group duplicate items in array

group.js

function groupItems(data) {
  const group1 = data.reduce((sums, item) => {
    const existingIndex = sums.findIndex(sum => sum.sku === item.sku);
    const existing = sums[existingIndex];
    if (!existing) return sums.concat(item);
    return [
      ...sums.slice(0, existingIndex),
      {
        sku: item.sku,
        quantity: existing.quantity + item.quantity
      },
      ...sums.slice(existingIndex + 1)
    ];
  }, []);
  return group1
}