ReactJS ReactDOM

In the previous article on introduction to JSX, we learned that we can use JSX to store HTML markups within Javascript variables. Now, ReactJS is a library to build active User Interfaces thus rendering is one of the integral parts of ReactJS. React provides the developers with a package react-dom a.k.a ReactDOM to access and modify the DOM. Let’s see in brief what is the need of having the package. 

What is DOM?

DOM, abbreviated as Document Object Model, is a World Wide Web Consortium standard logical representation of any webpage. In easier words, DOM is a tree-like structure that contains all the elements and it’s properties of a website as its nodes. DOM provides a language-neutral interface that allows accessing and updating of the content of any element of a webpage. 

Before React, Developers directly manipulated the DOM elements which resulted in frequent DOM manipulation, and each time an update was made the browser had to recalculate and repaint the whole view according to the particular CSS of the page, which made the total process to consume a lot of time. As a betterment, React brought into the scene the virtual DOM. The Virtual DOM can be referred to as a copy of the actual DOM representation that is used to hold the updates made by the user and finally reflect it over to the original Browser DOM at once consuming much lesser time. 

What is ReactDOM?

ReactDOM is a package that provides DOM specific methods that can be used at the top level of a web app to enable an efficient way of managing DOM elements of the web page. ReactDOM provides the developers with an API containing the following methods and a few more. 

• render()
• findDOMNode()
• unmountComponentAtNode()
• hydrate()
• createPortal()

Pre-requisite: To use the ReactDOM in any React web app we must first import ReactDOM from the react-dom package by using the following code snippet:  

import ReactDOM from 'react-dom'

render() Function

This is one of the most important methods of ReactDOM. This function is used to render a single React Component or several Components wrapped together in a Component or a div element. This function uses the efficient methods of React for updating the DOM by being able to change only a subtree, efficient diff methods, etc. 

Syntax: 

ReactDOM.render(element, container, callback)

Parameters: This method can take a maximum of three parameters as described below.  

• element: This parameter expects a JSX expression or a React Element to be rendered.
• container: This parameter expects the container in which the element has to be rendered.
• callback: This is an optional parameter that expects a function that is to be executed once the render is complete.

Return Type: This function returns a reference to the component or null if a stateless component was rendered. 

findDOMNode() Function

This function is generally used to get the DOM node where a particular React component was rendered. This method is very less used like the following can be done by adding a ref attribute to each component itself. 

Syntax: 

ReactDOM.findDOMNode(component)

Parameters: This method takes a single parameter component that expects a React Component to be searched in the Browser DOM. 

Return Type: This function returns the DOM node where the component was rendered on success otherwise null. 

unmountComponentAtNode() Function

This function is used to unmount or remove the React Component that was rendered to a particular container. As an example, you may think of a notification component, after a brief amount of time it is better to remove the component making the web page more efficient. 

Syntax:  

ReactDOM.unmountComponentAtNode(container)

Parameters: This method takes a single parameter container which expects the DOM container from which the React component has to be removed. 

Return Type: This function returns true on success otherwise false. 

hydrate() Function

This method is equivalent to the render() method but is implemented while using server-side rendering. 

Syntax: 

ReactDOM.hydrate(element, container, callback)

Parameters: This method can take a maximum of three parameters as described below. 

• element: This parameter expects a JSX expression or a React Component to be rendered.
• container: This parameter expects the container in which the element has to be rendered.
• callback: This is an optional parameter that expects a function that is to be executed once the render is complete.

Return Type: This function attempts to attach event listeners to the existing markup and returns a reference to the component or null if a stateless component was rendered. 

createPortal() Function

Usually, when an element is returned from a component’s render method, it’s mounted on the DOM as a child of the nearest parent node which in some cases may not be desired. Portals allow us to render a component into a DOM node that resides outside the current DOM hierarchy of the parent component. 

Syntax: 

ReactDOM.createPortal(child, container)

Parameters: This method takes two parameters as described below.  

• child: This parameter expects a JSX expression or a React Component to be rendered.
• container: This parameter expects the container in which the element has to be rendered.

Return Type: This function returns nothing. 

Important Points to Note:

• ReactDOM.render() replaces the child of the given container if any. It uses a highly efficient diff algorithm and can modify any subtree of the DOM.
• findDOMNode() function can only be implemented upon mounted components thus Functional components can not be used in findDOMNode() method.
• ReactDOM uses observables thus provides an efficient way of DOM handling.
• ReactDOM can be used on both the client-side and server-side.

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