Topic
Definition
Controlled inputs receive their displayed value from React state and report edits through event handlers.
In simpler words
State owns the field value, so typing updates state and state updates the input.
Controlled inputs, change events, and form submission.
Controlled inputs, change events, and form submission.
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.
const [name, setName] = useState("");
const [error, setError] = useState<string | null>(null);
<input
value={name}
onChange={e => setName(e.target.value)}
onBlur={() => setError(name.trim() ? null : "Required")}
onFocus={() => setError(null)}
/>;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
<input value={name} />
// Right
<input value={name} onChange={e => setName(e.target.value)} />Controlled inputs need both value and onChange.
// Wrong
onChange={e => {
setEmail(e.target.value);
setError(valid(e.target.value) ? null : "bad");
}}
// Right
onChange={e => setEmail(e.target.value)}
onBlur={() => setError(valid(email) ? null : "bad")}Blur validates the finished edit; change keeps the value.
// Wrong
<form>
<button type="button" onClick={save}>Save</button>
</form>
// Right
<form onSubmit={e => { e.preventDefault(); save(); }}>
<button type="submit">Save</button>
</form>Enter key users need onSubmit.
Live playground
Change one input at a time and predict the next render.
const [count, setCount] = useState(0); setCount(c => c + 1);
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