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const myFilter = products.filter(el =>
  el.name.toLowerCase().includes(filterText.toLowerCase()) && (inStockOnly ? el.stocked : true) || !inStockOnly
);

const finalList = myFilter.reduce((a, c) => (c.category !== a.lastCategory && a.output.push(<ProductCategoryRow key={c.category} category={c.category} />),
  a.output.push(<ProductRow key={c.name} product={c} />), a.lastCategory = c.category, a), { lastCategory: null, output: [] });