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What is React Fiber and what problem does it solve?

javascript

// React 15: Stack-based rendering
// Recursive, synchronous — can't be interrupted

// React 16+: Fiber-based rendering
// Linked-list of work units, can pause/resume/abort

AFiber is a CSS layout algorithm used by React Native

BFiber reimplements the reconciler as interruptible units of work — each component maps to a "fiber" node; React can pause high-priority work, do urgent updates, then resume

CFiber is only used in React Native, not React DOM

DFiber enables multi-threading by moving rendering to a Web Worker

When should you use startTransition?

javascript

const [query, setQuery] = useState("");
const [results, setResults] = useState([]);

function handleSearch(e) {
  // Urgent: update the input immediately
  setQuery(e.target.value);

  // Non-urgent: filter large list (can be deferred)
  startTransition(() => {
    setResults(filterLargeList(e.target.value));
  });
}

AstartTransition marks state updates as non-urgent — React may interrupt them to handle more urgent updates (like user input), preventing UI jank

BstartTransition makes the update run in a separate thread

CstartTransition delays the update by 300ms automatically

DstartTransition is equivalent to setTimeout(fn, 0)

What is tearing in the context of concurrent React?

javascript

// External store (outside React) changes during concurrent rendering
const store = createExternalStore();

function useStore() {
  return store.getSnapshot(); // reads external state
}

function A() { const val = useStore(); /* renders with val=1 */ }
function B() { const val = useStore(); /* store changes mid-render → reads val=2 */ }
// A shows 1, B shows 2 — inconsistent!

ATearing is when different components read different values from an external store during the same render pass — the UI shows an inconsistent snapshot

BTearing means CSS styles are applied unevenly

CTearing only happens in React 17 and earlier

DTearing is prevented automatically by all state management libraries

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