Excises of Eloquent JavaScript 4th edition. Chapter 04 Problem 01. https://eloquentjavascript.net/04_data.html#i_8ZspxiCEC/

Eloquent JavaScript 0401 The sum of a range.js

// Your code here.
const range = (start, end, step = 1) => {
  const ans = [];
  if (step > 0 && start < end) {
    for (let i = start; i <= end; i += step) {
      ans.push(i);
    }
  } else {
    if (step > 0) step = -1;
    for (let i = start; i >= end; i += step) {
      ans.push(i);
    }
  }
  return ans;
}

const sum = (arr) => {
  let ans = 0;
  for (let index in arr) {
    ans += arr[index];
  }
  return ans;
}

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