What is React Fiber and why did React rewrite its reconciler?
Frontend mid-senior interviews ask about Fiber to test whether you understand React deeper than "it uses a virtual DOM".
Pre-Fiber (stack reconciler): recursion walks the whole tree synchronously. Big trees = long frames = jank.
Fiber: each node becomes a "fiber" (work unit) in a linked-list-like structure with parent/child/sibling pointers — enables pausing/resuming.
Two phases: render/reconciliation (interruptible, builds work-in-progress tree) and commit (synchronous, applies DOM changes).
Priority lanes: user input > animations > data fetch. High-priority updates can preempt low-priority ones.
Enables Suspense, concurrent rendering, automatic batching — all features that need interruptible work.
Pre-16 React used recursive reconciliation — one long synchronous walk of the tree. On big trees that blocks the main thread and drops frames. Fiber turns that recursion into an iteration over a linked list of fiber nodes, each a unit of work the scheduler can pause at. The render phase is interruptible and builds a work-in-progress tree; the commit phase is synchronous and applies all DOM mutations atomically. The architecture unlocks Suspense, transitions, and concurrent rendering — none of which are possible when you can't pause. The practical interview takeaway: whenever React needs to be "smart" about what to render first, Fiber is the engine that makes it possible.
Don't confuse Fiber (the architecture) with concurrent mode (the feature set it enables).
The double-buffering "current tree vs work-in-progress tree" is a key mental model.
HireMe AI (即答侠) pairs Fiber explanations with the Suspense/Transition example so the answer feels concrete, not theoretical.
Not always — on small trees the bookkeeping overhead slightly hurts. The win is on large trees and interactive UIs where pausing matters.
React 18 finally turned on concurrent features that Fiber had been preparing for since 16. Fiber is the "engine", 18's APIs are the "car".
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