range and sum from http://eloquentjavascript.net/04_data.html#h_TcUD2vzyMe

range-sum.js

function range(start, end, order) {
  var arr = [], i;

  if (order === -1) {
    for (i = start; i >= end; i--) {
      arr.push(i);
    }
  } else {
    for (i = start; i <= end; i++) {
      arr.push(i);
    }
  }

  return arr;
}

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

function sum(arr) {
  var total = 0, i = 0;
  for (; i < arr.length; i++) {
    total += arr[i];
  }

  return total;
}

console.log(sum(range(1, 10))); // 55
console.log(sum(range(5, 2, -1))); // 14