React Internals: Fiber Architecture and Reconciliation

FAANG technical rounds for senior frontend roles routinely dive into React internals. Candidates are expected to explain the heuristics of the O(n) diffing algorithm and why keys enable stable component identity across renders.

Step through the internal Fiber node data structure with child, sibling, and return pointers. Understand the cooperative work loop, the separation between interruptible render work and synchronous commit work, and how the SyntheticEvent system delegates events to the root container.