The Virtual DOM in React JS is an in-memory representation of the actual Document Object Model (DOM) that exists in the browser. The virtual DOM is a lightweight, abstract representation of the actual DOM that can be manipulated by React without directly affecting the browser DOM, which can be expensive in terms of performance.
The virtual DOM is used to compare the previous and current states of a component and determine the minimum set of changes needed to update the actual DOM. This process is known as reconciliation, and it involves comparing the old virtual DOM tree with the new virtual DOM tree and calculating the differences between them.
Once the differences have been calculated, React updates the browser DOM with only the changes that are needed, minimizing the number of updates required and improving performance.