Weekend in Javascript

isWeekend.js

function isWeekend(thisDate) {
  let tf = Object.prototype.toString.call(thisDate) === '[object Date]'
  if (tf)
    throw new Exception("input parameter is not a Date)

  //.getDay() returns a integer between 0 and 6 (0 being Sunday, 6 being Saturday). 
  if( thisDate.getDay() == 6 || thisDate.getDay() == 0 ) 
    return true;
  else
    return false;
}
/* *** USAGE *** */
let dt = new Date()
if isWeekend(dt)
  alert(`${dt} is weekend')
else
  alert(`${dt} is NOT weekend')