What is Virtual DOM? Why is it needed? When is it built? Can there be multiple React instances in one application?

Virtual DOM is an abstract representation of the real DOM in memory. React uses the Virtual DOM for efficient UI updates.

Why it is needed:

• Changes in the UI are first applied to the Virtual DOM.
• React compares the new state of the Virtual DOM with the previous one (this process is called "reconciliation").
• Based on the differences, React minimally updates the real DOM, which improves performance.

When it is built:

• The Virtual DOM is created during the first render of a component.
• When the state or props change, React recreates the Virtual DOM for the updated tree.

Multiple React instances in one application are possible, but usually not recommended due to increased bundle size and potential conflicts. However, in micro-frontend architectures or during gradual migration, multiple React versions or instances on a single page can be used.