What is the virtual DOM and why does React maintain one instead of writing to the DOM on every state change? Sketch the cycle in pseudocode.

Sketch the two big assumptions React's diffing algorithm makes to stay O(n). Show a snippet that triggers the bad path because of unstable keys.

This list loses input focus and state every time the user reorders rows. Why? Fix the keys.

When is using the array index as a key actually safe? Give one concrete example and one concrete counter-example.