Compute values instead of storing them

How to do it

Keep one source, such as the list of cart lines, and compute the total from it in a function. Memoise only when a profiler shows the computation is slow. A stored copy goes stale the first time someone forgets to update it.

Example

export function cartSummary(lines: CartLine[]) {
  const totalMinor = lines.reduce((sum, line) => sum + line.unitPriceMinor * line.quantity, 0);
  return { totalMinor, itemCount: lines.reduce((sum, line) => sum + line.quantity, 0) };
}

def cart_summary(lines: list[CartLine]) -> dict:
    return {
        "total_minor": sum(line.unit_price_minor * line.quantity for line in lines),
        "item_count": sum(line.quantity for line in lines),
    }

How to check

A search of components and stores for state named total, count or filtered, such as setTotal( or state.itemCount =, finds no matches.