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Glossary

Term	Definition
JSX	Syntax compiling to `React.createElement` — markup inside functions.
Component	Reusable UI unit returning React elements.
Props	Read-only inputs from parent to child.
State	Mutable component data; updates trigger re-render.
Hook	`use*` API for state, effects, context, transitions, etc.
Virtual DOM	In-memory element tree diffed before DOM patches.
Reconciliation	Diffing prev vs next trees to compute updates.
Fiber	Incremental render work unit — pause, resume, prioritize.
Render phase	Pure diff; interruptible, no DOM writes.
Commit phase	Apply DOM updates; run layout/paint effects.
Context	Share data without prop drilling at every level.
Prop drilling	Passing props through unused intermediaries.
Colocation	State placed closest to its consumers.
useEffect	Post-paint side effects (fetch, subscriptions, DOM sync).
useReducer	Complex state via reducer + dispatch.
useMemo	Cache computed value until deps change.
useCallback	Cache function reference until deps change.
React.memo	Skip re-render when props are shallow-equal.
Key	Sibling-scoped identity for list reconciliation.
Suspense	Loading boundary with `fallback` while children suspend.
React.lazy	Dynamic `import()` wrapper for code-split components.
useTransition	Mark updates non-urgent; returns `[isPending, startTransition]`.
startTransition	Mark low-priority updates outside hooks.
useDeferredValue	Lagging value copy for deferring expensive renders.
Concurrent rendering	Interruptible rendering — urgent work preempts transitions.
Streaming SSR	Progressive HTML flush as Suspense sections resolve.
Selective hydration	Hydrate interactive regions by priority.
Error Boundary	Catches render/lifecycle errors in child tree.
use()	Read Promise or Context during render; suspends (React 19+).
Batching	Group state updates into fewer renders (React 18+).
HOC / render props	Legacy composition — prefer hooks + compounds in new code.