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const myFilter = products.filter( el => (el.name.toLowerCase().indexOf(filterText.toLowerCase()) !== -1) && (!inStockOnly || el.stocked))

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