Question 1. When does React re-render?

• Re-renders happen when subscribed values change.
• Triggers:
  • state updates (useState, useReducer)
  • Parent component re-renders
  • Props change
  • Context value changes
  • External store (Redux, Zustand, etc.) updates

Notes

• Props are read-only. If parent re-renders, child re-renders unless wrapped in React.memo.
• React.memo skips re-render if props are shallow-equal.
• Don’t copy props into state (can cause inconsistencies).

Question 2. What is the DOM?

• Document Object Model → Browser API representing an HTML/XML document as a tree structure.
• Nodes form a hierarchy: parent/child/sibling.
• Allows JS to manipulate content (add/remove/update elements, attach event handlers).
• Without DOM, static HTML cannot be dynamically updated.

Question 3. Difference between any and unknown in TypeScript?

• Both are top types (can hold any value).

• Use unknown when type is uncertain at runtime (e.g., catch (err: unknown)).
• Narrow with type guards (typeof, instanceof).
• any = unsafe bypass, unknown = safer but requires explicit checks.