JavaScript get first and last date of the previous month (week?)

previous_month.js

function previousMonth(originalDate) {
    let f = new Date(originalDate)
    let l = new Date(originalDate)
    l.setDate(0)  //  set to last day of previous month
    f.setDate(0)
    f.setDate(1)  //  set to the first day of that month
    return [f, l]
}

function previousWeek(originalDate) {
    let f = new Date(originalDate)
    let l = new Date(originalDate)
    // set to Monday of this week
    f.setDate(f.getDate() - (f.getDay() + 6) % 7);
    f.setDate(f.getDate() - 8);
    l.setDate(f.getDate() + 6);
    return [f, l]
}
dtFormatShort( nDate ) {  //  MM/DD/YYYY
  let dDate = new Date( nDate * 1000 )
  let year = dDate.getFullYear()
  let month = (1 + dDate.getMonth()).toString()
  let day = dDate.getDate().toString()
  month = month.length > 1 ? month : '0' + month
  day = day.length > 1 ? day : '0' + day
  return month + '/' + day + '/' + year
}