function sortByThenBy(arr, props) {
  // apply custom sort function on array
  return arr.sort(function(a, b) {
    // generate compare function return value by
    // iterating over the properties array
    return props.reduce(function(bool, k) {
      // if previous compare result is `0` then compare
      // with the next property value and return result
      return bool || (a[k] - b[k]);
      // set initial value as 0
    }, 0);
  })
}