firstSundays.js

function monthDays(year){
  const m = (new Array(12)).fill(1).map((_,x)=>x%2?30:31);
  if (year % 4) m[1] = 28;
  else if (year % 100) m[1] = 29;
  else if (year % 400) m[1] = 28
  else m[1] = 28;
  return m;
}



function firstSundays(start, end){

  let r = 0;
  let sundays = 0;
  for (let y = 1900; y < end; y++){
    for (let days of monthDays(y)){
      r = (days + r) % 7;
      if (r === 0 && y >= start) sundays++
    }
  }
  return sundays;
}