Understanding the DOM Deeply

What is DOM?

The DOM (Document Object Model) is a tree-like structure of nodes, where each node represents a part of the document.

When we open the browser console, we can see the HTML of the website we visited. In reality, the HTML is passed to the browser as a stream of text, and then the DOM is built by:

  • Parsing the HTML using the HTML parser
  • Converting HTML tags into element nodes
  • Converting text into text nodes

Build Flow of the DOM in the Browser

Stream of text

==> Check for tag (if found, run script first)

==> HTML parser converts text

==> HTML tags converted into nodes

==> Build DOM tree

Flow diagram showing how the browser parses HTML and builds the DOM tree

Figure 1: Flow of DOM construction in the browser

Relation with JavaScript

  • The DOM is not part of JavaScript.
  • It is a Web API provided by browsers that allows JavaScript to interact with and manipulate websites.

Difference with Virtual DOM

  • The Virtual DOM is used in React.
  • It is a lightweight representation of the real DOM tree.
  • Essentially, it is a JavaScript object representation of the UI.
  • When an event happens in React:
    1. A new Virtual DOM is created
    2. It is compared with the old one using diffing
    3. Differences are found using the reconciliation algorithm, and only those parts are updated in the real DOM

Diagram comparing Real DOM and Virtual DOM updates in React

Figure 2: Real DOM vs Virtual DOM updates in React

Conclusion

The DOM is the foundation of how browsers render and interact with web pages. It provides a tree-like structure that JavaScript can manipulate to make websites dynamic and interactive. However, working directly with the real DOM can be slow and inefficient for large or frequently updated applications.

That’s where the Virtual DOM comes in. By keeping a lightweight copy of the real DOM in memory, React can efficiently determine what needs to change and update only those parts — resulting in faster, smoother performance.

Understanding both the DOM and the Virtual DOM is essential for every modern web developer. With this knowledge, you’ll have a clearer view of how browsers work under the hood and how frameworks like React optimize UI updates.

💡 What about you? Have you faced performance issues while manipulating the DOM directly? Or do you prefer working with frameworks like React that handle the Virtual DOM for you? Share your thoughts in the comments!