInitialState: function() {
    return {nameWithQualifier: 'Mr. ' + this.props.name};
  };
  render: function() {
    return <div>{this.state.nameWithQualifier}</div>;
  }
});
React.render(<MessageBox name="Rogers"/>, mountNode);

Try this:

var MessageBox = React.createClass({
  render: function() {
    return <div>{'Mr. ' + this.props.name}</div>;
  }
});
React.render(<MessageBox name="Rogers"/>, mountNode);

Note: It won’t be an anti-pattern if we make it clear that synchronization's not the goal.

Spread Operator in JSX:

  var props = {};
  props.foo = x;
  props.bar = y;
  var component = <Component {...props} />;

You can also use the spread operator (...) to override props:

  var props = { foo: 'default' };
  var component = <Component {...props} foo={'override'} />;
  console.log(component.props.foo); // 'override'

The spread operator is already supported for arrays in ES6. There is also an ES7 proposal for Object Rest and Spread Properties.

Flux plays a key role if your application use dynamic data.

Don't try to compare Flux to Model View Controller (MVC) architecture, Flux is just a term to describe smart uni -directional data flow.

Figure 3: An Overview of Flux Architecture

Keys ideas:

• Views "Dispatch" "Actions".

• "Store" Responds to Dispatched Events.

•  Store Emits a "Change" Event.

• View Responds to the "Change" Event

Points to remember:

• A "dispatcher" is essentially an event system.

• There is at most one global dispatcher.

• “Store” is specific collection of logic and data.

• A “Store” is a singleton.

• A store is not a model. A store containsmodels.

•  A store is the only entity in the application that is aware of how to update data.

• Only stores registers to dispatcher callbacks. store emits an event, but not using the dispatcher!

• When store data changes, your views shouldn't care if things were added, deleted, or modified but just re-render.

React version 0.14 separated out all DOM related functionality into a separate dependency, which can be fetched from npm under the name `react-dom`

React version 0.14 separated out all DOM related functionality into a separate dependency, which can be fetched from npm under the name `react-dom`

Every new change in 0.14, including the major changes, is introduced with a runtime warning and will work as before until 0.15.

The react package contains React.createElement, .createClass, .Component, .PropTypes, .Children, and the other helpers related to elements and component classes

The react-dom package has ReactDOM.render, .unmountComponentAtNode, and .findDOMNode. In react-dom/server there is a server-side rendering support with ReactDOMServer.renderToString and .renderToStaticMarkup.

So, if you wanted to duplicate the example Hello component from earlier with react v0.14, it would look like this:

var React = require('react');
var ReactDOM = require('react-dom');
var Hello = React.createClass({
render: function() {
return <div>Hello World</div>;
}
});

ReactDOM.render(<Hello/>, node);

Jest allows for painless JavaScript Unit Testing. It is built on top of the Jasmine test framework.

Consider a scenario where you want to test the following div.js file:

// sum.js
function div (value1, value2) {
      return value1 /  value2;
                 }
                module.exports = sum;

// __tests__/sum-test.js
jest.dontMock('../sum');
describe('sum', function() {
    it('divides 4 / 2 to equal 2', function() {
        var sum = require('../div);
        expect(div(4, 2)).toBe(2);
     });
});

2.    Run npm install jest-cli --save-dev

3.    Add the following to your package.json

{ ... "scripts": { "test": "jest" } ... }

4.    Run npm test

[PASS] __tests__/sum-test.js (0.015s)

Just follow the above steps, and you are ready to run with Jest!

Note: For a react component, we follow the same steps along with it, we will have to use ‘React.addons.TestUtils`.