What is the difference between Virtual DOM, Shallow DOM, and DOM in React js?

In ReactJS, the Virtual DOM, Shallow DOM, and DOM are all concepts related to how the framework manages and manipulates the Document Object Model (DOM) of a web page, but they serve different purposes and operate at different levels.

1. DOM (Document Object Model):

   • The DOM is a programming interface for web documents. It represents the structure of HTML and XML documents as a tree-like structure where each node represents an element, attribute, or text in the document.
   • The DOM is what browsers use to render web pages and allows JavaScript to interact with the structure and content of the web page dynamically.

2. Virtual DOM:

   • The Virtual DOM is a lightweight, in-memory representation of the actual DOM.
   • When you update the state of a component in React, the entire component tree gets re-rendered in the Virtual DOM.
   • React then calculates the difference between the current Virtual DOM and the previous one (a process called reconciliation).
   • Once the differences are calculated, React updates only the parts of the actual DOM that need to change, minimizing the number of DOM manipulations required and thus improving performance.

3. Shallow DOM:

   • Shallow DOM is not a term typically associated with ReactJS. However, it's worth mentioning that shallow rendering is a concept used in testing libraries like Enzyme (commonly used with React) to render React components one level deep.
   • Shallow rendering allows you to test a component as a unit without deeply rendering its child components. This can be useful for isolating the behavior of the component under test.
   • Shallow rendering does not deeply render child components or trigger lifecycle methods on them, which can simplify testing and improve test performance.

In summary:

• DOM is the actual tree structure representing the HTML document.
• Virtual DOM is a lightweight, in-memory representation of the actual DOM that React uses to efficiently update the browser's DOM.
• Shallow DOM is not a React concept, but shallow rendering is a testing technique used to render components one level deep, without deeply rendering child components.