What is React?

React is a JavaScript tool that makes it easy to reason about, construct, and maintain stateless and stateful user interfaces. It provides the means to declaratively define and divide a UI into UI components (a.k.a., React components) using HTML-like nodes called React nodes. React nodes eventually get transformed into a format for UI rendering (e.g., HTML/DOM, canvas, svg, etc.).

I could ramble on trying to express in words what React is, but I think it best to just show you. What follows is a whirlwind tour of React and React components from thirty thousand feet. Don't try and figure out all the details yet as I describe React in this section. The entire book is meant to unwrap the details showcased in the following overview. Just follow along grabbing a hold of the big concepts for now.

Using React to create UI components similar to a <select>

Below is an HTML <select> element that encapsulates child HTML <option> elements. Hopefully the creation and functionality of an HTML <select> is already familiar.

source code

When a browser parses the above tree of elements it will produce a UI containing a textual list of items that can be selected. Click on the "Result" tab in the above JSFiddle, to see what the browser produces.

The browser, the DOM, and the shadow DOM are working together behind the scenes to turn the <select> HTML into a UI component. Note that the <select> component allows the user to make a selection thus storing the state of that selection (i.e., click on "Volvo", and you have selected it instead of "Mercedes").

Using React you can create a custom <select> by using React nodes to make a React component that eventually will result in HTML elements in an HTML DOM.

Let's create our own <select>-like UI component using React.

Defining a React component

Below I am creating a React component by invoking the React.createClass function in order to create a MySelect React component.

As you can see, the MySelect component is made up of some styles and an empty React <div> node element.

var MySelect = React.createClass({ //define MySelect component
    render: function(){
        var mySelectStyle = {
            border: '1px solid #999',
            display: 'inline-block',
            padding: '5px'
        };
        // using {} to reference a JS variable inside of JSX
        return <div style={mySelectStyle}></div>; //react div element, via JSX
    }
});

That <div> is an HTML-like tag, yes in JavaScript, called JSX. JSX is an optional custom JavaScript syntax used by React to express React nodes that can map to real HTML elements, custom elements, and text nodes. React nodes, defined using JSX should not be considered a one to one match to HTML elements. There are differences and some gotchas.

JSX syntax must be transformed from JSX to real JavaScript in order to be parsed by ES5 JS engines. The above code, if not transformed would of course cause a JavaScript error.

The official tool used to transform JSX to actual JavaScript code is called Babel.

After Babel (or something similar) transforms the JSX <div> in the above code into real JavaScript, it will look like this:

return React.createElement('div', { style: mySelectStyle });

instead of this:

return <div style={mySelectStyle}></div>;

For now, just keep in mind that when you write HTML-like tags in React code, eventually it must be transformed into real JavaScript, along with any ES6 syntax.

The <MySelect> component at this point consist of an empty React <div> node element. That's a rather trivial component, so let's change it.

I'm going to define another component called <MyOption> and then use the <MyOption> component within the <MySelect> component (a.k.a., composition).

Examine the updated JavaScript code below which defines both the <MySelect> and <MyOption> React components.

var MySelect = React.createClass({
    render: function(){
        var mySelectStyle = {
            border: '1px solid #999',
            display: 'inline-block',
            padding: '5px'
        };
        return ( //react div element, via JSX, containing <MyOption> component
            <div style={mySelectStyle}>
                <MyOption value="Volvo"></MyOption>
                <MyOption value="Saab"></MyOption>
                <MyOption value="Mercedes"></MyOption>
                <MyOption value="Audi"></MyOption>
            </div>
        );
    }
});

var MyOption = React.createClass({  //define MyOption component
    render: function(){
        return <div>{this.props.value}</div>; //react div element, via JSX
    }
});

You should note how the <MyOption> component is used inside of the <MySelect> component and that both are created using JSX.

Passing Component Options Using React attributes/props

Notice that the <MyOption> component is made up of one <div> containing the expression {this.props.value}. The {} brackets indicate to JSX that a JavaScript expression is being used. In other words, inside of {} you can write JavaScript.

