Just like python range

range.js

function range(start, end, step = 1) {
  const rangeOfNums = []
  if (step < 0) {
    for (let i = start; i >= end; i -= Math.abs(step)) {
      rangeOfNums.push(i);
    };
  } else {
    for (let i = start; i <= end; i += step) {
      rangeOfNums.push(i);
    };
  }

  return rangeOfNums;
}

function sum(nums) {
  let sumOfNums = 0
  for (let i = 0; i <nums.length; i++) {
    sumOfNums += nums[i]
  }

  return sumOfNums;
}

console.log(sum(range(5, 2, -1)))