dateformatter.js

const getLastDayOfMonth = (year, month, locale = 'en') => {
  const endDate = new Date(+year, +month+1, 0);
  return endDate.toLocaleDateString(locale);
};

const getFirstDayOfMonth = (year, month, locale = 'en') => {
  const endDate = new Date(+year, +month, 1);
  return endDate.toLocaleDateString(locale);
};

const formatYYYYMMDDwithHyphens = (date) => {
  const objDate = new Date(date);
  const month = objDate.getMonth().toString().padStart(2, '0');
  const day = objDate.getDate().toString().padStart(2, '0');
  const year = objDate.getFullYear();
  return `${year}-${month}-${day}`;
};

const totest = new Date('1/2/2021');// mois / jour / année

const monthAsString = totest.getMonth().toString();
const yearAsString = totest.getFullYear().toString();
const dayAsString = totest.getDate().toString();
console.log(dayAsString, monthAsString, yearAsString );

const lastDayOfMonth = getLastDayOfMonth(yearAsString, monthAsString);
const firstDayOfMonth = getFirstDayOfMonth(yearAsString, monthAsString);
console.log(firstDayOfMonth, lastDayOfMonth);

const formatedFirst = formatYYYYMMDDwithHyphens(firstDayOfMonth);
const formatedLast = formatYYYYMMDDwithHyphens(lastDayOfMonth);

console.log(formatedFirst, formatedLast);