How Virtual DOM is Powering React?
Tags: react, javascript, webdev, beginners
Published: 2021-06-02 20:19:09

I have been doing front-end web development for last year only. Lately, I started playing with ReactJS. I have worked on vanilla JavaScript and jQuery for quite some time. My expert colleague at Amazon suggested I explore React. My only sin was I said, "Can't we have an abstraction over jQuery?"

I have heard about React for a long time. The overwhelming number of tutorials on React made my learning very easy. I apologize to myself for not starting early. But better late, than never.

I have worked on compilers for a long time. So, while exploring a new language, I try to think how an interpreter or compiler might do the trick. After learning syntax, I started exploring how React is improving performance. And I encountered Virtual DOM.

[In this article, I will try to explain how virtual DOM might work with respect to a compiler. I will not go into any specific details. I am still learning how React is implemented. I will keep posting details.]

## Document Object Model or DOM ##
A tree of individual components that the browser creates upon receiving an HTML response from a server. JavaScript provides us ways to manipulate DOM to get the desired effect like displaying a div on clicking a button (Codepen sample).

The above operation is expensive as the browser manipulating the DOM tree each time some changes are applied. I believe the browser has to do some smart computation for identifying the changed element and making the changes (like suppose, keeping a reference to each element in a hash map. I am still exploring it.). It is definitely an expensive operation - time and memory-wise. Also, the browser has other tasks to do like displaying the page.

## What is Virtual DOM? ##
"The virtual DOM (VDOM) is a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM by a library such as ReactDOM." - React Documentation .

I did not even try to define it with my words. It is such a concise definition. What I have understood so far - React virtual DOM is an in-memory optimized representation of DOM. And React engine deals with low-level APIs to apply the changes.

## How React brings Performance into Picture? ##
Now, with the above concept, let's bring the compiler into the picture.

Every compiler stores an entire program in some form of representation (Abstract Syntax Tree). I am mapping Virtual DOM to AST (I am still learning how React optimizes). As virtual DOM is an in-memory representation, React engine or compiler can access it fast. Now, let's go step by step.

Now, if there is some change - React changes virtual DOM.
React engine compares the virtual DOM with real DOM (aka "diffing") and applies the changes to the changed part only. It finds out the changes.
It applies the changes using low-level APIs directly (like the JavaScript engine does for our code).
React engine can apply optimizations in all three steps because virtual DOM representation is in its control. React engine is actually a very advanced JavaScript engine. With syntax and semantics of React, it can fasten the performance.

Now, in JS, the browser is doing all the work. Also, to some extent, it is dependent on how code is being written. In the case of React, React engine is abstracting (remember the word I mentioned at the beginning) out all optimization.

This is one of the reasons, React is faster than traditional HTML JS webpages. I know technically everything I mentioned might not be true and I have not touched upon a lot of things. As I keep exploring, I will keep posting my findings in detail. I will see how my concepts get cleared on the way. I hope you enjoyed the article.

Happy learning!