/react-component-performance

Examples

Isolate a ticking timer from a long list

Scenario: A message list re-renders every second because a timer (elapsedMs) lives in the parent component. This causes visible jank on large lists.

Goal: Keep UI identical but limit re-renders to the timer area.

Before (problematic pattern):

function Messages({ items, isThinking, processingStartedAt }) {
  const [elapsedMs, setElapsedMs] = useState(0);

  useEffect(() => {
    if (!isThinking || !processingStartedAt) {
      setElapsedMs(0);
      return;
    }
    setElapsedMs(Date.now() - processingStartedAt);
    const interval = window.setInterval(() => {
      setElapsedMs(Date.now() - processingStartedAt);
    }, 1000);
    return () => window.clearInterval(interval);
  }, [isThinking, processingStartedAt]);

  return (
    <div>
      {items.map((item) => (
        <MessageRow key={item.id} item={item} />
      ))}
      <div>{formatDurationMs(elapsedMs)}</div>
    </div>
  );
}

After (isolated ticking state):

type WorkingIndicatorProps = {
  isThinking: boolean;
  processingStartedAt?: number | null;
};

const WorkingIndicator = memo(function WorkingIndicator({
  isThinking,
  processingStartedAt = null,
}: WorkingIndicatorProps) {
  const [elapsedMs, setElapsedMs] = useState(0);

  useEffect(() => {
    if (!isThinking || !processingStartedAt) {
      setElapsedMs(0);
      return;
    }
    setElapsedMs(Date.now() - processingStartedAt);
    const interval = window.setInterval(() => {
      setElapsedMs(Date.now() - processingStartedAt);
    }, 1000);
    return () => window.clearInterval(interval);
  }, [isThinking, processingStartedAt]);

  return <div>{formatDurationMs(elapsedMs)}</div>;
});

function Messages({ items, isThinking, processingStartedAt }) {
  return (
    <div>
      {items.map((item) => (
        <MessageRow key={item.id} item={item} />
      ))}
      <WorkingIndicator
        isThinking={isThinking}
        processingStartedAt={processingStartedAt}
      />
    </div>
  );
}

Why it helps: Only the WorkingIndicator subtree re-renders every second. The list remains stable unless its props change.

Optional follow-ups:

• Wrap MessageRow in memo if props are stable.
• Use useCallback for handlers passed to rows to avoid re-render churn.
• Consider list virtualization if the list is very large.