The {} brackets are used to gain access (i.e., this.props.value) to the properties or attributes passed by the <MyOption> component. In other words, when the <MyOption> component is rendered the value option, passed using an HTML-like attribute (i.e., value="Volvo"), will be placed into the <div>. These HTML looking attributes are considered React attributes/props. React uses them to pass stateless/immutable options into components. In this case we are simply passing the value prop to the <MyOption> component. Not unlike how an argument is passed to a JavaScript function. And in fact that is exactly what is going on behind the JSX.

Rendering a component to the Virtual DOM, then HTML DOM

At this point our JavaScript only defines two React Components. We have yet to actually render these components to the Virtual DOM and thus to the HTML DOM.

Before we do that I'd like to mention that up to this point all we have done is define two React components using JavaScript. In theory, the JavaScript we have so far is just the definition of a UI component. It doesn't strictly have to go into a DOM, or even a Virtual DOM. This same definition, in theory, could also be used to render this component to a native mobile platform or an HTML canvas. But we're not going to do that, even though we could. Just be aware that React is a pattern for organizing a UI that can transcend the DOM, front-end applications, and even the web platform.

Let's now render the <MySelect> component to the virtual DOM which in turn will render it to the actual DOM inside of an HTML page.

In the JavaScript below notice I added a call to the ReactDOM.render() function on the last line. Here I am passing the ReactDOM.render() function the component we want to render (i.e., <MySelect>) and a reference to the HTML element already in the HTML DOM (i.e., <div id="app"></div>) where I want to render my React <MySelect> component.

Click on the "Result" tab and you will see our custom React <MySelect> component rendered to the HTML DOM.

source code

Note that all I did was tell React where to start rendering components and which component to start with. React will then render any children components (i.e., <MyOption>) contained within the starting component (i.e., <MySelect>).

Hold up, you might be thinking. We haven't actually created a <select> at all. All we have done is create a static/stateless list of text strings. We'll fix that next.

Before I move on I want to point out that no implicit DOM interactions were written to get the <MySelect> component into the real DOM. In other words, no jQuery code was invoked during the creation of this component. The dealings with the actual DOM have all been abstracted by the React virtual DOM. In fact, when using React what you are doing is describing a virtual DOM that React takes and uses to create a real DOM for you.

Using React state

In order for our <MySelect> component to mimic a native <select> element we are going to have to add state. After all what good is a custom <select> element if it can't keep the state of the selection.

State typically gets involved when a component contains snapshots of information. In regards to our custom <MyOption> component, its state is the currently selected text or the fact that no text is selected at all. Note that state will typically involve user events (i.e., mouse, keyboard, clipboard, etc.) or network events (i.e., AJAX) and its value is used to determine when the UI needs to be re-rendered (i.e., when value changes re-render).

State is typically found on the top most component which makes up a UI component. Using the React getInitialState() function we can set the default state of our component to false (i.e., nothing selected) by returning a state object when getInitialState is invoked (i.e., return {selected: false};). The getInitialState lifecycle method gets invoked once before the component is mounted. The return value will be used as the initial value of this.state.

I've updated the code below accordingly to add state to the component. As I am making updates to the code, make sure you read the JavaScript comments which call attention to the changes in the code.

var MySelect = React.createClass({
    getInitialState: function(){ //add selected, default state
        return {selected: false}; //this.state.selected = false;
    },
    render: function(){
        var mySelectStyle = {
            border: '1px solid #999',
            display: 'inline-block',
            padding: '5px'
        };
        return (
            <div style={mySelectStyle}>
                <MyOption value="Volvo"></MyOption>
                <MyOption value="Saab"></MyOption>
                <MyOption value="Mercedes"></MyOption>
                <MyOption value="Audi"></MyOption>
            </div>
        );
    }
});

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

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

With the default state set, next we'll add a callback function called select that gets called when a user clicks on an option. Inside of this function we get the text of the option that was selected (via the event parameter) and use that to determine how to setState on the current component. Notice that I am using event details passed to the select callback. This pattern should look familiar if you've had any experience with jQuery.

var MySelect = React.createClass({
    getInitialState: function(){
        return {selected: false};
    },
    select:function(event){// added select function
        if(event.target.textContent === this.state.selected){//remove selection
            this.setState({selected: false}); //update state
        }else{//add selection
            this.setState({selected: event.target.textContent}); //update state
        }   
    },
    render: function(){
        var mySelectStyle = {
            border: '1px solid #999',
            display: 'inline-block',
            padding: '5px'
        };
        return (
            <div style={mySelectStyle}>
                <MyOption value="Volvo"></MyOption>
                <MyOption value="Saab"></MyOption>
                <MyOption value="Mercedes"></MyOption>
                <MyOption value="Audi"></MyOption>
            </div>
        );
    }
});

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

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

In order for our <MyOption> components to gain access to the select function we'll have to pass a reference to it, via props, from the <MySelect> component to the <MyOption> component. To do this we add select={this.select} to the <MyOption> components.

With that in place we can add onClick={this.props.select} to the <MyOption> components. Hopefully it's obvious that all we have done is wired up a click event that will call the select function. React takes care of wiring up the real click handler in the real DOM for you.

var MySelect = React.createClass({
    getInitialState: function(){
        return {selected: false};
    },
    select:function(event){
        if(event.target.textContent === this.state.selected){
            this.setState({selected: false});
        }else{
            this.setState({selected: event.target.textContent});
        }   
    },
    render: function(){
        var mySelectStyle = {
            border: '1px solid #999',
            display: 'inline-block',
            padding: '5px'
        };
        return (//pass reference, using props, to select callback to <MyOption>
            <div style={mySelectStyle}>
                <MyOption select={this.select} value="Volvo"></MyOption>
                <MyOption select={this.select} value="Saab"></MyOption>
                <MyOption select={this.select} value="Mercedes"></MyOption>
                <MyOption select={this.select} value="Audi"></MyOption>
            </div>
        );
    }
});

var MyOption = React.createClass({
    render: function(){//add event handler that will invoke select callback
        return <div onClick={this.props.select}>{this.props.value}</div>;
    }
});

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

By doing all this we can now set the state by clicking on one of the options. In other words, when you click on an option the select function will now run and set the state of the MySelect component. However, the user of the component has no idea this is being done because all we have done is update our code so that state is managed by the component. At this point we have no feedback visually that anything is selected. Let's fix that.

The next thing we will need to do is pass the current state down to the <MyOption> component so that it can respond visually to the state of the component.

Using props, again, we will pass the selected state from the <MySelect> component down to the <MyOption> component by placing the property state={this.state.selected} on all of the <MyOption> components. Now that we know the state (i.e., this.props.state) and the current value (i.e., this.props.value) of the option we can verify if the state matches the value in a given <MyOption> component . If it does, we then know that this option should be selected. We do this by writing a simple if statement which adds a styled selected state (i.e., selectedStyle) to the JSX <div> if the state matches the value of the current option. Otherwise, we return a React element with unSelectedStyle styles.

source code

Make sure you click on the "Result" tab above and use the custom React select component to verify the new functioning.

While our React UI select component is not as pretty or feature complete as you might hope, I think you can see still where all this is going. React is a tool that can help you reason about, construct, and maintain stateless and stateful UI components, in a structure tree (i.e., a tree of components).

