// step parameter is optional
// if step is not passed in,
// and start is less than or equal to end,
// then step = 1, else step = -1
function range(start, end, step = start <= end ? 1 : -1) {
  let result = [];
  // loop iterates up for positive step values
  // and iterates down for negative step values
  for (let i = start; step >= 0 ? i <= end : i >= end; i+=step) {
    result.push(i);
  }
  return result;
}

function sum(numbers) {
  result = 0;
  for (let num of numbers) {
    result += num;
  }
  return result;
}

console.log(range(1, 10));
// → [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
console.log(range(1, 10, 2));
// → [1, 3, 5, 7, 9]
console.log(range(5, 2, -1));
// → [5, 4, 3, 2]
console.log(range(5, 2));
// → [5, 4, 3, 2]
console.log(range(10, 1, -3));
// → [10, 7, 4, 1]
console.log(sum(range(1, 10)));
// → 55
console.log(sum(range(1, 10, 2)));
// → 25
console.log(sum(range(10, 1, -3)));
// → 22 (10 + 7 + 4 + 1)