maximumKey = a => {
  let max;
  for (let i = 1; i < a.length; i++) {
    if (max === undefined || a[i].key > max) {
      max = a[i].key;
    }
  }
  return max;
}
Finds the maximum key from an array of objects

Asymptotic Complexity: O(array.length)

@param Array
@return Integer if array non-empty
        Undefined otherwise