Rendering lists in code
{tickets.map(ticket => (
<li key={ticket.id}>{ticket.title}</li>
))}Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 1
Topic
Rendering lists
Definition
List rendering maps an array to React elements with stable keys that preserve item identity across updates.
In simpler words
Turn each item into JSX with map, and give React an ID it can trust.
map, key placement, and why array indexes can break dynamic lists.
After this you can
- Render API collections without identity bugs.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Rendering lists
map, key placement, and why array indexes can break dynamic lists.
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 Rendering lists
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: Index as key on dynamic lists
// Wrong
items.map((item, i) => <Row key={i} item={item} />)
// Right
items.map((item) => <Row key={item.id} item={item} />)Indexes shift on insert/delete — wrong state sticks to wrong row.
Mistake: Random keys
// Wrong
items.map((item) => <Row key={Math.random()} item={item} />)
// Right
items.map((item) => <Row key={item.id} item={item} />)Random keys remount every render and wipe local state.
Mistake: forEach as children
// Wrong
return <ul>{items.forEach(i => <li>{i}</li>)}</ul>;
// Right
return <ul>{items.map(i => <li key={i.id}>{i.title}</li>)}</ul>;forEach returns undefined; map returns elements.
Live playground
Rendering lists sandbox
Change one input at a time and predict the next render.
- Auth cookie (open)
- Query keys (done)
- Layouts (open)
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