JavaScript function that adds numbers

addNumbers.js

function addNumbers(startNumber, endNumber) {
      var sum = 0;

      if (startNumber < endNumber) {
        for (var i = startNumber; i <= endNumber; i++) {
          sum += i;
        }
      } else {
        for (var i = startNumber; i >= endNumber; i--) {
          sum += i;
        }
      }

      return sum;
    }