React Components

A Component is one of the core building blocks of React. In other words, we can say that every application you will develop in React will be made up of pieces called components. Components make the task of building UIs much easier. You can see a UI broken down into multiple individual pieces called components and work on them independently and merge them all in a parent component which will be your final UI.
You can see in the below image we have broken down the UI of GeeksforGeeks’s homepage into individual components.

The google’s custom search at the top can be seen as an individual component, the navigation bar can be seen as an individual component, the sidebar is an individual component, the list of articles or post is also an individual component and finally we can merge all of these individual components to make a parent component which will be the final UI for the homepage.

Components in React basically returns a piece of JSX code which tells what should be rendered on the screen. In React we mainly have two types of components:

Functional Components: Functional components are simply javascript functions. We can create a functional component in React by writing a javascript function. These functions may or may not receive data as parameters, we will discuss this later in the tutorial. Below example shows a valid functional component in React:

function Democomponent()
{
    return <h1>Welcome Message!</h1>;
}

Class Components: The class components are little more complex than the functional components. The functional components are not aware about the other components in your program where as the class components can work with each other. We can pass data from one class component to other class component. We can use javascript ES6 classes to create class based components in React. Below example shows a valid class-based component in React:

class Democomponent extends React.Component
{
    render(){
          return <h1>Welcome Message!</h1>;
    }
}

In this post, we will mainly write functional components to make things easier to understand. We will discuss class-based components in details later in the tutorial.

Rendering Components

In our previous post on rendering elements in React we have seen how elements intialized with DOM tags are rendered using ReactDOM.render() method. React is also capable of rendering user-defined components. To render a component in React we can initialize an element with a user-defined component and pass this element as the first parameter to ReactDOM.render() or directly pass the component as first argument to the ReactDOM.render() method.
Below syntax shows how to initialize a component to an element:

const elementName = <ComponentName />;

In the above syntax the ComponentName is the name of the user-defined component.
Note: The name of a component should always start with a capital letter. This is done to differentiate a component tag with html tags.

Below example renders a component named Welcome to the screen:

import React from 'react'; 
import ReactDOM from 'react-dom'; 
  
// This is a functional component 
function Welcome() 
{ 
        return <h1>Hello World!</h1> 
} 
   
ReactDOM.render( 
    <Welcome />,  
    document.getElementById("root") 
); 

Let us see step-wise what is happening in the above example:

• We call the ReactDOM.render() with as the first parameter.
• React then calls the component Welcome, which returns <h1>Hello World!</h1>; as the result.
• Then the ReactDOM efficiently updates the DOM to match with the returned element and renders that element to the DOM element with id as “root”.