React JS for Beginners

React JS

React is a front-end library developed by Facebook. It is used for handling the view layer for web and mobile apps. ReactJS allows us to create reusable UI components. It is currently one of the most popular JavaScript libraries and has a strong foundation and large community behind it.

Is React JS a Library or a Framework?

This is one of the most unclear subjects of React. Let’s make this clear from the beginning. React is a Library, not a Framework.

What is a Library?

A library in programming can be explained as a collection of codes. We use a library to write code in a much simpler way or to import a feature from it into our project. JQuery is a library for example.

We can write JavaScript much simpler by using JQuery, or we can import written JQuery features to our project. The project itself is not dependent on a library.

What is a Framework?

A Framework, on the other hand, is a complete package of code with its own functionalities & libraries. A Framework has its own rules, you don’t have much flexibility and the project is dependent on the Framework you use. Angular is an example of a framework.

So React is for building User Interfaces, and how you program the rest of the project is up to you. Like JQuery, you can include React in your project partially, or completely. So React JS a library.

Now let’s move on with how React actually works…

The setup

When getting started with React, you should use the simplest setup possible: an HTML file which imports the React and the ReactDOM libraries using script tags.

It looks like this:

We’ve also imported Babel, as React uses something called JSX to write markup. We’ll need to transform the JSX into plain JavaScript, so that the browser can understand it.

There are more two things I want you to notice:

1. The <div> with the id of #root. This is the entry point for our app. This is where our entire app will live.
2. The <script type="text/babel"> tag in the body. This is where we’ll write our React code.

Components

Everything in React is a component, and these usually take the form of JavaScript classes. You create a component by extending upon the React-Component class. Let’s create a component called Hello:

class Hello extends React.Component {
    render() {
        return <h1>Hello world!</h1>;
    }
}

You then define the methods for the component. In our example, we only have one method, and it’s called render().

Inside render() you’ll return a description of what you want React to draw on the page. In the case above, we simply want it to display an h1 tag with the text Hello world! inside it.

To get our tiny application to render on the screen, we also have to use ReactDOM.render():

ReactDOM.render(
    <Hello />, 
    document.getElementById("root")
);

So this is where we connect our Hello component with the entry point for the app (<div id="root"></div>).

So we’re simply saying: Hey React! Please render the Hello component inside the DOM node with an id of root!

It results in the following:

Everything in React is a component, and these usually take the form of JavaScript classes. You create a component by extending upon the React-Component class. Let’s create a component called Hello:

class Hello extends React.Component {
    render() {
        return <h1>Hello world!</h1>;
    }
}

You then define the methods for the component. In our example, we only have one method, and it’s called render().

Inside render() you’ll return a description of what you want React to draw on the page. In the case above, we simply want it to display an h1 tag with the text Hello world! inside it.

To get our tiny application to render on the screen, we also have to use ReactDOM.render():

ReactDOM.render(
    <Hello />, 
    document.getElementById("root")
);

So this is where we connect our Hello component with the entry point for the app (<div id="root"></div>).

So we’re simply saying: Hey React! Please render the Hello component inside the DOM node with an id of root!

It results in the following:

The HTML’ish syntax we just looked at (<h1> and <Hello/>) is the JSX code I mentioned earlier. It’s not actually HTML, it’s a lot more powerful. Though what you write there does end up as HTML tags in the DOM.

The next step is to get our app to handle data.

Handling data

There are two types of data in React: props and state. The difference between the two is a bit tricky to understand in the beginning, so don’t worry if you find it a bit confusing. It’ll become easier once you start working with them.

The key difference is that the state is private and can be changed from within the component itself. Props are external, and not controlled by the component itself. It’s passed down from components higher up the hierarchy, who also control the data.

A component can change its internal state directly. It can not change its props directly.

Let’s take a closer look at props first.

Props

Our Hello component is completely static. It renders out the same message no matter what. However, a big part of React is reusability, meaning the ability to write a component once, and then reuse it in different use cases. For example to display different messages.

To achieve this type of reusability, we’ll add props. This is how you pass props to a component:

ReactDOM.render(
    <Hello message="my friend" />, 
    document.getElementById("root")
);

This prop is called message and has the value “my friend”. We can access this prop inside the Hello component by referencing this.props.message, like this:

class Hello extends React.Component {
    render() {
        return <h1>Hello {this.props.message}!</h1>;
    }
}

As a result, this is rendered on the screen:

The reason we’re writing {this.props.message} with curly braces is because we need to tell the JSX that we want to add a JavaScript expression. This is called escaping.

So now we have a reusable component which can render whatever message we want on the page. Woohoo!

However, what if we want the component to be able to change its own data? Then we have to use state instead!

State

The other way of storing data in React is in the component’s state. And unlike props — which can’t be changed directly by the component — the state can.

So if you want the data in your app to change — for example based on user interactions — it must be stored in a component’s state somewhere in the app.

Initializing state

To initialize the state, simply set this.state in the constructor() method of the class. Our state is an object which in our case only has one key called message.

class Hello extends React.Component {
    
    constructor(){
        super();
        this.state = {
            message: "my friend (from state)!"
        };
    }
    
    render() {
        return <h1>Hello {this.state.message}!</h1>;
    }
}

Before we set the state, we have to call super() in the constructor. This is because this is uninitialized before super() has been called.

Changing the state

To modify the state, simply call this.setState(), passing in the new state object as the argument. We’ll do this inside a method which we’ll call updateMessage.

class Hello extends React.Component {
    
    constructor(){
        super();
        this.state = {
            message: "my friend (from state)!"
        };
        this.updateMessage = this.updateMessage.bind(this);
   }    updateMessage() {
        this.setState({
            message: "my friend (from changed state)!"
        });
    }        render() {
        return <h1>Hello {this.state.message}!</h1>;
    }
}

Note: To make this work, we also had to bind the this keyword to the updateMessage method. Otherwise we couldn’t have accessed this in the method.

Event Handlers

The next step is to create a button to click on, so that we can trigger the updateMessage() method.

So let’s add a button to the render() method:

render() {
  return (
     <div>
       <h1>Hello {this.state.message}!</h1>
       <button onClick={this.updateMessage}>Click me!</button>
     </div>   
  )
}

Here, we’re hooking an event listener onto the button, listening for the onClick event. When this is triggered, we call the updateMessage method.

Here’s the entire component:

class Hello extends React.Component {
    
    constructor(){
        super();
        this.state = {
            message: "my friend (from state)!"
        };
        this.updateMessage = this.updateMessage.bind(this);
    }    updateMessage() {
        this.setState({
            message: "my friend (from changed state)!"
        });
    }    render() {
         return (
           <div>
             <h1>Hello {this.state.message}!</h1>
             <button onClick={this.updateMessage}>Click me!</button>
           </div>   
        )
    }
}

The updateMessage method then calls this.setState() which changes the this.state.message value. And when we click the button, here’s how that will play out: