JavaScript - get weeks in a month as array of start and end days

getWeeksInMonth.js

//note: month is 0 based, just like Dates in js
function getWeeksInMonth(year, month) {
  const weeks = [],
    firstDate = new Date(year, month, 1),
    lastDate = new Date(year, month + 1, 0),
    numDays = lastDate.getDate();

  let dayOfWeekCounter = firstDate.getDay();

  for (let date = 1; date <= numDays; date++) {
    if (dayOfWeekCounter === 0 || weeks.length === 0) {
      weeks.push([]);
    }
    weeks[weeks.length - 1].push(date);
    dayOfWeekCounter = (dayOfWeekCounter + 1) % 7;
  }

  return weeks
    .filter((w) => !!w.length)
    .map((w) => ({
      start: w[0],
      end: w[w.length - 1],
      dates: w,
    }));
}

exports.getWeeksInMonth = getWeeksInMonth;

getWeeksInMonth.spec.js

const getWeeksInMonth = require("./getWeeksInMonth").getWeeksInMonth;

describe("getWeeksInMonth", () => {
  test("June 2019 (starts on Saturday)", () => {
    const weeks = getWeeksInMonth(2019, 5);
    expect(weeks.length).toBe(6);
    expect(weeks[0].start).toBe(1);
    expect(weeks[0].end).toBe(1);
  });

  test("Feb 2021 (starts on Monday)", () => {
    const weeks = getWeeksInMonth(2021, 1);
    expect(weeks.length).toBe(5);
    expect(weeks[0].start).toBe(1);
    expect(weeks[0].end).toBe(6);

    expect(weeks[4].start).toBe(28);
    expect(weeks[4].end).toBe(28);
  });

  test("Aug 2021 (starts on Sunday)", () => {
    const weeks = getWeeksInMonth(2021, 7);
    expect(weeks.length).toBe(5);
    expect(weeks[0].start).toBe(1);
    expect(weeks[0].end).toBe(7);

    expect(weeks[4].start).toBe(29);
    expect(weeks[4].end).toBe(31);
  });
});