What is React? in code
const title = "Tickets";
return <h1>{title}</h1>;Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 1
Topic
What is React?
Definition
React is a JavaScript library for building user interfaces from reusable components and state.
In simpler words
React lets you describe the screen from data; it updates the browser when that data changes.
Declarative UI, component trees, and one-way data flow.
After this you can
- Return UI from state instead of manually changing DOM nodes.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand What is React?
Declarative UI, component trees, and one-way data flow.
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.
Apply What is React?
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?
Where bugs hide
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.
Mistake: Treating React as the backend
// Wrong
// In a Client Component
const rows = await db.query("select * from tickets");
// Right
const { data } = useQuery({
queryKey: ["tickets"],
queryFn: () => ticketsApi.list(),
});Nest owns the database. React displays Nest responses.
Mistake: querySelector instead of state
// Wrong
useEffect(() => {
document.getElementById("label")!.innerText = title;
}, [title]);
// Right
return <p id="label">{title}</p>;Put the value in state/props and render it — do not poke the DOM.
Mistake: Assuming full page remount
// Wrong
// "I must reload the page to refresh the list"
// Right
// Invalidate Query / update state → React re-renders the subtreeUpdates usually refresh a component tree, not the whole document.
Live playground
What is React? sandbox
Change one input at a time and predict the next render.
Hello, Ada
function Hello({ name }) {
return <h1>Hello, {name}</h1>;
}Keep in mind
- Keep the formal definition in mind; it explains which tool belongs where.
- Prefer one source of truth over synchronized copies of the same value.
- When behavior surprises you, trace: input → update → render → committed UI.
- Study the Wrong vs Right examples in “Where bugs hide” before you merge.
Test
Check your understanding
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