Calculate the the daily reproduction numbers (R) from an object array

repro-number.js

function reproNumber(data, steps = 4, key = 'value') {
  return data.map((obj, i) => {
    const currentDays = data.slice(i - steps, i);
    const previousDays = data.slice(i - steps * 2, i - steps);
    const currentDaysSum = currentDays.reduce((sum, curr) => { return sum + curr[key]; }, 0);
    const previousDaysSum = previousDays.reduce((sum, curr) => { return sum + curr[key]; }, 0);

    return Object.assign({}, obj, {
      r: currentDaysSum / previousDaysSum
    });
  }).filter(d => d.r && isFinite(d.r));
}