Updating old code, arrow function doesn't work

var upper = 10;
var randomNumber = getRandomNumber(upper);
var guess;
var attempts = 0;

const getRandomNumber = upper => Math.floor(Math.random() * upper) + 1;

while (guess !== randomNumber){
  guess = getRandomNumber(upper);
  attempts += 1;
}
document.write(getRandomNumber(upper));
document.write("The number was " + randomNumber + "<br />");
document.write(" It took the computer " + attempts + " attempts");