Unit test newbie - how a novice writes his first unit tests

I did have to test if that stack overflow code would worked. I made some changes to follow the recommendations of the moment js documentation

function weekDays(month, year) {
    const endDate = moment.utc().year(year).month(month).endOf('month');

    return Array(endDate.date()).fill(0).map((_, i) => moment.utc().year(year).month(month).date(i + 1))
        .map((day) => ({day, week: day.week()}))
        .filter(({week}, i, arr) => arr.findIndex((info) => info.week === week) === i)
        .map(({day, week}) => ({
            week,
            days: Array(7).fill(0).map((_, i) => moment.utc(day).week(week).startOf('week').add(i, 'day'))
        }));

}

Test with Jest Framework

let testAll = (month, year) => {
    const monthYear = moment().year(year).month(month)
    const firstDay = monthYear.startOf('month');
    // console.log(firstDay)
    const endDay = moment().year(year).month(month).endOf("month");
    const calendar = weekDays(month, year);

    describe("testing calendar", () => {
        it('check if first date is in first week / should be true', () => {
            expect(calendar[0]["days"].filter((day) => day.date() === firstDay.date())[0].format("YYYY-MM-DD") === firstDay.format("YYYY-MM-DD")).toBe(true);
        });

       it('check if end date is in end week / should be true', () => {
            expect(calendar[calendar.length - 1]["days"].filter((day) => day.date() === endDay.date())[0].format("YYYY-MM-DD") === endDay.format("YYYY-MM-DD")).toBe(true);
        });
    });
}

Once modified I had to know if it would work, and so the need arise to test the code until 80 years

let locale = ["en", "es", "fr", "de"]
for (let i in locale){
    moment.locale(`${locale[i]}`)
    for (let y = 2000; y < 2100; y++) {
        for (let m = 0; m <= 11; m++) {
            testAll(m, y)
            console.log(m)
        }
    }
}

P.S: I'm not an expert in javascript and it's my first unit test