React - Getting Started (The Welcome Element)

React is a virtual dom library that:

• permits to define the HTML dom programmatically
• will update HTML dom incrementally

There is basically two big steps when you work with React (or any virtual dom library)

• You write your React DOM element (ie the equivalent of HTML element) with the createElement function.
• You render them where React will turn the React DOM into an HTML dom making the needed modification.

In the below code, you see:

• A html h1 react element created with the create element function (in the welcome function)
• A Welcome user defined react element (also known as component) that accepts arguments (called props)
  • making the h1 text dynamic with a name
  • allowing to add children

function Welcome(props) {
  return React.createElement(
     'h1',  // the type of element (html element type)
     {},  // the props (html such as element can get attribute such as  class)
     [ // the children of the element build the DOM tree
        `Hello, ${props.name}`, // the first child is a text node
        props.children // the other node passed with the arguments
     ]
     );
}

You can compose the element to create the React DOM Tree. Below we define our top node of the tree:

• with a welcome element
• that accepts a welcome element as child

const MyRootNode = Welcome( 
    { 
        name:"Bar",
        children: Welcome({name:"Foo"})
    }
);

The react library ¹⁾needs to be present.

<script crossorigin src="https://unpkg.com/[email protected]/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/[email protected]/umd/react-dom.production.min.js"></script>

To render in a HTML page, React needs to now where to put the HTML that it will create.

This html element is generally a div called the root Id.

In your HTML page:

<div id="root"></div>

The creation of the HTML (rendering) is done via the ReactDOM.render that takes two parameters:

• a React element (component, html element). We pass our root node but you can pass any other element.
• The HTML node where to add the generated HTML (in our case root)

ReactDOM.render(
  MyRootNode,
  document.getElementById('root')
);

The above rendering function will output:

If you inspect the generated HTML DOM, you will see:

<h1>Hello, Bar
    <h1>Hello, Foo</h1>
</h1>

You can write your javascript in jsx

• to replace the create element function.
• by markup xhtml notation

making your code less verbose.

The above React Element written in JSX would become:

• The definition of Welcome tag in Jsx

function Welcome(props) {
  return <h1>Hello, {props.name} {props.children}</h1>;
}

• The root node

const MyRootNode = 
<Welcome name="Foo">
    <Welcome name="Bar"></Welcome>
</Welcome>;

• Output: