I have an array of objects that contains numeric values. When I loop through it I need to set in the following logic the values in a way that each value starting from second element is equal to the previous value -80

  result.data.forEach((dataItem) => {
        if (dataItem.values && dataItem.values.length) {
          dataItem.sumValue = dataItem.values.reduce((sum, item) => {
            return (sum + item.value)
          }, 0)
        } else dataItem.sumValue = dataItem.value

So each dataItem.sumValue should be equal to the previous value -80.

How can it be achieved?