Here's what we generate from a real JD.

What does this useMemo prevent?

Given the component below, what does removing the `useMemo` hook change about its behavior?

function PriceTable({ orders, taxRate }: Props) {
  const totals = useMemo(() => {
    return orders.map((o) => ({
      id: o.id,
      total: o.quantity * o.unitPrice * (1 + taxRate),
    }))
  }, [orders, taxRate])

  return <Table rows={totals} />
}

• A) The component would no longer re-render when `orders` or `taxRate` change.
• B) `totals` would be recomputed on every render of `PriceTable`, causing `<Table>` to receive a new array reference each time and re-render even when its inputs are structurally unchanged.
• C) React would throw a warning because `orders.map` cannot be called outside of a memoization hook.
• D) The component would crash on the first render because `useMemo` is required for any derived data in React 18.

Fix the useEffect leak

A junior engineer wrote the component below to poll an endpoint. It works, but the team is getting alerts about memory pressure in long-lived dashboards. Identify the bug(s) and fix them. Add a comment above each fix explaining what it addresses.

function LiveCount({ id }: { id: string }) {
  const [count, setCount] = useState(0)

  useEffect(() => {
    setInterval(async () => {
      const res = await fetch(`/api/count/${id}`)
      const data = await res.json()
      setCount(data.count)
    }, 1000)
  }, [])

  return <span>{count}</span>
}

function LiveCount({ id }: { id: string }) {
  const [count, setCount] = useState(0)

  useEffect(() => {
    // Your fix here
  }, [])

  return <span>{count}</span>
}

Write a PR description

You're refactoring a data table from client-side sorting to server-side sorting because profiling showed 800ms blocking renders on tables with >10k rows. Write a concise PR description (4–6 sentences) that explains: what changed, why, what reviewers should pay attention to, and any follow-up work. Assume your audience is another senior engineer who hasn't seen the profiling data.