Before moving on to the role of the virtual DOM I do want to stress that you don't have to use JSX and Babel. You can always bypass these tools and just write straight JavaScript. Below, I'm showing the final state of the code after the JSX has been transformed by Babel. If you choose not to use JSX, then you'll have to write the following code yourself instead of the code I've written throughout this section.

var MySelect = React.createClass({
  displayName: 'MySelect',

  getInitialState: function getInitialState() {
    return { selected: false };
  },
  select: function select(event) {
    if (event.target.textContent === this.state.selected) {
      this.setState({ selected: false });
    } else {
      this.setState({ selected: event.target.textContent });
    }
  },
  render: function render() {
    var mySelectStyle = {
      border: '1px solid #999',
      display: 'inline-block',
      padding: '5px'
    };
    return React.createElement(
      'div',
      { style: mySelectStyle },
      React.createElement(MyOption, { state: this.state.selected, select: this.select, value: 'Volvo' }),
      React.createElement(MyOption, { state: this.state.selected, select: this.select, value: 'Saab' }),
      React.createElement(MyOption, { state: this.state.selected, select: this.select, value: 'Mercedes' }),
      React.createElement(MyOption, { state: this.state.selected, select: this.select, value: 'Audi' })
    );
  }
});

var MyOption = React.createClass({
  displayName: 'MyOption',

  render: function render() {
    var selectedStyle = { backgroundColor: 'red', color: '#fff', cursor: 'pointer' };
    var unSelectedStyle = { cursor: 'pointer' };
    if (this.props.value === this.props.state) {
      return React.createElement(
        'div',
        { style: selectedStyle, onClick: this.props.select },
        this.props.value
      );
    } else {
      return React.createElement(
        'div',
        { style: unSelectedStyle, onClick: this.props.select },
        this.props.value
      );
    }
  }
});

ReactDOM.render(React.createElement(MySelect, null), document.getElementById('app'));

Understanding the role of the Virtual DOM

I'm going to end this whirlwind tour where most people typically start talking about React. I'll finish off this React overview by talking about the merits of the React virtual DOM.

Hopefully you notice the only interaction with the real DOM we had during the creation of our custom select UI is when we told the ReactDOM.render() function where to render our UI component in the HTML page (i.e., render it to <div id="app"></div>). This might just be the only interaction you ever have with the real DOM when building out a React application from a tree of React components. And herein lies much of the value of React. By using React, you really don't ever have to think about the DOM like you once did when you were writing jQuery code. React replaces jQuery, as a complete DOM abstraction, by removing most, if not all, implicit DOM interactions from your code. Of course, that's not the only benefit, or even the best benefit.

Because the DOM has been completely abstracted by the Virtual DOM this allows for a heavy handed performance pattern of updating the real DOM when state is changed. The Virtual DOM keeps track of UI changes based on state and props. It then compares that to the real DOM, and then makes only the minimal changes required to update the UI. In other words, the real DOM is only ever patched with the minimal changes needed when state or props change.

Seeing these performant updates in real time will often clarify any confusion about the performant DOM diffing. Look at the animated image below showcasing the usage (i.e., changing state) of the UI component we created in this chapter.

Notice that as the UI component changes state only the minimally needed changes to the real DOM are occurring. We know that React is doing it's job because the only parts of the real DOM that are actually being updated are the parts with a green outline/background. The entire UI component is not being updated on each state change, only the parts that require a change.

Let me be clear, this isn't a revolutionary concept. You could accomplish the same thing with some carefully crafted and performant minded jQuery code. However, by using React you'll rarely, if ever, have to think about it. The Virtual DOM is doing all the performance work for you. In a sense, this is the best type of jQuery/DOM abstraction possible. One where you don't even have to worry about, or code for, the DOM. It all just happens behinds the scene without you ever implicitly having to interact with the DOM itself.

Now, it might be tempting to leave this overview thinking the value of React is contained in the fact that it almost eliminates the need for something like jQuery. And while the Virtual DOM is certainly a relief when compared to implicit jQuery code, the value of React does not rest alone on the Virtual DOM. The Virtual DOM is just the icing on the cake. Simply stated, the value of React rests upon the fact it provides a simple and maintainable pattern for creating a tree of UI components. Imagine how simple programming a UI could be by defining the entire interface of your application using reusable React components alone.

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