function alignNumber(num) {
  return num < 10 ? `0${num}` : `${num}`;
}

function dateToString(date) {
  const years = date.getFullYear();
  const months = alignNumber(date.getMonth() + 1);
  const days = alignNumber(date.getDate());

  return `${years}-${months}-${days}`;
}

function groupWeeks(data) {
  const dateCounts = data.reduce((acc, item) => {
    const date = new Date(item.date);

    const monday = dateToString(
      new Date(date.setDate(date.getDate() - date.getDay() + 1))
    );

    if (!acc[monday]) {
      acc[monday] = 0;
    }

    acc[monday] += item.count;
    return acc;
  }, {});

  return Object.keys(dateCounts).reduce((acc, date) => {
    const item = {
      weekStart: date,
      count: dateCounts[date]
    };

    acc.push(item);
    return acc;
  }, []);
}