Arrays tiene método reduce

 arrOfProducts.reduce((acc, curr) => { const foundedIndex = acc.findIndex(i => curr.id === i.id) if(foundedIndex !== -1){ const item = acc[foundedIndex] return [...acc.slice(0, foundedIndex), {...item, count: item.count+1}, ...acc.slice(foundedIndex+1)] } return [...acc, {...curr, count: 1}] }, [])