Conditional rendering in code
return isAdmin ? <DeleteButton /> : <p>Read-only</p>;Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 1
Topic
Conditional rendering
Definition
Conditional rendering selects which React elements to return based on current props or state.
In simpler words
Use conditions to choose the current UI branch.
Ternary, &&, early return, and empty states.
After this you can
- Show loading, error, empty, and success branches deliberately.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Conditional rendering
Ternary, &&, early return, and empty states.
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 Conditional rendering
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: && with zero
// Wrong
{count && <Badge />}
// count===0 renders "0"
// Right
{count > 0 ? <Badge /> : null}0 is falsy but still a valid React child.
Mistake: Silent null for errors
// Wrong
if (error) return null;
// Right
if (error) return <p role="alert">{error.message}</p>;Show error/empty/loading branches explicitly.
Mistake: Unreadable nested ternaries
// Wrong
return a ? b ? c : d : e ? f : g;
// Right
if (!a) return <E />;
if (b) return <C />;
return <D />;Prefer early returns or small components.
Live playground
Conditional rendering sandbox
Change one input at a time and predict the next render.
Please sign in
{loggedIn ? <Welcome /> : <Login />}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