Topic
Definition
Lifting state up moves shared state to the closest common ancestor so children receive a single source of truth through props.
In simpler words
When siblings must agree, their parent keeps the value and passes it down.
Single source of truth, controlled children, and callbacks.
Single source of truth, controlled children, and callbacks.
Start by identifying which value or browser behavior changes. Then describe the UI from that current input instead of editing the DOM as a separate source of truth.
function Accordion() {
const [openId, setOpenId] = useState(null);
return <Panel open={openId === "a"} onOpen={() => setOpenId("a")} />;
}Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Keep rendering as a calculation. Put user-triggered changes in event handlers, preserve UI memory in state, and reserve external synchronization for Effects or the server-state layer.
Name values by their UI meaning, test the loading and error path when data is remote, and avoid keeping two editable copies of the same value.
Ask before adding code: is this local UI memory, shared client state, or Nest-owned server state?
Definition
High-bug areas are places where a small API misuse looks correct but produces stale UI, duplicate work, or silent failures.
In simpler words
Each mistake below shows Wrong vs Right code — compare them side by side.
When something misbehaves, match the symptom to a pattern below before rewriting the feature.
Prefer fixing the ownership or update path over adding another Effect or sync step.
// Wrong
// ChildA and ChildB each hold openId and Effects copy them
// Right
// Parent holds openId and passes props/callbacksLift shared state instead of syncing copies.
// Wrong
// Every keystroke in a tiny input re-renders the whole app shell state
// Right
// Keep local draft state near the input; lift only what siblings shareLift to the closest common owner.
// Wrong
<Panel open={open} /> // cannot request close
// Right
<Panel open={open} onClose={() => setOpen(false)} />Children notify parents via callbacks.
Live playground
Change one input at a time and predict the next render.
Lifted state — both panels read the same index.
Active panel: A
Test
At least 10 questions — mix of concept, syntax, practical, and logic. Score ≥ 80% (enforced by the API) to save progress.
Checking your session…
10 questions · concept 3 · syntax 3 · practical 2 · logic 2