What is a virtual DOM? Why is it needed? What are the three conditions of the tree comparison algorithm (diffing)?

The Virtual DOM is an abstract representation of the real DOM tree in memory. Instead of directly modifying the real DOM, changes are first applied to the virtual DOM, and then the minimal set of changes needed to update the real DOM is calculated.

This improves performance because operations on the real DOM are costly and slow.

Three conditions of the tree comparison (diffing) algorithm:

1. Different node types are considered different: if the element types differ (e.g., <div> and <span>), the nodes are considered different, and the old node is replaced with the new one.

2. Comparison by keys: for lists of elements, a key is used to efficiently match elements and minimize reordering.

3. Recursive comparison of children: if nodes are of the same type, their properties are compared, and their children are recursively compared to determine the minimal changes.

Thus, the virtual DOM allows efficient UI updates by minimizing operations on the real DOM and improving application responsiveness.