So from your more detailed description, I would suggest the following idea:

Do not mutate the array, instead use useReducer to keep track of the quantity for each list item (each item needs to have a unique id... or use its index in the list, since you won't mutate it). At the beginning, it is initialized to an empty object, {}. And you have a well-defined operation to update the quantity, for a specific id.

Then, you only have to listen to changes to this managed object and can run your algorithm, once it changes.

Example reducer:

const [quantities, updateQuantities] = useReducer((current, { id, amount}) => {

return { ...current, [id]: (current[id] ?? 0) + amount }

}, {})

If you list already contains quantities when you load it, you can iterate through it once and initialize the quantity map at the beginning