#3 most askedVery commonEasyLists & performance

FlatList vs ScrollView

🏢 Asked in real interviews

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You need to show 5,000 messages. Why use FlatList instead of ScrollView, and how do you keep it smooth?

🔮 Predict the output

// A list "virtualizer": which rows render for this scroll position?
const rowHeight = 50, screenHeight = 300, buffer = 2;
function visibleRows(scrollY, total) {
  const first = Math.max(0, Math.floor(scrollY / rowHeight) - buffer);
  const last = Math.min(total - 1, Math.ceil((scrollY + screenHeight) / rowHeight) + buffer);
  return [first, last];
}
console.log(visibleRows(0, 5000), visibleRows(10000, 5000));

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