Google Sheets custom function to get month name (either full or 3-character abbreviation), given an integer

getMonth.gs

function getMonthText(val, short) {
  if(val !== parseInt(val,10)) {
    return "";
  }
  if(val < 1 || val > 12) {
    return "";
  }
  var x = "";
  switch(val) {
    case 1:
      x= "January";
      break;
    case 2:
      x= "February";
      break;
    case 3:
      x= "March";
      break;
    case 4:
      x= "April";
      break;
    case 5:
      x= "May";
      break;
    case 6:
      x= "June";
      break;
    case 7:
      x= "July";
      break;
    case 8:
      x= "August";
      break;
    case 9:
      x= "September";
      break;
    case 10:
      x= "October";
      break;
    case 11:
      x= "November";
      break;
    case 12:
      x= "December";
      break;
  }
  if(short) {
    x= x.slice(0,3);
  }
  return x;
}