Javascript Array sum all numbers in a range

function sumAll(arr) {
  // sort the array
  function sortNumber(a,b)
  {//from   w  ww.j a  v  a  2 s .  co  m
    return a - b;
  }
  
  arr = arr.sort(sortNumber);

  var sum = 0;
  for (var i = arr[0]; i <= arr[1]; i++) {
    sum = sum + i;
  }
    return sum;
}

console.log(sumAll([1, 4]));
console.log(sumAll([4, 1]));
console.log(sumAll([5, 10]));
console.log(sumAll([10, 5]));