Updating arrays in state in code
setTickets(items => items.filter(ticket => ticket.id !== id));Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 1
Topic
Updating arrays in state
Definition
Array state should be updated by creating a new array with non-mutating methods such as map, filter, or spread.
In simpler words
Build a new list for add, edit, remove, or reorder actions instead of changing the old array.
Adding, removing, replacing, and sorting immutable arrays.
After this you can
- Update list UI without corrupting existing state.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Updating arrays in state
Adding, removing, replacing, and sorting immutable arrays.
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 Updating arrays in state
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: push on state array
// Wrong
tickets.push(newTicket);
setTickets(tickets);
// Right
setTickets([...tickets, newTicket]);Build a new array.
Mistake: sort in place
// Wrong
tickets.sort();
setTickets(tickets);
// Right
setTickets([...tickets].sort());sort mutates; copy first.
Mistake: Fighting Query with a local copy
// Wrong
const [tickets, setTickets] = useState([]);
// manual merge after every mutation
// Right
const q = useQuery({ queryKey: ["tickets"], queryFn: list });
// mutate + invalidateQueriesLet Query own server arrays.
Live playground
Updating arrays in state sandbox
Change one input at a time and predict the next render.
- Auth
- Query
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